[7/7] riscv: Add support for str(n)cmp inline expansion

Message ID 20221113230521.712693-8-christoph.muellner@vrull.eu
State Unresolved
Headers
Series riscv: Improve builtins expansion |

Checks

Context Check Description
snail/gcc-patch-check warning Git am fail log

Commit Message

Christoph Müllner Nov. 13, 2022, 11:05 p.m. UTC
  From: Christoph Müllner <christoph.muellner@vrull.eu>

This patch implements expansions for the cmpstrsi and the cmpstrnsi
builtins using Zbb instructions (if available).
This allows to inline calls to strcmp() and strncmp().

The expansion basically emits a peeled comparison sequence (i.e. a peeled
comparison loop) which compares XLEN bits per step if possible.

The emitted sequence can be controlled, by setting the maximum number
of compared bytes (-mstring-compare-inline-limit).

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (riscv_expand_strn_compare): New
	  prototype.
	* config/riscv/riscv-string.cc (GEN_EMIT_HELPER3): New helper
	  macros.
	(GEN_EMIT_HELPER2): New helper macros.
	(expand_strncmp_zbb_sequence): New function.
	(riscv_emit_str_compare_zbb): New function.
	(riscv_expand_strn_compare): New function.
	* config/riscv/riscv.md (cmpstrnsi): Invoke expansion functions
	  for strn_compare.
	(cmpstrsi): Invoke expansion functions for strn_compare.
	* config/riscv/riscv.opt: Add new parameter
	  '-mstring-compare-inline-limit'.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
---
 gcc/config/riscv/riscv-protos.h               |   1 +
 gcc/config/riscv/riscv-string.cc              | 344 ++++++++++++++++++
 gcc/config/riscv/riscv.md                     |  46 +++
 gcc/config/riscv/riscv.opt                    |   5 +
 .../gcc.target/riscv/zbb-strcmp-unaligned.c   |  36 ++
 gcc/testsuite/gcc.target/riscv/zbb-strcmp.c   |  55 +++
 6 files changed, 487 insertions(+)
 create mode 100644 gcc/testsuite/gcc.target/riscv/zbb-strcmp-unaligned.c
 create mode 100644 gcc/testsuite/gcc.target/riscv/zbb-strcmp.c
  

Comments

Jeff Law Nov. 14, 2022, 7:28 p.m. UTC | #1
On 11/13/22 16:05, Christoph Muellner wrote:
> From: Christoph Müllner <christoph.muellner@vrull.eu>
>
> This patch implements expansions for the cmpstrsi and the cmpstrnsi
> builtins using Zbb instructions (if available).
> This allows to inline calls to strcmp() and strncmp().
>
> The expansion basically emits a peeled comparison sequence (i.e. a peeled
> comparison loop) which compares XLEN bits per step if possible.
>
> The emitted sequence can be controlled, by setting the maximum number
> of compared bytes (-mstring-compare-inline-limit).
>
> gcc/ChangeLog:
>
> 	* config/riscv/riscv-protos.h (riscv_expand_strn_compare): New
> 	  prototype.
> 	* config/riscv/riscv-string.cc (GEN_EMIT_HELPER3): New helper
> 	  macros.
> 	(GEN_EMIT_HELPER2): New helper macros.
> 	(expand_strncmp_zbb_sequence): New function.
> 	(riscv_emit_str_compare_zbb): New function.
> 	(riscv_expand_strn_compare): New function.
> 	* config/riscv/riscv.md (cmpstrnsi): Invoke expansion functions
> 	  for strn_compare.
> 	(cmpstrsi): Invoke expansion functions for strn_compare.
> 	* config/riscv/riscv.opt: Add new parameter
> 	  '-mstring-compare-inline-limit'.

Presumably the hybrid inline + out of line approach is to capture the 
fact that most strings compare unequal early, then punt out to the 
library if they don't follow that model?  It looks like we're structured 
for that case by peeling iterations rather than having a fully inlined 
approach.  Just want to confirm...


I was a bit worried about the "readahead" problem that arises when 
reading more than a byte and a NUL is found in the first string.  If 
you're not careful, the readahead of the second string could fault.  But 
it looks like we avoid that by requiring word alignment on both strings.


> +
> +/* Emit a string comparison sequence using Zbb instruction.
> +
> +   OPERANDS[0] is the target (result).
> +   OPERANDS[1] is the first source.
> +   OPERANDS[2] is the second source.
> +   If NO_LENGTH is zero, then:
> +   OPERANDS[3] is the length.
> +   OPERANDS[4] is the alignment in bytes.
> +   If NO_LENGTH is nonzero, then:
> +   OPERANDS[3] is the alignment in bytes.

Ugh.  I guess it's inevitable unless we want to drop the array and pass 
each element individually (in which case we'd pass a NULL_RTX in the 
case we don't have a length argument).


I'd like to give others a chance to chime in here.  Everything looks 
sensible here, but I may have missed something.  So give the other 
maintainers a couple days to chime in before committing.


Jeff
  
Christoph Müllner Nov. 14, 2022, 9:49 p.m. UTC | #2
On Mon, Nov 14, 2022 at 8:28 PM Jeff Law <jeffreyalaw@gmail.com> wrote:

