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

[Jakub Jelinek <jakub@redhat.com>]

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.

--- gcc/combine.cc.jj	2024-03-04 10:01:21.054937316 +0100
+++ gcc/combine.cc	2024-03-04 17:40:51.556052647 +0100
@@ -12554,6 +12554,8 @@ simplify_comparison (enum rtx_code code,
 	  if (paradoxical_subreg_p (op0))
 	    {
 	      if (WORD_REGISTER_OPERATIONS
+		  && is_a <scalar_int_mode> (GET_MODE (SUBREG_REG (op0)),
+					     &inner_mode)
 		  && GET_MODE_PRECISION (inner_mode) < BITS_PER_WORD
 		  /* On WORD_REGISTER_OPERATIONS targets the bits
 		     beyond sub_mode aren't considered undefined,


	Jakub