https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117448
Bug ID: 117448
Summary: gcc failed to optimize dividing to subtracting with
simple assumption
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: xiaohuba2021 at 163 dot com
Target Milestone: ---
The following code:
```
constexpr unsigned int mod = 998244353;
unsigned int add(unsigned int x, unsigned int y) {
[[assume(x < mod)]];
[[assume(y < mod)]];
return (x + y) % mod;
}
```
could be optimized to simply sum up x and y, and then subtract mod if
(x+y)>=mod. In fact, clang does such optimization even if -O2 is enabled.
However, it seems gcc failed to recognize that, and generated a bunch of
instructions to simulate dividing by multiplying and subtracting, even with -O3
enabled.
Godbolt link: https://godbolt.org/z/bn5avY8n3
g++ version:
```
Using built-in specs.
COLLECT_GCC=/opt/compiler-explorer/gcc-snapshot/bin/g++
Target: x86_64-linux-gnu
Configured with: ../gcc-trunk-20241105/configure
--prefix=/opt/compiler-explorer/gcc-build/staging
--enable-libstdcxx-backtrace=yes --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu --disable-bootstrap
--enable-multiarch --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --enable-clocale=gnu
--enable-languages=c,c++,fortran,ada,objc,obj-c++,go,d,rust,m2 --enable-ld=yes
--enable-gold=yes --enable-libstdcxx-debug --enable-libstdcxx-time=yes
--enable-linker-build-id --enable-lto --enable-plugins --enable-threads=posix
--with-pkgversion=Compiler-Explorer-Build-gcc-ad1f112980040ba7bb8b8d0b9273268d4d710c9e-binutils-2.42
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 15.0.0 20241105 (experimental)
(Compiler-Explorer-Build-gcc-ad1f112980040ba7bb8b8d0b9273268d4d710c9e-binutils-2.42)
```