>
> On 11/13/22 16:05, Christoph Muellner wrote:
> > From: Christoph Müllner <christoph.muellner@vrull.eu>
> >
> > This patch implements expansions for the cmpstrsi and the cmpstrnsi
> > builtins using Zbb instructions (if available).
> > This allows to inline calls to strcmp() and strncmp().
> >
> > The expansion basically emits a peeled comparison sequence (i.e. a peeled
> > comparison loop) which compares XLEN bits per step if possible.
> >
> > The emitted sequence can be controlled, by setting the maximum number
> > of compared bytes (-mstring-compare-inline-limit).
> >
> > gcc/ChangeLog:
> >
> >       * config/riscv/riscv-protos.h (riscv_expand_strn_compare): New
> >         prototype.
> >       * config/riscv/riscv-string.cc (GEN_EMIT_HELPER3): New helper
> >         macros.
> >       (GEN_EMIT_HELPER2): New helper macros.
> >       (expand_strncmp_zbb_sequence): New function.
> >       (riscv_emit_str_compare_zbb): New function.
> >       (riscv_expand_strn_compare): New function.
> >       * config/riscv/riscv.md (cmpstrnsi): Invoke expansion functions
> >         for strn_compare.
> >       (cmpstrsi): Invoke expansion functions for strn_compare.
> >       * config/riscv/riscv.opt: Add new parameter
> >         '-mstring-compare-inline-limit'.
>
> Presumably the hybrid inline + out of line approach is to capture the
> fact that most strings compare unequal early, then punt out to the
> library if they don't follow that model?  It looks like we're structured
> for that case by peeling iterations rather than having a fully inlined
> approach.  Just want to confirm...
>

Yes, this was inspired by gcc/config/rs6000/rs6000-string.cc
(e.g. expand_strncmp_gpr_sequence).

The current implementation emits an unrolled loop to process up to N
characters.
For longer strings, we do a handover to libc to process the remainder there.
The hand-over implies a call overhead and, of course, a well-optimized
str(n)cmp
implementation would be beneficial (once we have the information in user
space for ifuncs).

We can take this further, but then the following questions pop up:
* how much data processing per loop iteration?
* what about unaligned strings?

Happy to get suggestions/opinions for improvement.


> I was a bit worried about the "readahead" problem that arises when
> reading more than a byte and a NUL is found in the first string.  If
> you're not careful, the readahead of the second string could fault.  But
> it looks like we avoid that by requiring word alignment on both strings.
>

Yes, aligned strings are not affected by the readahead.

I wonder if we should add dynamic tests in case the compiler cannot derive
XLEN-alignment so we capture more cases (e.g. character-arrays have
guaranteed alignment 1, but are allocated with a higher actual alignment on
the stack).


> > +
> > +/* Emit a string comparison sequence using Zbb instruction.
> > +
> > +   OPERANDS[0] is the target (result).
> > +   OPERANDS[1] is the first source.
> > +   OPERANDS[2] is the second source.
> > +   If NO_LENGTH is zero, then:
> > +   OPERANDS[3] is the length.
> > +   OPERANDS[4] is the alignment in bytes.
> > +   If NO_LENGTH is nonzero, then:
> > +   OPERANDS[3] is the alignment in bytes.
>
> Ugh.  I guess it's inevitable unless we want to drop the array and pass
> each element individually (in which case we'd pass a NULL_RTX in the
> case we don't have a length argument).
>

I will split the array into individual rtx arguments as suggested.


> I'd like to give others a chance to chime in here.  Everything looks
> sensible here, but I may have missed something.  So give the other
> maintainers a couple days to chime in before committing.
>
>
> Jeff
>
>
  
Jeff Law Nov. 15, 2022, 12:22 a.m. UTC | #3
On 11/14/22 14:49, Christoph Müllner wrote:
>
>
> We can take this further, but then the following questions pop up:
> * how much data processing per loop iteration?

I have no idea because I don't have any real data.  Last time I gathered 
any data on this issue was circa 1988 :-)


> * what about unaligned strings?

I'd punt.  I don't think we can depend on having a high performance 
unaligned access.  You could do a dynamic check of alignment, but you'd 
really need to know that they're aligned often enough that the dynamic 
check can often be recovered.


>
> Happy to get suggestions/opinions for improvement.

I think this is pretty good without additional data that would indicate 
that handling unaligned cases or a different number of loop peels would 
be a notable improvement.

Jeff
  
Kito Cheng Nov. 15, 2022, 12:46 a.m. UTC | #4
Hi Christoph:

> This patch implements expansions for the cmpstrsi and the cmpstrnsi
> builtins using Zbb instructions (if available).
> This allows to inline calls to strcmp() and strncmp().
>
> The expansion basically emits a peeled comparison sequence (i.e. a peeled
> comparison loop) which compares XLEN bits per step if possible.
>
> The emitted sequence can be controlled, by setting the maximum number
> of compared bytes (-mstring-compare-inline-limit).

I would like to have a unified option interface,
maybe -m[no-]inline-str[n]cmp and -minline-str[n]cmp-limit.
And add some option like this:
-minline-str[n]cmp=[bitmanip|vector|auto] in future,
since I assume we'll have different versions of those things.

>
> gcc/ChangeLog:
>
>         * config/riscv/riscv-protos.h (riscv_expand_strn_compare): New
>           prototype.
>         * config/riscv/riscv-string.cc (GEN_EMIT_HELPER3): New helper
>           macros.
>         (GEN_EMIT_HELPER2): New helper macros.
>         (expand_strncmp_zbb_sequence): New function.
>         (riscv_emit_str_compare_zbb): New function.
>         (riscv_expand_strn_compare): New function.
>         * config/riscv/riscv.md (cmpstrnsi): Invoke expansion functions
>           for strn_compare.
>         (cmpstrsi): Invoke expansion functions for strn_compare.
>         * config/riscv/riscv.opt: Add new parameter
>           '-mstring-compare-inline-limit'.

We need to document this option.
  
Palmer Dabbelt Nov. 15, 2022, 12:53 a.m. UTC | #5
On Mon, 14 Nov 2022 16:46:37 PST (-0800), Kito Cheng wrote:
> Hi Christoph:
>
>> This patch implements expansions for the cmpstrsi and the cmpstrnsi
>> builtins using Zbb instructions (if available).
>> This allows to inline calls to strcmp() and strncmp().
>>
>> The expansion basically emits a peeled comparison sequence (i.e. a peeled
>> comparison loop) which compares XLEN bits per step if possible.
>>
>> The emitted sequence can be controlled, by setting the maximum number
>> of compared bytes (-mstring-compare-inline-limit).
>
> I would like to have a unified option interface,
> maybe -m[no-]inline-str[n]cmp and -minline-str[n]cmp-limit.
> And add some option like this:
> -minline-str[n]cmp=[bitmanip|vector|auto] in future,
> since I assume we'll have different versions of those things.

