Do not erase warning data in gimple_set_location

  Hi,

gimple_set_location is mostly invoked on newly built GIMPLE statements, so 
their location is UNKNOWN_LOCATION and setting it will clobber the warning 
data of the passed location, if any.

Tested on x86-64/Linux, OK for mainline and 12 branch?


2022-06-10  Eric Botcazou  <ebotcazou@adacore.com>

	* gimple.h (gimple_set_location): Do not copy warning data from
	the previous location when it is UNKNOWN_LOCATION.


2022-06-10  Eric Botcazou  <ebotcazou@adacore.com>

testsuite/
	* c-c++-common/nonnull-1.c: Remove XFAIL for C++.
  

On 6/10/2022 4:57 AM, Eric Botcazou via Gcc-patches wrote:
> Hi,
>
> gimple_set_location is mostly invoked on newly built GIMPLE statements, so
> their location is UNKNOWN_LOCATION and setting it will clobber the warning
> data of the passed location, if any.
>
> Tested on x86-64/Linux, OK for mainline and 12 branch?
>
>
> 2022-06-10  Eric Botcazou  <ebotcazou@adacore.com>
>
> 	* gimple.h (gimple_set_location): Do not copy warning data from
> 	the previous location when it is UNKNOWN_LOCATION.
>
>
> 2022-06-10  Eric Botcazou  <ebotcazou@adacore.com>
>
> testsuite/
> 	* c-c++-common/nonnull-1.c: Remove XFAIL for C++.
OK for trunk and gcc-12.
jeff
  

On Fri, Jun 10, 2022 at 12:58 PM Eric Botcazou via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Hi,
>
> gimple_set_location is mostly invoked on newly built GIMPLE statements, so
> their location is UNKNOWN_LOCATION and setting it will clobber the warning
> data of the passed location, if any.

Hmm, I think instead of special-casing UNKNOWN_LOCATION
what gimple_set_location should probably do is either not copy
warnings at all or union them.  Btw, gimple_set_location also
removes a previously set BLOCK (but gimple_set_block preserves
the location locus and diagnostic override).

So I'd be tempted to axe the copy_warning () completely here.  Martin,
there were
probably cases that warranted it - do you remember anything specific here?

Thanks,
Richard.

> Tested on x86-64/Linux, OK for mainline and 12 branch?
>
>
> 2022-06-10  Eric Botcazou  <ebotcazou@adacore.com>
>
>         * gimple.h (gimple_set_location): Do not copy warning data from
>         the previous location when it is UNKNOWN_LOCATION.
>
>
> 2022-06-10  Eric Botcazou  <ebotcazou@adacore.com>
>
> testsuite/
>         * c-c++-common/nonnull-1.c: Remove XFAIL for C++.
>
> --
> Eric Botcazou
  

On 6/13/22 05:15, Richard Biener wrote:
> On Fri, Jun 10, 2022 at 12:58 PM Eric Botcazou via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>>
>> Hi,
>>
>> gimple_set_location is mostly invoked on newly built GIMPLE statements, so
>> their location is UNKNOWN_LOCATION and setting it will clobber the warning
>> data of the passed location, if any.
> 
> Hmm, I think instead of special-casing UNKNOWN_LOCATION
> what gimple_set_location should probably do is either not copy
> warnings at all or union them.  Btw, gimple_set_location also
> removes a previously set BLOCK (but gimple_set_block preserves
> the location locus and diagnostic override).
> 
> So I'd be tempted to axe the copy_warning () completely here.  Martin,
> there were
> probably cases that warranted it - do you remember anything specific here?

Nothing specific, just that the assumption behind the warning group
design was that a location must exist in order to suppress a warning
(a location is one of the first things that's set early on by the FE
and it makes little sense to issue a warning without one).

There was and in all likelihood still is code sets TREE_NO_WARNING
or gimple_no_warning on new trees/statements before setting their
location.  That interferes with the design when the new tree or
statement is meant to be a replacement of another.  I fixed a few
cases like that to set the location first but didn't have a way
of finding all such instances.  My expectation was to over time
change GCC to make sure a location would always be set before
the no-warning bit, and asserting that on every call to these
routines.  Adding tests like in the patch below goes in the opposite
direction and effectively papers over the problem.  I can't think
of a way to make the suppression work completely reliably without
ensuring that a location is always set before suppressing
a warning.

Martin

> 
> Thanks,
> Richard.
> 
>> Tested on x86-64/Linux, OK for mainline and 12 branch?
>>
>>
>> 2022-06-10  Eric Botcazou  <ebotcazou@adacore.com>
>>
>>          * gimple.h (gimple_set_location): Do not copy warning data from
>>          the previous location when it is UNKNOWN_LOCATION.
>>
>>
>> 2022-06-10  Eric Botcazou  <ebotcazou@adacore.com>
>>
>> testsuite/
>>          * c-c++-common/nonnull-1.c: Remove XFAIL for C++.
>>
>> --
>> Eric Botcazou
>
  

> Hmm, I think instead of special-casing UNKNOWN_LOCATION
> what gimple_set_location should probably do is either not copy
> warnings at all or union them.  Btw, gimple_set_location also
> removes a previously set BLOCK (but gimple_set_block preserves
> the location locus and diagnostic override).
> 
> So I'd be tempted to axe the copy_warning () completely here. 

