[PATCH] ext-dce: Fix subreg_lsb is_constant assumption

[Richard Sandiford]

ext-dce had:

          if (SUBREG_P (dst) && SUBREG_BYTE (dst).is_constant ())
            {
              bit = subreg_lsb (dst).to_constant ();
              if (bit >= HOST_BITS_PER_WIDE_INT)
                bit = HOST_BITS_PER_WIDE_INT - 1;
              dst = SUBREG_REG (dst);

But a constant SUBREG_BYTE doesn't guarantee a constant subreg_lsb.
If the SUBREG_REG is a pair of N-bit registers on a big-endian target,
the most significant end has a SUBREG_BYTE of 0 but a subreg_lsb of N.
This N would then be non-constant for variable-length registers.

The patch fixes gcc.dg/torture/pr120276.c and other failures on
aarch64_be-elf.

Tested on aarch64-linux-gnu & aarch64_be-elf.  OK to install?

Richard


gcc/
        * ext-dce.cc (ext_dce_process_uses): Apply is_constant directly
        to the subreg_lsb.
---
 gcc/ext-dce.cc | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/gcc/ext-dce.cc b/gcc/ext-dce.cc
index afe7afe21fd..e7635fb7a39 100644
--- a/gcc/ext-dce.cc
+++ b/gcc/ext-dce.cc
@@ -651,9 +651,8 @@ ext_dce_process_uses (rtx_insn *insn, rtx obj,
 
          /* ?!? How much of this should mirror SET handling, potentially
             being shared?   */
-         if (SUBREG_P (dst) && SUBREG_BYTE (dst).is_constant ())
+         if (SUBREG_P (dst) && subreg_lsb (dst).is_constant (&bit))
            {
-             bit = subreg_lsb (dst).to_constant ();
              if (bit >= HOST_BITS_PER_WIDE_INT)
                bit = HOST_BITS_PER_WIDE_INT - 1;
              dst = SUBREG_REG (dst);
-- 
2.43.0