Can we just decide that from mtune?  We'll probably have uarch-specific 
string functions at some point, might as well start planning for it now.

>> gcc/ChangeLog:
>>
>>         * config/riscv/riscv-protos.h (riscv_expand_strn_compare): New
>>           prototype.
>>         * config/riscv/riscv-string.cc (GEN_EMIT_HELPER3): New helper
>>           macros.
>>         (GEN_EMIT_HELPER2): New helper macros.
>>         (expand_strncmp_zbb_sequence): New function.
>>         (riscv_emit_str_compare_zbb): New function.
>>         (riscv_expand_strn_compare): New function.
>>         * config/riscv/riscv.md (cmpstrnsi): Invoke expansion functions
>>           for strn_compare.
>>         (cmpstrsi): Invoke expansion functions for strn_compare.
>>         * config/riscv/riscv.opt: Add new parameter
>>           '-mstring-compare-inline-limit'.
>
> We need to document this option.
  
Kito Cheng Nov. 15, 2022, 1:55 a.m. UTC | #6
On Tue, Nov 15, 2022 at 8:53 AM Palmer Dabbelt <palmer@dabbelt.com> wrote:
>
> On Mon, 14 Nov 2022 16:46:37 PST (-0800), Kito Cheng wrote:
> > Hi Christoph:
> >
> >> This patch implements expansions for the cmpstrsi and the cmpstrnsi
> >> builtins using Zbb instructions (if available).
> >> This allows to inline calls to strcmp() and strncmp().
> >>
> >> The expansion basically emits a peeled comparison sequence (i.e. a peeled
> >> comparison loop) which compares XLEN bits per step if possible.
> >>
> >> The emitted sequence can be controlled, by setting the maximum number
> >> of compared bytes (-mstring-compare-inline-limit).
> >
> > I would like to have a unified option interface,
> > maybe -m[no-]inline-str[n]cmp and -minline-str[n]cmp-limit.
> > And add some option like this:
> > -minline-str[n]cmp=[bitmanip|vector|auto] in future,
> > since I assume we'll have different versions of those things.
>
> Can we just decide that from mtune?  We'll probably have uarch-specific
> string functions at some point, might as well start planning for it now.

I assume you mean the -minline-str[n]cmp=[bitmanip|vector|auto] part?
I think this part should have more discussion and could defer that until
we reach consensus.

But -m[no-]inline-str[n]cmp and -minline-str[n]cmp-limit part I favor having
those two options to disable and/or fine tune those parameters.

>
> >> gcc/ChangeLog:
> >>
> >>         * config/riscv/riscv-protos.h (riscv_expand_strn_compare): New
> >>           prototype.
> >>         * config/riscv/riscv-string.cc (GEN_EMIT_HELPER3): New helper
> >>           macros.
> >>         (GEN_EMIT_HELPER2): New helper macros.
> >>         (expand_strncmp_zbb_sequence): New function.
> >>         (riscv_emit_str_compare_zbb): New function.
> >>         (riscv_expand_strn_compare): New function.
> >>         * config/riscv/riscv.md (cmpstrnsi): Invoke expansion functions
> >>           for strn_compare.
> >>         (cmpstrsi): Invoke expansion functions for strn_compare.
> >>         * config/riscv/riscv.opt: Add new parameter
> >>           '-mstring-compare-inline-limit'.
> >
> > We need to document this option.
  
Jeff Law Nov. 15, 2022, 3:41 a.m. UTC | #7
On 11/14/22 17:53, Palmer Dabbelt wrote:
> On Mon, 14 Nov 2022 16:46:37 PST (-0800), Kito Cheng wrote:
>> Hi Christoph:
>>
>>> This patch implements expansions for the cmpstrsi and the cmpstrnsi
>>> builtins using Zbb instructions (if available).
>>> This allows to inline calls to strcmp() and strncmp().
>>>
>>> The expansion basically emits a peeled comparison sequence (i.e. a 
>>> peeled
>>> comparison loop) which compares XLEN bits per step if possible.
>>>
>>> The emitted sequence can be controlled, by setting the maximum number
>>> of compared bytes (-mstring-compare-inline-limit).
>>
>> I would like to have a unified option interface,
>> maybe -m[no-]inline-str[n]cmp and -minline-str[n]cmp-limit.
>> And add some option like this:
>> -minline-str[n]cmp=[bitmanip|vector|auto] in future,
>> since I assume we'll have different versions of those things.
>
> Can we just decide that from mtune?  We'll probably have 
> uarch-specific string functions at some point, might as well start 
> planning for it now.

Sure, though the implementation isn't terribly tied to any uarch at the 
moment and I doubt uarch approaches would make significant impacts here 
-- we're peeling off some small number of iterations fairly 
generically.  The uarch specific stuff would be the code in glibc 
selected by an ifunc.  uarch variants for block copiers seem inevitable 
though :-)


I don' t have strong opinions here, so if we want to key off mtune, 
sure.  If we want to have variants for scalar vs vector that's quite 
reasonable too.  Or if we want to go all the way to uarch specific 
implementations, I won't object.






> th
>>> gcc/ChangeLog:
>>>
>>>         * config/riscv/riscv-protos.h (riscv_expand_strn_compare): New
>>>           prototype.
>>>         * config/riscv/riscv-string.cc (GEN_EMIT_HELPER3): New helper
>>>           macros.
>>>         (GEN_EMIT_HELPER2): New helper macros.
>>>         (expand_strncmp_zbb_sequence): New function.
>>>         (riscv_emit_str_compare_zbb): New function.
>>>         (riscv_expand_strn_compare): New function.
>>>         * config/riscv/riscv.md (cmpstrnsi): Invoke expansion functions
>>>           for strn_compare.
>>>         (cmpstrsi): Invoke expansion functions for strn_compare.
>>>         * config/riscv/riscv.opt: Add new parameter
>>>           '-mstring-compare-inline-limit'.
>>
>> We need to document this option.

