Fix bogus -Wstringop-overflow warning

Message ID 3194055.aeNJFYEL58@fomalhaut
State Accepted, archived
Headers
Series Fix bogus -Wstringop-overflow warning |

Checks

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

Commit Message

Eric Botcazou Oct. 13, 2022, 12:06 p.m. UTC
  Hi,

if you compile the attached testcase with -O2 -fno-inline -Wall, you get:

In function 'process_array3':
cc1: warning: 'process_array4' accessing 4 bytes in a region of size 3 [-
Wstringop-overflow=]
cc1: note: referencing argument 1 of type 'char[4]'
t.c:6:6: note: in a call to function 'process_array4'
    6 | void process_array4 (char a[4], int n)
      |      ^~~~~~~~~~~~~~
cc1: warning: 'process_array4' accessing 4 bytes in a region of size 3 [-
Wstringop-overflow=]
cc1: note: referencing argument 1 of type 'char[4]'
t.c:6:6: note: in a call to function 'process_array4'

That's because the ICF IPA pass has identified the two functions and turned 
process_array3 into a wrapper of process_array4.  This looks sensible to me 
given that the only difference between them is an "access" attribute on their 
type describing the access size of the parameter and the "access" attribute 
does not affect type identity (struct attribute_spec.affects_type_identity).

Hence the proposed fix, tested on x86-64/Linux, OK for the mainline?


2022-10-13  Eric Botcazou  <ebotcazou@adacore.com>

	* gimple-ssa-warn-access.cc (pass_waccess::check_call): Return
	early for calls made from thunks.


2022-10-13  Eric Botcazou  <ebotcazou@adacore.com>

	* gcc.dg/Wstringop-overflow-89.c: New test.
  

Comments

Jeff Law Oct. 13, 2022, 10:31 p.m. UTC | #1
On 10/13/22 06:06, Eric Botcazou via Gcc-patches wrote:
> Hi,
>
> if you compile the attached testcase with -O2 -fno-inline -Wall, you get:
>
> In function 'process_array3':
> cc1: warning: 'process_array4' accessing 4 bytes in a region of size 3 [-
> Wstringop-overflow=]
> cc1: note: referencing argument 1 of type 'char[4]'
> t.c:6:6: note: in a call to function 'process_array4'
>      6 | void process_array4 (char a[4], int n)
>        |      ^~~~~~~~~~~~~~
> cc1: warning: 'process_array4' accessing 4 bytes in a region of size 3 [-
> Wstringop-overflow=]
> cc1: note: referencing argument 1 of type 'char[4]'
> t.c:6:6: note: in a call to function 'process_array4'
>
> That's because the ICF IPA pass has identified the two functions and turned
> process_array3 into a wrapper of process_array4.  This looks sensible to me
> given that the only difference between them is an "access" attribute on their
> type describing the access size of the parameter and the "access" attribute
> does not affect type identity (struct attribute_spec.affects_type_identity).
>
> Hence the proposed fix, tested on x86-64/Linux, OK for the mainline?
>
>
> 2022-10-13  Eric Botcazou  <ebotcazou@adacore.com>
>
> 	* gimple-ssa-warn-access.cc (pass_waccess::check_call): Return
> 	early for calls made from thunks.
>
>
> 2022-10-13  Eric Botcazou  <ebotcazou@adacore.com>
>
> 	* gcc.dg/Wstringop-overflow-89.c: New test.

Not a fan as it could potentially hide a real issue, but I don't really 
have a better solution.  I pondered suggesting "access" affect type 
identity, but the cases where that's really important are probably 
better handled by the "fn spec" attribute, leaving "access" strictly 
impacting diagnostics.

OK

jeff
  
Eric Botcazou Oct. 13, 2022, 10:53 p.m. UTC | #2
> Not a fan as it could potentially hide a real issue, but I don't really
> have a better solution.

Thanks.

> I pondered suggesting "access" affect type identity, but the cases where
> that's really important are probably better handled by the "fn spec"
> attribute, leaving "access" strictly impacting diagnostics.

I can expand a bit here, because I tried to change the "access" attribute that 
way and this badly breaks the C compiler, for example:

int foo (int n, char m[1][n]);

int foo (int n, char m[1][n]) {}

no longer compiles with an error about different function types.
  
Richard Biener Oct. 14, 2022, 6:12 a.m. UTC | #3
On Fri, Oct 14, 2022 at 12:54 AM Eric Botcazou via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> > Not a fan as it could potentially hide a real issue, but I don't really
> > have a better solution.
>
> Thanks.
>
> > I pondered suggesting "access" affect type identity, but the cases where
> > that's really important are probably better handled by the "fn spec"
> > attribute, leaving "access" strictly impacting diagnostics.
>
> I can expand a bit here, because I tried to change the "access" attribute that
> way and this badly breaks the C compiler, for example:
>
> int foo (int n, char m[1][n]);
>
> int foo (int n, char m[1][n]) {}
>
> no longer compiles with an error about different function types.

