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.

1 files changed, 29 insertions, 13 deletions

diff --git a/gcc/combine.cc b/gcc/combine.cc
index 468b7fd..360aa2f 100644
--- a/gcc/combine.cc
+++ b/gcc/combine.cc
@@ -13411,27 +13411,43 @@ record_dead_and_set_regs_1 (rtx dest, const_rtx setter, void *data)
 
   if (REG_P (dest))
     {
-      /* If we are setting the whole register, we know its value.  Otherwise
-	 show that we don't know the value.  We can handle a SUBREG if it's
-	 the low part, but we must be careful with paradoxical SUBREGs on
-	 RISC architectures because we cannot strip e.g. an extension around
-	 a load and record the naked load since the RTL middle-end considers
-	 that the upper bits are defined according to LOAD_EXTEND_OP.  */
+      /* If we are setting the whole register, we know its value.  */
       if (GET_CODE (setter) == SET && dest == SET_DEST (setter))
 	record_value_for_reg (dest, record_dead_insn, SET_SRC (setter));
+      /* We can handle a SUBREG if it's the low part, but we must be
+	 careful with paradoxical SUBREGs on RISC architectures because
+	 we cannot strip e.g. an extension around a load and record the
+	 naked load since the RTL middle-end considers that the upper bits
+	 are defined according to LOAD_EXTEND_OP.  */
       else if (GET_CODE (setter) == SET
 	       && GET_CODE (SET_DEST (setter)) == SUBREG
 	       && SUBREG_REG (SET_DEST (setter)) == dest
 	       && known_le (GET_MODE_PRECISION (GET_MODE (dest)),
 			    BITS_PER_WORD)
 	       && subreg_lowpart_p (SET_DEST (setter)))
-	record_value_for_reg (dest, record_dead_insn,
-			      WORD_REGISTER_OPERATIONS
-			      && word_register_operation_p (SET_SRC (setter))
-			      && paradoxical_subreg_p (SET_DEST (setter))
-			      ? SET_SRC (setter)
-			      : gen_lowpart (GET_MODE (dest),
-					     SET_SRC (setter)));
+	{
+	  if (WORD_REGISTER_OPERATIONS
+	      && word_register_operation_p (SET_SRC (setter))
+	      && paradoxical_subreg_p (SET_DEST (setter)))
+	    record_value_for_reg (dest, record_dead_insn, SET_SRC (setter));
+	  else if (!partial_subreg_p (SET_DEST (setter)))
+	    record_value_for_reg (dest, record_dead_insn,
+				  gen_lowpart (GET_MODE (dest),
+					       SET_SRC (setter)));
+	  else
+	    {
+	      record_value_for_reg (dest, record_dead_insn,
+				    gen_lowpart (GET_MODE (dest),
+						 SET_SRC (setter)));
+
+	      unsigned HOST_WIDE_INT mask;
+	      reg_stat_type *rsp = &reg_stat[REGNO (dest)];
+	      mask = GET_MODE_MASK (GET_MODE (SET_DEST (setter)));
+	      rsp->last_set_nonzero_bits |= ~mask;
+	      rsp->last_set_sign_bit_copies = 1;
+	    }
+	}
+      /* Otherwise show that we don't know the value.  */
       else
 	record_value_for_reg (dest, record_dead_insn, NULL_RTX);
     }