Yes, definitely needs documentation.  Thanks for catching that.


jeff
  
Christoph Müllner Nov. 15, 2022, 10:22 p.m. UTC | #8
On Tue, Nov 15, 2022 at 1:46 AM Kito Cheng <kito.cheng@gmail.com> wrote:

> Hi Christoph:
>
> > This patch implements expansions for the cmpstrsi and the cmpstrnsi
> > builtins using Zbb instructions (if available).
> > This allows to inline calls to strcmp() and strncmp().
> >
> > The expansion basically emits a peeled comparison sequence (i.e. a peeled
> > comparison loop) which compares XLEN bits per step if possible.
> >
> > The emitted sequence can be controlled, by setting the maximum number
> > of compared bytes (-mstring-compare-inline-limit).
>
> I would like to have a unified option interface,
> maybe -m[no-]inline-str[n]cmp and -minline-str[n]cmp-limit.
>

Ok, I don't mind (in fact, I thought about this as well).
The reason why it is how it is: I took inspiration from the rs6000 backend.


> And add some option like this:
> -minline-str[n]cmp=[bitmanip|vector|auto] in future,
> since I assume we'll have different versions of those things.
>
> >
> > gcc/ChangeLog:
> >
> >         * config/riscv/riscv-protos.h (riscv_expand_strn_compare): New
> >           prototype.
> >         * config/riscv/riscv-string.cc (GEN_EMIT_HELPER3): New helper
> >           macros.
> >         (GEN_EMIT_HELPER2): New helper macros.
> >         (expand_strncmp_zbb_sequence): New function.
> >         (riscv_emit_str_compare_zbb): New function.
> >         (riscv_expand_strn_compare): New function.
> >         * config/riscv/riscv.md (cmpstrnsi): Invoke expansion functions
> >           for strn_compare.
> >         (cmpstrsi): Invoke expansion functions for strn_compare.
> >         * config/riscv/riscv.opt: Add new parameter
> >           '-mstring-compare-inline-limit'.
>
> We need to document this option.
>
  
Philipp Tomsich Nov. 16, 2022, 12:15 a.m. UTC | #9
On Tue, 15 Nov 2022 at 01:46, Kito Cheng <kito.cheng@gmail.com> wrote:
>
> Hi Christoph:
>
> > This patch implements expansions for the cmpstrsi and the cmpstrnsi
> > builtins using Zbb instructions (if available).
> > This allows to inline calls to strcmp() and strncmp().
> >
> > The expansion basically emits a peeled comparison sequence (i.e. a peeled
> > comparison loop) which compares XLEN bits per step if possible.
> >
> > The emitted sequence can be controlled, by setting the maximum number
> > of compared bytes (-mstring-compare-inline-limit).
>
> I would like to have a unified option interface,
> maybe -m[no-]inline-str[n]cmp and -minline-str[n]cmp-limit.

For the basic option (-m[no-]inline-str[n]cmp), I would punt to
-fno-builtin-str[n]cmp.
The limit-one sounds more like a --param?

> And add some option like this:
> -minline-str[n]cmp=[bitmanip|vector|auto] in future,

If we want to follow the lead of others, then x86 has a -mstringop-strategy=alg


> since I assume we'll have different versions of those things.
>
> >
> > gcc/ChangeLog:
> >
> >         * config/riscv/riscv-protos.h (riscv_expand_strn_compare): New
> >           prototype.
> >         * config/riscv/riscv-string.cc (GEN_EMIT_HELPER3): New helper
> >           macros.
> >         (GEN_EMIT_HELPER2): New helper macros.
> >         (expand_strncmp_zbb_sequence): New function.
> >         (riscv_emit_str_compare_zbb): New function.
> >         (riscv_expand_strn_compare): New function.
> >         * config/riscv/riscv.md (cmpstrnsi): Invoke expansion functions
> >           for strn_compare.
> >         (cmpstrsi): Invoke expansion functions for strn_compare.
> >         * config/riscv/riscv.opt: Add new parameter
> >           '-mstring-compare-inline-limit'.
>
> We need to document this option.
  
Kito Cheng Nov. 21, 2022, 3:24 a.m. UTC | #10
> > I would like to have a unified option interface,
> > maybe -m[no-]inline-str[n]cmp and -minline-str[n]cmp-limit.
>
> For the basic option (-m[no-]inline-str[n]cmp), I would punt to
> -fno-builtin-str[n]cmp.

-fno-bulitin-* will also suppress middle-end optimization for those builtins.

see:
https://github.com/gcc-mirror/gcc/blob/master/gcc/tree-ssa-strlen.cc#L5372

and
https://github.com/gcc-mirror/gcc/blob/master/gcc/tree-loop-distribution.cc
for -fno-bulitin-memcpy/memset/memmove

> The limit-one sounds more like a --param?

Use -param=inline-*-limit= sound good idea, aarch64 and x86 have few
options like that.

>
> > And add some option like this:
> > -minline-str[n]cmp=[bitmanip|vector|auto] in future,
>
> If we want to follow the lead of others, then x86 has a -mstringop-strategy=alg

Using same option name as x86 is SGTM.

