diff mbox series

RISC-V: Use .p2align for code-alignment

Message ID 20221113204119.4061447-1-philipp.tomsich@vrull.eu
State Accepted
Headers
Series RISC-V: Use .p2align for code-alignment |

Checks

Context Check Description
snail/gcc-patch-check success Github commit url

Commit Message

Philipp Tomsich Nov. 13, 2022, 8:41 p.m. UTC 
  RISC-V's .p2align (currently) ignores the max-skip argument.  As we
have experimental patches underway to address this in a
backwards-compatible manner, let's prepare GCC for the day when
binutils gets updated.

gcc/ChangeLog:

	* config/riscv/riscv.h (ASM_OUTPUT_MAX_SKIP_ALIGN): Implement.

Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
---

 gcc/config/riscv/riscv.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

Comments

Jeff Law Nov. 15, 2022, 4:37 p.m. UTC | #1 
On 11/13/22 13:41, Philipp Tomsich wrote:
> RISC-V's .p2align (currently) ignores the max-skip argument.  As we
> have experimental patches underway to address this in a
> backwards-compatible manner, let's prepare GCC for the day when
> binutils gets updated.
>
> gcc/ChangeLog:
>
> 	* config/riscv/riscv.h (ASM_OUTPUT_MAX_SKIP_ALIGN): Implement.


What are the implications if we start using p2align immediately when the 
current (broken?) state of the assembler?  I'm pretty sure configure is 
already turning on HAVE_GAS_SKIP_P2ALIGN.  From a native risc-v build:


auto-host.h:#define HAVE_GAS_MAX_SKIP_P2ALIGN 1



jeff
Philipp Tomsich Nov. 15, 2022, 9:14 p.m. UTC | #2 
Jeff,

On Tue, 15 Nov 2022 at 17:37, Jeff Law <jeffreyalaw@gmail.com> wrote:
>
>
> On 11/13/22 13:41, Philipp Tomsich wrote:
>
> RISC-V's .p2align (currently) ignores the max-skip argument.  As we
> have experimental patches underway to address this in a
> backwards-compatible manner, let's prepare GCC for the day when
> binutils gets updated.
>
> gcc/ChangeLog:
>
> * config/riscv/riscv.h (ASM_OUTPUT_MAX_SKIP_ALIGN): Implement.
>
>
> What are the implications if we start using p2align immediately when the current (broken?) state of the assembler?  I'm pretty sure configure is already turning on HAVE_GAS_SKIP_P2ALIGN.  From a native risc-v build:
>
>
> auto-host.h:#define HAVE_GAS_MAX_SKIP_P2ALIGN 1

This is your tree, which has the (partial fix — i.e., the best we can
do without breaking backward compatibility) for .p2align.
When building against upstream binutils, this should not be defined.

Philipp.
Philipp Tomsich Nov. 16, 2022, 12:36 a.m. UTC | #3 
On Tue, 15 Nov 2022 at 22:14, Philipp Tomsich <philipp.tomsich@vrull.eu> wrote:
>
> Jeff,
>
> On Tue, 15 Nov 2022 at 17:37, Jeff Law <jeffreyalaw@gmail.com> wrote:
> >
> >
> > On 11/13/22 13:41, Philipp Tomsich wrote:
> >
> > RISC-V's .p2align (currently) ignores the max-skip argument.  As we
> > have experimental patches underway to address this in a
> > backwards-compatible manner, let's prepare GCC for the day when
> > binutils gets updated.
> >
> > gcc/ChangeLog:
> >
> > * config/riscv/riscv.h (ASM_OUTPUT_MAX_SKIP_ALIGN): Implement.
> >
> >
> > What are the implications if we start using p2align immediately when the current (broken?) state of the assembler?  I'm pretty sure configure is already turning on HAVE_GAS_SKIP_P2ALIGN.  From a native risc-v build:
> >
> >
> > auto-host.h:#define HAVE_GAS_MAX_SKIP_P2ALIGN 1
>
> This is your tree, which has the (partial fix — i.e., the best we can
> do without breaking backward compatibility) for .p2align.
> When building against upstream binutils, this should not be defined.

After rereading the generated configure-script, I've changed my mind
The check the does nothing to ensure that the max-skip is actually
honored and only tests whether it is accepted.

Having the max-skip silently ignored may still be the lesser evil.

Philipp.
diff mbox series

Patch

diff --git a/gcc/config/riscv/riscv.h b/gcc/config/riscv/riscv.h
index 2d0d170645c..c216173cf6b 100644
--- a/gcc/config/riscv/riscv.h
+++ b/gcc/config/riscv/riscv.h
@@ -938,6 +938,24 @@  typedef struct {
   fprintf (STREAM, "\t.word\t%sL%d-%sL%d\n",				\
 	   LOCAL_LABEL_PREFIX, VALUE, LOCAL_LABEL_PREFIX, REL)
 
+#ifdef HAVE_GAS_MAX_SKIP_P2ALIGN
+/* Support for -falign-* switches.  Use .p2align to ensure that code
+   sections are padded with NOP instructions, rather than zeros.  */
+#define ASM_OUTPUT_MAX_SKIP_ALIGN(FILE, LOG, MAX_SKIP)		\
+  do								\
+    {								\
+      if ((LOG) != 0)						\
+	{							\
+	  if ((MAX_SKIP) == 0)					\
+	    fprintf ((FILE), "\t.p2align %d\n", (int) (LOG));	\
+	  else							\
+	    fprintf ((FILE), "\t.p2align %d,,%d\n",		\
+		     (int) (LOG), (int) (MAX_SKIP));		\
+	}							\
+    } while (0)
+
+#endif /* HAVE_GAS_MAX_SKIP_P2ALIGN */
+
 /* This is how to output an assembler line
    that says to advance the location counter
    to a multiple of 2**LOG bytes.  */