Note in discussion with IPA folks we agreed that IPA cloning that modifies
arguments either has to remove access attributes, adjust them or refrain
from cloning.

Martin - has anything been done to this respect?

I suppose there's also a way to figure if a clone has arguments
changed in any way?

Thanks,
Richard.

> --
> Eric Botcazou
>
>
  
Martin Liška Oct. 14, 2022, 10:27 a.m. UTC | #4
On 10/14/22 08:12, Richard Biener wrote:
> On Fri, Oct 14, 2022 at 12:54 AM Eric Botcazou via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>>
>>> Not a fan as it could potentially hide a real issue, but I don't really
>>> have a better solution.
>>
>> Thanks.
>>
>>> I pondered suggesting "access" affect type identity, but the cases where
>>> that's really important are probably better handled by the "fn spec"
>>> attribute, leaving "access" strictly impacting diagnostics.
>>
>> I can expand a bit here, because I tried to change the "access" attribute that
>> way and this badly breaks the C compiler, for example:
>>
>> int foo (int n, char m[1][n]);
>>
>> int foo (int n, char m[1][n]) {}
>>
>> no longer compiles with an error about different function types.
> 
> Note in discussion with IPA folks we agreed that IPA cloning that modifies
> arguments either has to remove access attributes, adjust them or refrain
> from cloning.
> 
> Martin - has anything been done to this respect?

I think it's more for Martin Jambor who's the IPA specialist when it comes
to parameter manipulation.

Martin

> 
> I suppose there's also a way to figure if a clone has arguments
> changed in any way?
> 
> Thanks,
> Richard.
> 
>> --
>> Eric Botcazou
>>
>>
  
Martin Jambor Oct. 14, 2022, 11:59 a.m. UTC | #5
Hello,

On Fri, Oct 14 2022, Martin Liška wrote:
> On 10/14/22 08:12, Richard Biener wrote:
>> On Fri, Oct 14, 2022 at 12:54 AM Eric Botcazou via Gcc-patches
>> <gcc-patches@gcc.gnu.org> wrote:
>>>
>>>> Not a fan as it could potentially hide a real issue, but I don't really
>>>> have a better solution.
>>>
>>> Thanks.
>>>
>>>> I pondered suggesting "access" affect type identity, but the cases where
>>>> that's really important are probably better handled by the "fn spec"
>>>> attribute, leaving "access" strictly impacting diagnostics.
>>>
>>> I can expand a bit here, because I tried to change the "access" attribute that
>>> way and this badly breaks the C compiler, for example:
>>>
>>> int foo (int n, char m[1][n]);
>>>
>>> int foo (int n, char m[1][n]) {}
>>>
>>> no longer compiles with an error about different function types.
>> 
>> Note in discussion with IPA folks we agreed that IPA cloning that modifies
>> arguments either has to remove access attributes, adjust them or refrain
>> from cloning.
>> 
>> Martin - has anything been done to this respect?
>
> I think it's more for Martin Jambor who's the IPA specialist when it comes
> to parameter manipulation.
>

They are being dropped since 2af63f0f53a

Adjusting them accordingly is an item buried quite deep in my TODO list.

>> 
>> I suppose there's also a way to figure if a clone has arguments
>> changed in any way?

Look whether clone_info::get (node) exists and its param_adjustments is
non-NULL.

In theory the param_adjustments could contain description of the very
same signature the original function has but in practice it does not
currently happen and is unlikely to happen ever.

Martin
  

Patch

diff --git a/gcc/gimple-ssa-warn-access.cc b/gcc/gimple-ssa-warn-access.cc
index 04aa849a4b1..59a70530600 100644
--- a/gcc/gimple-ssa-warn-access.cc
+++ b/gcc/gimple-ssa-warn-access.cc
@@ -4291,14 +4291,18 @@  pass_waccess::check_pointer_uses (gimple *stmt, tree ptr,
 void
 pass_waccess::check_call (gcall *stmt)
 {
-  if (gimple_call_builtin_p (stmt, BUILT_IN_NORMAL))
-    check_builtin (stmt);
+  /* Skip special calls generated by the compiler.  */
+  if (gimple_call_from_thunk_p (stmt))
+    return;
 
   /* .ASAN_MARK doesn't access any vars, only modifies shadow memory.  */
   if (gimple_call_internal_p (stmt)
       && gimple_call_internal_fn (stmt) == IFN_ASAN_MARK)
     return;
 
+  if (gimple_call_builtin_p (stmt, BUILT_IN_NORMAL))
+    check_builtin (stmt);
+
   if (!m_early_checks_p)
     if (tree callee = gimple_call_fndecl (stmt))
       {