> > since I assume we'll have different versions of those things.
> >
> > >
> > > gcc/ChangeLog:
> > >
> > >         * config/riscv/riscv-protos.h (riscv_expand_strn_compare): New
> > >           prototype.
> > >         * config/riscv/riscv-string.cc (GEN_EMIT_HELPER3): New helper
> > >           macros.
> > >         (GEN_EMIT_HELPER2): New helper macros.
> > >         (expand_strncmp_zbb_sequence): New function.
> > >         (riscv_emit_str_compare_zbb): New function.
> > >         (riscv_expand_strn_compare): New function.
> > >         * config/riscv/riscv.md (cmpstrnsi): Invoke expansion functions
> > >           for strn_compare.
> > >         (cmpstrsi): Invoke expansion functions for strn_compare.
> > >         * config/riscv/riscv.opt: Add new parameter
> > >           '-mstring-compare-inline-limit'.
> >
> > We need to document this option.
  

Patch

diff --git a/gcc/config/riscv/riscv-protos.h b/gcc/config/riscv/riscv-protos.h
index 18187e3bd78..7f334be333c 100644
--- a/gcc/config/riscv/riscv-protos.h
+++ b/gcc/config/riscv/riscv-protos.h
@@ -97,6 +97,7 @@  rtl_opt_pass * make_pass_shorten_memrefs (gcc::context *ctxt);
 /* Routines implemented in riscv-string.c.  */
 extern bool riscv_expand_block_move (rtx, rtx, rtx);
 extern bool riscv_expand_strlen (rtx[]);
+extern bool riscv_expand_strn_compare (rtx[], int);
 
 /* Information about one CPU we know about.  */
 struct riscv_cpu_info {
diff --git a/gcc/config/riscv/riscv-string.cc b/gcc/config/riscv/riscv-string.cc
index bf96522b608..f157e04ac0c 100644
--- a/gcc/config/riscv/riscv-string.cc
+++ b/gcc/config/riscv/riscv-string.cc
@@ -84,6 +84,11 @@  GEN_EMIT_HELPER2(one_cmpl) /* do_one_cmpl2  */
 GEN_EMIT_HELPER2(clz) /* do_clz2  */
 GEN_EMIT_HELPER2(ctz) /* do_ctz2  */
 GEN_EMIT_HELPER2(zero_extendqi) /* do_zero_extendqi2  */
+GEN_EMIT_HELPER3(xor) /* do_xor3  */
+GEN_EMIT_HELPER3(ashl) /* do_ashl3  */
+GEN_EMIT_HELPER2(bswap) /* do_bswap2  */
+GEN_EMIT_HELPER3(riscv_ior_not) /* do_riscv_ior_not3  */
+GEN_EMIT_HELPER3(riscv_and_not) /* do_riscv_and_not3  */
 
 /* Helper function to load a byte or a Pmode register.
 
@@ -268,6 +273,345 @@  riscv_expand_block_move (rtx dest, rtx src, rtx length)
   return false;
 }
 
