https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65932
--- Comment #18 from Jim Wilson <wilson at gcc dot gnu.org> ---
Ultimately, I believe that this is an ARM backend bug. PROMOTE_MODE and
TARGET_PROMOTE_FUNCTION_MODE should not behave differently. It would help if
the PROMOTE_MODE macro was merged with the TARGET_PROMOTE_FUNCTION_MODE hook,
to avoid accidents like this.
I've tested my patch to modify PROMOTE_MODE so that it no longer sets UNSIGNEDP
for char and short.
For the SPEC CPU2000 benchmarks, individual benchmarks are within 1% which is
within the noise range, and the full benchmark results have almost identical
performance.
I get 3 additional failures in the gcc testsuite, for
gcc.target/arm/wmul-[123].c. These are testcases to verify generation of the
smulbb and smlabb instruction. However, they only work currently because of
the extra sign-extends emitted by PROMOTE_MODE. We currently emit an unsigned
short load, a sign-extend, and a multiply. The sign-extend gets merged into
the multiply. But with the patch, we emit a signed short load and a multiply,
and hence can't form smulbb. Unpatched, for wmul-1.c we get
ldrh r1, [r4, #2]!
ldrh r6, [r0, #2]!
smlabb r5, r1, r6, r5
smlabb r2, r1, r1, r2
and patched we get
ldrsh r1, [r4, #2]!
ldrsh r6, [r0, #2]!
mla r5, r1, r6, r5
mla r2, r1, r1, r2
wmul-2.c is similar. There is a bigger difference with wmul-3.c. Unpatched is
ldrh r1, [r5, #2]!
ldrh r4, [r0, #2]!
smulbb r4, r1, r4
subs r6, r6, r4
smulbb r1, r1, r1
subs r2, r2, r1
whereas patched is
ldrsh r1, [r4, #2]!
ldrsh r6, [r0, #2]!
mls r5, r1, r6, r5
mls r2, r1, r1, r2
The patched code is better or equivalent to the unpatched code in all cases,
but these testcases no longer serve their purpose. I can fix wmul-1.c by
changing types to int and casting to signed short. This doesn't work for
wmul-2.c because the scalar sign-extend is moved out of the loop, and no longer
available to merge with the multiple. This also doesn't work for wmul-3.c, but
only because it is cse'd differently. I get unpatched
smulbb r4, r1, r4
subs r6, r6, r4
and patched
sxth r4, r4
mls r6, r1, r4, r6
At the moment the only option I have to make wmul-2.c and wmul-3.c work is to
remove them
