https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96034
Bug ID: 96034
Summary: missed optimization with extended registers
Product: gcc
Version: 9.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: sshannin at gmail dot com
Target Milestone: ---
Noticed in example for PR96009.
Consider this simple function:
double bar(char i) {
return i;
}
Compiled with -O3, we get:
movsbl %dil, %edi
pxor %xmm0, %xmm0
cvtsi2sdl %edi, %xmm0
ret
But aren't the movsb and pxor unnecessary? I think we should be able to just
cvtsi2sd and then ret.
Interestingly, compiling with -OS instead of -O3 manages to remove the pxor:
movsbl %dil, %edi
cvtsi2sdl %edi, %xmm0
ret
Which is a one instruction better (unless -O3 is trying to keep the pxor for
alignment?), but even here I think the movsb could still go too.
Closest thing I find is PR48701, in that it also doesn't seem to understand
which registers are the same.
seth@fr-dev3:$ /toolchain14/bin/gcc -v
Using built-in specs.
COLLECT_GCC=/toolchain14/bin/gcc
COLLECT_LTO_WRAPPER=/toolchain14/libexec/gcc/x86_64-pc-linux-gnu/9.1.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc_9_1_0/configure --prefix=/toolchain14
--enable-languages=c,c++,fortran --enable-lto --disable-plugin
--program-suffix=-9.1.0 --disable-multilib
Thread model: posix
gcc version 9.1.0 (GCC)