+/* Generate the sequence of compares for strcmp/strncmp using zbb instructions.
+   BYTES_TO_COMPARE is the number of bytes to be compared.
+   BASE_ALIGN is the smaller of the alignment of the two strings.
+   ORIG_SRC1 is the unmodified rtx for the first string.
+   ORIG_SRC2 is the unmodified rtx for the second string.
+   DATA1 is the register for loading the first string.
+   DATA2 is the register for loading the second string.
+   HAS_NUL is the register holding non-NUL bytes for NUL-bytes in the string.
+   TARGET is the rtx for the result register (SImode)
+   EQUALITY_COMPARE_REST if set, then we hand over to libc if string matches.
+   END_LABEL is the location before the calculation of the result value.
+   FINAL_LABEL is the location after the calculation of the result value.  */
+
+static void
+expand_strncmp_zbb_sequence (unsigned HOST_WIDE_INT bytes_to_compare,
+			     rtx src1, rtx src2, rtx data1, rtx data2,
+			     rtx target, rtx orc, bool equality_compare_rest,
+			     rtx end_label, rtx final_label)
+{
+  const unsigned HOST_WIDE_INT p_mode_size = GET_MODE_SIZE (Pmode);
+  rtx src1_addr = force_reg (Pmode, XEXP (src1, 0));
+  rtx src2_addr = force_reg (Pmode, XEXP (src2, 0));
+  unsigned HOST_WIDE_INT offset = 0;
+
+  rtx m1 = gen_reg_rtx (Pmode);
+  emit_insn (gen_rtx_SET (m1, constm1_rtx));
+
+  /* Generate a compare sequence.  */
+  while (bytes_to_compare > 0)
+    {
+      machine_mode load_mode = QImode;
+      unsigned HOST_WIDE_INT load_mode_size = 1;
+      if (bytes_to_compare > 1)
+	{
+	  load_mode = Pmode;
+	  load_mode_size = p_mode_size;
+	}
+      unsigned HOST_WIDE_INT cmp_bytes = 0;
+
+      if (bytes_to_compare >= load_mode_size)
+	cmp_bytes = load_mode_size;
+      else
+	cmp_bytes = bytes_to_compare;
+
+      unsigned HOST_WIDE_INT remain = bytes_to_compare - cmp_bytes;
+
+      /* load_mode_size...bytes we will read
+	 cmp_bytes...bytes we will compare (might be less than load_mode_size)
+	 bytes_to_compare...bytes we will compare (incl. cmp_bytes)
+	 remain...bytes left to compare (excl. cmp_bytes)  */
+
+      rtx addr1 = gen_rtx_PLUS (Pmode, src1_addr, GEN_INT (offset));
+      rtx addr2 = gen_rtx_PLUS (Pmode, src2_addr, GEN_INT (offset));
+
+      do_load_from_addr (load_mode, data1, addr1, src1);
+      do_load_from_addr (load_mode, data2, addr2, src2);
+
+      if (load_mode_size == 1)
+	{
+	  /* Special case for comparing just single (last) byte.  */
+	  gcc_assert (remain == 0);
+
+	  if (!equality_compare_rest)
+	    {
+	      /* Calculate difference and jump to final_label.  */
+	      rtx result = gen_reg_rtx (Pmode);
+	      do_sub3 (result, data1, data2);
+	      emit_insn (gen_movsi (target, gen_lowpart (SImode, result)));
+	      emit_jump_insn (gen_jump (final_label));
+	    }
+	  else
+	    {
+	      /* Compare both bytes and jump to final_label if not equal.  */
+	      rtx result = gen_reg_rtx (Pmode);
+	      do_sub3 (result, data1, data2);
+	      emit_insn (gen_movsi (target, gen_lowpart (SImode, result)));
+	      /* Check if str1[i] is NULL.  */
+	      rtx cond1 = gen_rtx_EQ (VOIDmode, data1, const0_rtx);
+	      riscv_emit_unlikely_jump (gen_cbranch4 (Pmode, cond1,
+					data1, const0_rtx, final_label));
+	      /* Check if str1[i] == str2[i].  */
+	      rtx cond2 = gen_rtx_NE (VOIDmode, data1, data2);
+	      riscv_emit_unlikely_jump (gen_cbranch4 (Pmode, cond2,
+					data1, data2, final_label));
+	      /* Processing will fall through to libc calls.  */
+	    }
+	}
+      else
+	{
+	  /* Eliminate irrelevant data (behind the N-th character).  */
+	  if (bytes_to_compare < p_mode_size)
+	    {
+	      gcc_assert (remain == 0);
+	     /* Set a NUL-byte after the relevant data (behind the string).  */
+	      unsigned long im = 0xffUL;
+	      rtx imask = gen_rtx_CONST_INT (Pmode, im);
+	      rtx m_reg = gen_reg_rtx (Pmode);
+	      emit_insn (gen_rtx_SET (m_reg, imask));
+	      do_ashl3 (m_reg, m_reg, GEN_INT (cmp_bytes * BITS_PER_UNIT));
+	      do_riscv_and_not3 (data1, m_reg, data1);
+	      do_riscv_and_not3 (data2, m_reg, data2);
+	      do_orcb2 (orc, data1);
+	      emit_jump_insn (gen_jump (end_label));
+	    }
+	  else
+	    {
+	      /* Check if data1 contains a NUL character.  */
+	      do_orcb2 (orc, data1);
+	      rtx cond1 = gen_rtx_NE (VOIDmode, orc, m1);
+	      riscv_emit_unlikely_jump (gen_cbranch4 (Pmode, cond1, orc, m1,
+						      end_label));
+
+	      /* Break out if u1 != u2 */
+	      rtx cond2 = gen_rtx_NE (VOIDmode, data1, data2);
+	      riscv_emit_unlikely_jump (gen_cbranch4 (Pmode, cond2, data1,
+						      data2, end_label));
+
+	      /* Fast-exit for complete and equal strings.  */
+	      if (remain == 0 && !equality_compare_rest)
+		{
+		  /* All compared and everything was equal.  */
+		  emit_insn (gen_rtx_SET (target, gen_rtx_CONST_INT (SImode, 0)));
+		  emit_jump_insn (gen_jump (final_label));
+		}
+	    }
+	}
+
+      offset += cmp_bytes;
+      bytes_to_compare -= cmp_bytes;
+    }
+  /* Processing will fall through to libc calls.  */
+}
+
+/* Emit a string comparison sequence using Zbb instruction.
+
+   OPERANDS[0] is the target (result).
+   OPERANDS[1] is the first source.
+   OPERANDS[2] is the second source.
+   If NO_LENGTH is zero, then:
+   OPERANDS[3] is the length.
+   OPERANDS[4] is the alignment in bytes.
+   If NO_LENGTH is nonzero, then:
+   OPERANDS[3] is the alignment in bytes.
+   BYTES_TO_COMPARE is the maximum number of bytes to compare.
+   EQUALITY_COMPARE_REST defines if str(n)cmp should be called on equality.
+ */
+
+static bool
+riscv_emit_str_compare_zbb (rtx operands[], int no_length,
+			    unsigned HOST_WIDE_INT bytes_to_compare,
+			    bool equality_compare_rest)
+{
+  const unsigned HOST_WIDE_INT p_mode_size = GET_MODE_SIZE (Pmode);
+  rtx target = operands[0];
+  rtx src1 = operands[1];
+  rtx src2 = operands[2];
