ext-dce: Fix subreg_lsb is_constant assumption
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.
gcc/
* ext-dce.cc (ext_dce_process_uses): Apply is_constant directly
to the subreg_lsb.
(cherry picked from commit bf3037e923e9f91d93ab64bdf73a37f64f659fb9)
diff --git a/gcc/ext-dce.cc b/gcc/ext-dce.cc
index afe7afe..e7635fb 100644
--- a/gcc/ext-dce.cc
+++ b/gcc/ext-dce.cc
@@ -651,9 +651,8 @@
/* ?!? 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);
