This fixes a testsuite regression seen on m68k after some of the recent
ext-dce changes. Ultimately Richard S and I have concluded the bug was
a latent issue in subreg simplification.
Essentially when simplifying something like
(set (target:M1) (subreg:M1 (subreg:M2 (reg:M1) 0) 0))
Where M1 > M2. We'd simplify to:
(set (target:M1) (reg:M1))
The problem is on a big endian target that's wrong. Consider if M1 is
DI and M2 is SI. The original should extract bits 32..63 from the
source register and store them into bits 0..31 of the target register.
In the simplified form it's just a copy, so bits 0..63 of the source end
up bits 0..63 of the target.
This shows up as the following regressions on the m68k:
Tests that now fail, but worked before (3 tests):
gcc: gcc.c-torture/execute/960416-1.c -O2 execution test
gcc: gcc.c-torture/execute/960416-1.c -O2 -flto -fno-use-linker-plugin
-flto-partition=none execution test
gcc: gcc.c-torture/execute/960416-1.c -Os execution test
The fix is pretty trivial, instead of hardcoding "0" as the byte offset
in the test for the simplification, instead we need to use the
subreg_lowpart_offset.
Anyway, bootstrapped and regression tested on m68k and x86_64 and tested
on the other embedded targets as well without regressions. Naturally it
fixes the regression noted above. I haven't see other testsuite
improvements when I spot checked some of the big endian crosses.
OK for the trunk?
Jeff
PR rtl-optimization/116136
gcc/
* simplify-rtx.cc (simplify_context::simplify_subreg): Check
that we're working with the lowpart offset rather than byte 0.
diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc
index a49eefb34d4..a20a61c5ddd 100644
--- a/gcc/simplify-rtx.cc
+++ b/gcc/simplify-rtx.cc
@@ -7744,8 +7744,9 @@ simplify_context::simplify_subreg (machine_mode
outermode, rtx op,
return NULL_RTX;
if (outermode == innermostmode
- && known_eq (byte, 0U)
- && known_eq (SUBREG_BYTE (op), 0))
+ && known_eq (byte, subreg_lowpart_offset (outermode, innermode))
+ && known_eq (SUBREG_BYTE (op),
+ subreg_lowpart_offset (innermode, innermostmode)))
return SUBREG_REG (op);
/* Work out the memory offset of the final OUTERMODE value relative