+  rtx bytes_rtx = NULL;
+  rtx align_rtx = operands[3];
+
+  if (!no_length)
+    {
+      bytes_rtx = operands[3];
+      align_rtx = operands[4];
+    }
+
+  gcc_assert (TARGET_ZBB);
+
+  /* Enable only if we can access at least one XLEN-register.  */
+  if (bytes_to_compare < p_mode_size)
+    return false;
+
+  /* Limit to 12-bits (maximum load-offset).  */
+  if (bytes_to_compare > IMM_REACH)
+    return false;
+
+  /* We don't support big endian.  */
+  if (BYTES_BIG_ENDIAN)
+    return false;
+
+  /* We need to know the alignment.  */
+  if (!CONST_INT_P (align_rtx))
+    return false;
+
+  unsigned HOST_WIDE_INT base_align = UINTVAL (align_rtx);
+  unsigned HOST_WIDE_INT required_align = p_mode_size;
+  if (base_align < required_align)
+    return false;
+
+  rtx data1 = gen_reg_rtx (Pmode);
+  rtx data2 = gen_reg_rtx (Pmode);
+  rtx orc = gen_reg_rtx (Pmode);
+  rtx end_label = gen_label_rtx ();
+  rtx final_label = gen_label_rtx ();
+
+  /* Generate a sequence of zbb instructions to compare out
+     to the length specified.  */
+  expand_strncmp_zbb_sequence (bytes_to_compare, src1, src2, data1, data2,
+			       target, orc, equality_compare_rest,
+			       end_label, final_label);
+
+  if (equality_compare_rest)
+    {
+      /* Update pointers past what has been compared already.  */
+      rtx src1_addr = force_reg (Pmode, XEXP (src1, 0));
+      rtx src2_addr = force_reg (Pmode, XEXP (src2, 0));
+      unsigned HOST_WIDE_INT offset = bytes_to_compare;
+      rtx src1 = force_reg (Pmode,
+			    gen_rtx_PLUS (Pmode, src1_addr, GEN_INT (offset)));
+      rtx src2 = force_reg (Pmode,
+			    gen_rtx_PLUS (Pmode, src2_addr, GEN_INT (offset)));
+
+      /* Construct call to strcmp/strncmp to compare the rest of the string.  */
+      if (no_length)
+	{
+	  tree fun = builtin_decl_explicit (BUILT_IN_STRCMP);
+	  emit_library_call_value (XEXP (DECL_RTL (fun), 0),
+				   target, LCT_NORMAL, GET_MODE (target),
+				   src1, Pmode, src2, Pmode);
+	}
+      else
+	{
+	  unsigned HOST_WIDE_INT bytes = UINTVAL (bytes_rtx);
+	  unsigned HOST_WIDE_INT delta = bytes - bytes_to_compare;
+	  gcc_assert (delta > 0);
+	  rtx len_rtx = gen_reg_rtx (Pmode);
+	  emit_move_insn (len_rtx, gen_int_mode (delta, Pmode));
+	  tree fun = builtin_decl_explicit (BUILT_IN_STRNCMP);
+	  emit_library_call_value (XEXP (DECL_RTL (fun), 0),
+				   target, LCT_NORMAL, GET_MODE (target),
+				   src1, Pmode, src2, Pmode, len_rtx, Pmode);
+	}
+
+      emit_jump_insn (gen_jump (final_label));
+    }
+
+  emit_barrier (); /* No fall-through.  */
+
+  emit_label (end_label);
+
+  /* Convert non-equal bytes into non-NUL bytes.  */
+  rtx diff = gen_reg_rtx (Pmode);
+  do_xor3 (diff, data1, data2);
+  do_orcb2 (diff, diff);
+
+  /* Convert non-equal or NUL-bytes into non-NUL bytes.  */
+  rtx syndrome = gen_reg_rtx (Pmode);
+  do_riscv_ior_not3 (syndrome, orc, diff);
+
+  /* Count the number of equal bits from the beginning of the word.  */
+  rtx shift = gen_reg_rtx (Pmode);
+  do_ctz2 (shift, syndrome);
+
+  do_bswap2 (data1, data1);
+  do_bswap2 (data2, data2);
+
+  /* The most-significant-non-zero bit of the syndrome marks either the
+     first bit that is different, or the top bit of the first zero byte.
+     Shifting left now will bring the critical information into the
+     top bits.  */
+  do_ashl3 (data1, data1, gen_lowpart (QImode, shift));
+  do_ashl3 (data2, data2, gen_lowpart (QImode, shift));
+
+  /* But we need to zero-extend (char is unsigned) the value and then
+     perform a signed 32-bit subtraction.  */
+  unsigned int shiftr = p_mode_size * BITS_PER_UNIT - 8;
+  do_lshr3 (data1, data1, GEN_INT (shiftr));
+  do_lshr3 (data2, data2, GEN_INT (shiftr));
+
+  rtx result = gen_reg_rtx (Pmode);
+  do_sub3 (result, data1, data2);
+  emit_insn (gen_movsi (target, gen_lowpart (SImode, result)));
+
+  /* And we are done.  */
+  emit_label (final_label);
+  return true;
+}
+
+/* Expand a string compare operation with length, and return
+   true if successful.  Return false if we should let the
+   compiler generate normal code, probably a strncmp call.
+   If NO_LENGTH is set, there is no upper bound of the strings.
+
+   OPERANDS[0] is the target (result).
+   OPERANDS[1] is the first source.
+   OPERANDS[2] is the second source.
+   If NO_LENGTH is zero, then:
+   OPERANDS[3] is the length.
+   OPERANDS[4] is the alignment in bytes.
+   If NO_LENGTH is nonzero, then:
+   OPERANDS[3] is the alignment in bytes.  */
+
+bool
+riscv_expand_strn_compare (rtx operands[], int no_length)
+{
+  rtx bytes_rtx = NULL;
+  const unsigned HOST_WIDE_INT compare_max = riscv_string_compare_inline_limit;
+  unsigned HOST_WIDE_INT compare_length; /* How much to compare inline.  */
+  bool equality_compare_rest = false; /* Call libc to compare remainder.  */
+
+  if (riscv_string_compare_inline_limit == 0)
+    return false;
+
+  /* Decide how many bytes to compare inline and what to do if there is
+     no difference detected at the end of the compared bytes.
+     We might call libc to continue the comparison.  */
+  if (no_length)
+    {
+      compare_length = compare_max;
+      equality_compare_rest = true;
+    }
+  else
+    {
+      /* If we have a length, it must be constant.  */
+      bytes_rtx = operands[3];
+      if (!CONST_INT_P (bytes_rtx))
+	return false;
+
+      unsigned HOST_WIDE_INT bytes = UINTVAL (bytes_rtx);
+      if (bytes <= compare_max)
+	{
+	  compare_length = bytes;
+	  equality_compare_rest = false;
+	}
+      else
+	{
+	  compare_length = compare_max;
+	  equality_compare_rest = true;
+	}
+    }
+
+  if (TARGET_ZBB)
+    {
+      return riscv_emit_str_compare_zbb (operands, no_length, compare_length,
+					 equality_compare_rest);
+    }
+
+  return false;
+}
+
 /* If the provided string is aligned, then read XLEN bytes
    in a loop and use orc.b to find NUL-bytes.  */
 
