https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112835
Bug ID: 112835
Summary: inverting the result of memcmp() produces inefficient
code
Product: gcc
Version: 13.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gnu at kosak dot com
Target Milestone: ---
Created attachment 56778
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=56778&action=edit
the .ii file from --save-temps
Hello,
The assembly output of this program does a suboptimal thing in two different
places. It moves a constant (0 or 1) into %eax and then does a `testl` and
`sete` to invert that constant. This would be reasonable code for `!x` when x
is a runtime `int` value. However, when x is a constant, it would be simpler to
emit a single load instruction.
```
#include <cstring>
bool calc(const void *a, const void *b) {
return !std::memcmp(a, b, 12);
}
```
Relevant assembly on x86_64, invoked with g++ -S -O3 test.cc
```
_Z4calcPKvS0_:
.LFB31:
.cfi_startproc
endbr64
movq (%rsi), %rax
cmpq %rax, (%rdi)
je .L5
.L2:
movl $1, %eax
testl %eax, %eax
sete %al
ret
.p2align 4,,10
.p2align 3
.L5:
movl 8(%rsi), %eax
cmpl %eax, 8(%rdi)
jne .L2
xorl %eax, %eax
testl %eax, %eax
sete %al
ret
.cfi_endproc
```
In the above, the code at label .L2 (movl/testl/sete) would be better off as
the single instruction "xorl %eax,%eax" and likewise the code at the latter
part of .L5 (xorl/testl/sete) would be better off as the single instruction
"movl $1, %eax". You can see the compiler doing this, though with inverted
logic, if you simply delete the ! in the source code.
Apologies if I've posted this to the wrong component.
Output from g++ -v:
```
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-linux-gnu/13/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 13.2.0-4ubuntu3'
--with-bugurl=file:///usr/share/doc/gcc-13/README.Bugs
--enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-13
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/libexec --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib
--enable-libphobos-checking=release --with-target-system-zlib=auto
--enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet
--with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-13-XYspKM/gcc-13-13.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-13-XYspKM/gcc-13-13.2.0/debian/tmp-gcn/usr
--enable-offload-defaulted --without-cuda-driver --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
--with-build-config=bootstrap-lto-lean --enable-link-serialization=2
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 13.2.0 (Ubuntu 13.2.0-4ubuntu3)
```
