Re: [PATCH] combine: Fix recent WORD_REGISTER_OPERATIONS check [PR113010]

[Jeff Law <jeffreyalaw@gmail.com>]

On 3/4/24 09:49, Jakub Jelinek wrote:
> On Mon, Mar 04, 2024 at 05:18:39PM +0100, Rainer Orth wrote:
>>> On 2/26/24 17:17, Greg McGary wrote:
>>>> The sign-bit-copies of a sign-extending load cannot be known until runtime on
>>>> WORD_REGISTER_OPERATIONS targets, except in the case of a zero-extending MEM
>>>> load.  See the fix for PR112758.
>>>> 2024-02-22  Greg McGary  <gkm@rivosinc.com>
>>>>           PR rtl-optimization/113010
>>>> 	* combine.cc (simplify_comparison): Simplify a SUBREG on
>>>> 	  WORD_REGISTER_OPERATIONS targets only if it is a zero-extending
>>>> 	  MEM load.
>>>> 	* gcc.c-torture/execute/pr113010.c: New test.
>>> I think this is fine for the trunk.  I'll do some final testing on it
>>> tomorrow.
>>
>> unfortunately, the patch broke Solaris/SPARC bootstrap
>> (sparc-sun-solaris2.11):
>>
>> /vol/gcc/src/hg/master/local/gcc/combine.cc: In function 'rtx_code simplify_comparison(rtx_code, rtx_def**, rtx_def**)':
>> /vol/gcc/src/hg/master/local/gcc/combine.cc:12101:25: error: '*(unsigned int*)((char*)&inner_mode + offsetof(scalar_int_mode, scalar_int_mode::m_mode))' may be used uninitialized [-Werror=maybe-uninitialized]
>> 12101 |   scalar_int_mode mode, inner_mode, tmode;
>>        |                         ^~~~~~~~~~
> 
> I don't see how it could ever work properly, inner_mode in that spot is
> just uninitialized.
> 
> I think we shouldn't worry about paradoxical subregs of non-scalar_int_mode
> REGs/MEMs and for the scalar_int_mode ones should initialize inner_mode
> before we use it.
> Another option would be to use
> maybe_lt (GET_MODE_PRECISION (GET_MODE (SUBREG_REG (op0))), BITS_PER_WORD)
> and
> load_extend_op (GET_MODE (SUBREG_REG (op0))) == ZERO_EXTEND,
> or set machine_mode smode = GET_MODE (SUBREG_REG (op0)); and use it in
> those two spots.
> 
> 2024-03-04  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR rtl-optimization/113010
> 	* combine.cc (simplify_comparison): Guard the
> 	WORD_REGISTER_OPERATIONS check on scalar_int_mode of SUBREG_REG
> 	and initialize inner_mode.
Egad.  Sorry.  OK for the trunk.  Thanks for picking this up.  Got 
distracted by an internal issue.

jeff