diff --git a/gcc/config/riscv/riscv.md b/gcc/config/riscv/riscv.md
index f05c764c3d4..dce33a4b638 100644
--- a/gcc/config/riscv/riscv.md
+++ b/gcc/config/riscv/riscv.md
@@ -3010,6 +3010,52 @@  (define_expand "cpymemsi"
     FAIL;
 })
 
+;; String compare N insn.
+;; Argument 0 is the target (result)
+;; Argument 1 is the source1
+;; Argument 2 is the source2
+;; Argument 3 is the length
+;; Argument 4 is the alignment
+
+(define_expand "cmpstrnsi"
+  [(parallel [(set (match_operand:SI 0)
+	      (compare:SI (match_operand:BLK 1)
+			  (match_operand:BLK 2)))
+	      (use (match_operand:SI 3))
+	      (use (match_operand:SI 4))])]
+  ""
+{
+  if (optimize_insn_for_size_p ())
+    FAIL;
+
+  if (riscv_expand_strn_compare (operands, 0))
+    DONE;
+  else
+    FAIL;
+})
+
+;; String compare insn.
+;; Argument 0 is the target (result)
+;; Argument 1 is the destination
+;; Argument 2 is the source
+;; Argument 3 is the alignment
+
+(define_expand "cmpstrsi"
+  [(parallel [(set (match_operand:SI 0)
+	      (compare:SI (match_operand:BLK 1)
+			  (match_operand:BLK 2)))
+	      (use (match_operand:SI 3))])]
+  ""
+{
+  if (optimize_insn_for_size_p ())
+    FAIL;
+
+  if (riscv_expand_strn_compare (operands, 1))
+    DONE;
+  else
+    FAIL;
+})
+
 ;; Search character in string (generalization of strlen).
 ;; Argument 0 is the resulting offset
 ;; Argument 1 is the string
diff --git a/gcc/config/riscv/riscv.opt b/gcc/config/riscv/riscv.opt
index 7c3ca48d1cc..fdf768ae9a7 100644
--- a/gcc/config/riscv/riscv.opt
+++ b/gcc/config/riscv/riscv.opt
@@ -249,3 +249,8 @@  Enum(isa_spec_class) String(20191213) Value(ISA_SPEC_CLASS_20191213)
 misa-spec=
 Target RejectNegative Joined Enum(isa_spec_class) Var(riscv_isa_spec) Init(TARGET_DEFAULT_ISA_SPEC)
 Set the version of RISC-V ISA spec.
+
+mstring-compare-inline-limit=
+Target Var(riscv_string_compare_inline_limit) Init(64) RejectNegative Joined UInteger Save
+Max number of bytes to compare.
+
diff --git a/gcc/testsuite/gcc.target/riscv/zbb-strcmp-unaligned.c b/gcc/testsuite/gcc.target/riscv/zbb-strcmp-unaligned.c
new file mode 100644
index 00000000000..2126c849e0a
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/zbb-strcmp-unaligned.c
@@ -0,0 +1,36 @@ 
+/* { dg-do compile } */
+/* { dg-options "-march=rv64gc_zbb -mabi=lp64 -mstring-compare-inline-limit=64" } */
+
+typedef long unsigned int size_t;
+
+int
+my_str_cmp (const char *s1, const char *s2)
+{
+  return __builtin_strcmp (s1, s2);
+}
+
+int
+my_str_cmp_const (const char *s1)
+{
+  return __builtin_strcmp (s1, "foo");
+}
+
+int
+my_strn_cmp (const char *s1, const char *s2, size_t n)
+{
+  return __builtin_strncmp (s1, s2, n);
+}
+
+int
+my_strn_cmp_const (const char *s1, size_t n)
+{
+  return __builtin_strncmp (s1, "foo", n);
+}
+
+int
+my_strn_cmp_bounded (const char *s1, const char *s2)
+{
+  return __builtin_strncmp (s1, s2, 42);
+}
+
+/* { dg-final { scan-assembler-not "orc.b\t" } } */
diff --git a/gcc/testsuite/gcc.target/riscv/zbb-strcmp.c b/gcc/testsuite/gcc.target/riscv/zbb-strcmp.c
new file mode 100644
index 00000000000..3465e7ffee3
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/zbb-strcmp.c
@@ -0,0 +1,55 @@ 
+/* { dg-do compile } */
+/* { dg-options "-march=rv64gc_zbb -mabi=lp64 -mstring-compare-inline-limit=64" } */
+/* { dg-skip-if "" { *-*-* } { "-O0" "-Os" "-Oz" "-Og" } } */
+
+typedef long unsigned int size_t;
+
+/* Emits 8+1 orc.b instructions.  */
+
+int
+my_str_cmp (const char *s1, const char *s2)
+{
+  s1 = __builtin_assume_aligned (s1, 4096);
+  s2 = __builtin_assume_aligned (s2, 4096);
+  return __builtin_strcmp (s1, s2);
+}
+
+/* 8+1 because the backend does not know the size of "foo".  */
+
+int
+my_str_cmp_const (const char *s1)
+{
+  s1 = __builtin_assume_aligned (s1, 4096);
+  return __builtin_strcmp (s1, "foo");
+}
+
+/* Emits 6+1 orc.b instructions.  */
+
+int
+my_strn_cmp (const char *s1, const char *s2)
+{
+  s1 = __builtin_assume_aligned (s1, 4096);
+  s2 = __builtin_assume_aligned (s2, 4096);
+  return __builtin_strncmp (s1, s2, 42);
+}
+
+/* Note expanded because the backend does not know the size of "foo".  */
+
+int
+my_strn_cmp_const (const char *s1, size_t n)
+{
+  s1 = __builtin_assume_aligned (s1, 4096);
+  return __builtin_strncmp (s1, "foo", n);
+}
+
+/* Emits 6+1 orc.b instructions.  */
+
+int
+my_strn_cmp_bounded (const char *s1, const char *s2)
+{
+  s1 = __builtin_assume_aligned (s1, 4096);
+  s2 = __builtin_assume_aligned (s2, 4096);
+  return __builtin_strncmp (s1, s2, 42);
+}
+
+/* { dg-final { scan-assembler-times "orc.b\t" 32 } } */