combine: Fix recent WORD_REGISTER_OPERATIONS check [PR113010]

combine: Fix recent WORD_REGISTER_OPERATIONS check [PR113010]

On Mon, Mar 04, 2024 at 05:18:39PM +0100, Rainer Orth wrote:
> unfortunately, the patch broke Solaris/SPARC bootstrap
> (sparc-sun-solaris2.11):
>
> .../gcc/combine.cc: In function 'rtx_code simplify_comparison(rtx_code, rtx_def**, rtx_def**)':
> .../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.