PR rtl-optimization/110701: Fix SUBREG SET_DEST handling in combine.

PR rtl-optimization/110701: Fix SUBREG SET_DEST handling in combine.

This patch is my proposed fix to PR rtl-optimization 110701, a latent bug
in combine's record_dead_and_set_regs_1 exposed by recent improvements to
simplify_subreg.

The issue involves the handling of (normal) SUBREG SET_DESTs as in the
instruction:

	(set (subreg:HI (reg:SI x) 0) (expr:HI y))

The semantics of this are that the bits specified by the SUBREG are set
to the SET_SRC, y, and that the other bits of the SET_DEST are left/become
undefined.  To simplify explanation, we'll only consider lowpart SUBREGs
(though in theory non-lowpart SUBREGS could be handled), and the fact that
bits outside of the lowpart WORD retain their original values (treating
these as undefined is a missed optimization rather than incorrect code
bug, that only affects targets with less than 64-bit words).

The bug is that combine simulates the behaviour of the above instruction,
for calculating nonzero_bits and set_sign_bit_copies, in the function
record_value_for_reg, by using the equivalent of:

	(set (reg:SI x) (subreg:SI (expr:HI y))

by calling gen_lowpart on the SET_SRC.  Alas, the semantics of this
revised instruction aren't always equivalent to the original.

In the test case for PR110701, the original instruction

	(set (subreg:HI (reg:SI x), 0)
             (and:HI (subreg:HI (reg:SI y) 0)
		     (const_int 340)))

which (by definition) leaves the top bits of x undefined, is mistakenly
considered to be equivalent to

	(set (reg:SI x) (and:SI (reg:SI y) (const_int 340)))

where gen_lowpart's freedom to do anything with paradoxical SUBREG bits,
has now cleared the high bits.  The same bug also triggers when the
SET_SRC is say (subreg:HI (reg:DI z)), where gen_lowpart transforms
this into (subreg:SI (reg:DI z)) which defines bits 16-31 to be the
same as bits 16-31 of z.

The fix is that after calling record_value_for_reg, we need to mark
the bits that should be undefined as undefined, in case gen_lowpart,
which performs transforms appropriate for r-values, has changed the
interpretation of the SUBREG when used as an l-value.

2023-10-04  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR rtl-optimization/110701
	* combine.cc (record_dead_and_set_regs_1): Split comment into
	pieces placed before the relevant clauses.  When the SET_DEST
	is a partial_subreg_p, mark the bits outside of the updated
	portion of the destination as undefined.

gcc/testsuite/ChangeLog
	PR rtl-optimization/110701
	* gcc.target/i386/pr110701.c: New test case.