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From: "gabravier at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/96930] Failure to optimize out arithmetic
 with bigger size when it can't matter with division transformed into right
 shift
Date: Sun, 03 Jan 2021 12:06:23 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96930

--- Comment #6 from Gabriel Ravier <gabravier at gmail dot com> ---
For this exact code :

unsigned f(unsigned a, unsigned b)
{
    return a / (unsigned long long)(1U << b);
}

compiled with a trunk-based GCC built yesterday for x86-64-linux-gnu config=
ured
with: ../gcc-trunk-20210102/configure
--prefix=3D/opt/compiler-explorer/gcc-build/staging --build=3Dx86_64-linux-=
gnu
--host=3Dx86_64-linux-gnu --target=3Dx86_64-linux-gnu --disable-bootstrap
--enable-multiarch --with-abi=3Dm64 --with-multilib-list=3Dm32,m64,mx32
--enable-multilib --enable-clocale=3Dgnu --enable-languages=3Dc,c++,fortran=
,ada,d
--enable-ld=3Dyes --enable-gold=3Dyes --enable-libstdcxx-debug
--enable-libstdcxx-time=3Dyes --enable-linker-build-id --enable-lto
--enable-plugins --enable-threads=3Dposix
--with-pkgversion=3DCompiler-Explorer-Build

with the specific compiler command line of `g++ -g -o
/tmp/compiler-explorer-compiler202103-4524-15mdddx.f4b4g/output.s -masm=3Di=
ntel
-S -fdiagnostics-color=3Dalways -O3
/tmp/compiler-explorer-compiler202103-4524-15mdddx.f4b4g/example.cpp`, I ge=
t:

f(unsigned int, unsigned int):
  mov eax, edi
  mov ecx, esi
  shr rax, cl
  ret

whereas LLVM uses `shr eax, cl` instead.

See also https://godbolt.org/z/Gqza7v for the exact setup I used (in case I
missed something and you rather avoid having to wait for me to answer).=