The first thing I tried, but it regressed the original testcase IIRC.

Even my minimal patch manages to break bootstrap on ARM:

buildslave/workspace/tcwg_gnu_1/abe/snapshots/gcc.git~master/libcpp/lex.cc:
1523:9: error: pointer used after ‘void operator delete(void*, std::size_t)’ 
[-Werror=use-after-free]
# 00:31:04 make[3]: *** [Makefile:227: lex.o] Error 1
# 00:31:04 make[2]: *** [Makefile:9527: all-stage3-libcpp] Error 2
# 00:31:35 make[1]: *** [Makefile:25887: stage3-bubble] Error 2
# 00:31:35 make: *** [Makefile:1072: all] Error 2

      /* Don't warn for cases like when a cdtor returns 'this' on ARM.  */
      else if (warning_suppressed_p (var, OPT_Wuse_after_free))
	return;

because warning-control.cc:copy_warning also clobbers the warning data of the 
destination.  We have in cp/decl.cc:maybe_return_this the lines:

      /* Return the address of the object.  */
      tree val = DECL_ARGUMENTS (current_function_decl);
      suppress_warning (val, OPT_Wuse_after_free);

-Wuse-after-free is suppressed for the location of VAL and the TREE_NO_WARNING 
bit set on it.  But other expressions may have the same location as VAL and 
the TREE_NO_WARNING bit _not_ set, so when you call copy_warning (expr, expr) 
(we do that a lot after failed folding) for them, copy_warning erases the 
warning data of the location.

I have installed the obvious fixlet after testing on x86-64/Linux, but the 
decoupling between TREE_NO_WARNING bit and location looks a bit problematic.


	* warning-control.cc (copy_warning) [generic version]: Do not erase
	the warning data of the destination location when the no-warning
	bit is not set on the source.
	(copy_warning) [tree version]: Return early if TO is equal to FROM.
	(copy_warning) [gimple version]: Likewise.
testsuite/
	* g++.dg/warn/Wuse-after-free5.C: New test.
  

On Tue, Jun 14, 2022 at 12:49 PM Eric Botcazou <botcazou@adacore.com> wrote:
>
> > Hmm, I think instead of special-casing UNKNOWN_LOCATION
> > what gimple_set_location should probably do is either not copy
> > warnings at all or union them.  Btw, gimple_set_location also
> > removes a previously set BLOCK (but gimple_set_block preserves
> > the location locus and diagnostic override).
> >
> > So I'd be tempted to axe the copy_warning () completely here.
>
> The first thing I tried, but it regressed the original testcase IIRC.
>
> Even my minimal patch manages to break bootstrap on ARM:
>
> buildslave/workspace/tcwg_gnu_1/abe/snapshots/gcc.git~master/libcpp/lex.cc:
> 1523:9: error: pointer used after ‘void operator delete(void*, std::size_t)’
> [-Werror=use-after-free]
> # 00:31:04 make[3]: *** [Makefile:227: lex.o] Error 1
> # 00:31:04 make[2]: *** [Makefile:9527: all-stage3-libcpp] Error 2
> # 00:31:35 make[1]: *** [Makefile:25887: stage3-bubble] Error 2
> # 00:31:35 make: *** [Makefile:1072: all] Error 2
>
>       /* Don't warn for cases like when a cdtor returns 'this' on ARM.  */
>       else if (warning_suppressed_p (var, OPT_Wuse_after_free))
>         return;
>
> because warning-control.cc:copy_warning also clobbers the warning data of the
> destination.  We have in cp/decl.cc:maybe_return_this the lines:
>
>       /* Return the address of the object.  */
>       tree val = DECL_ARGUMENTS (current_function_decl);
>       suppress_warning (val, OPT_Wuse_after_free);
>
> -Wuse-after-free is suppressed for the location of VAL and the TREE_NO_WARNING
> bit set on it.  But other expressions may have the same location as VAL and
> the TREE_NO_WARNING bit _not_ set, so when you call copy_warning (expr, expr)
> (we do that a lot after failed folding) for them, copy_warning erases the
> warning data of the location.
>
> I have installed the obvious fixlet after testing on x86-64/Linux, but the
> decoupling between TREE_NO_WARNING bit and location looks a bit problematic.

Thanks - that makes more sense.

>
>         * warning-control.cc (copy_warning) [generic version]: Do not erase
>         the warning data of the destination location when the no-warning
>         bit is not set on the source.
>         (copy_warning) [tree version]: Return early if TO is equal to FROM.
>         (copy_warning) [gimple version]: Likewise.
> testsuite/
>         * g++.dg/warn/Wuse-after-free5.C: New test.
>
> --
> Eric Botcazou