https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96535
Bug ID: 96535
Summary: GCC 10 ignoring function __attribute__ optimize for
all x86
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: danielhanchen at gmail dot com
Target Milestone: ---
Hey GCC team!
In GCC 10.x, it seems like any argument to __attribute__((optimize(...)) is
ignored at the function level. GCC 9.x and previous do not have this issue. [Or
maybe only -funroll-loops is ignored not 100% sure]
Detailed example at: https://gcc.godbolt.org/z/PTK4WE
3 Scenarios
1. [GCC 10.2: -O2 -ffast-math -march=haswell -std=c++2a -fopenmp] +
[__attribute__((optimize("O2","fast-math","unroll-loops")))] DOES NOT unroll.
2. [GCC 10.2: -funroll-loops -O2 -ffast-math -march=haswell -std=c++2a
-fopenmp] + [__attribute__((optimize("O2","fast-math","unroll-loops")))] DOES
unroll.
3. [GCC 9.3: -O2 -ffast-math -march=haswell -std=c++2a -fopenmp] +
[__attribute__((optimize("O2","fast-math","unroll-loops")))] DOES unroll.
It seems that in GCC 10.x, you have to place -funroll-loops in the compilation
string, and function level __attribute__s are ignored?
PS: Code in godbolt is a matrix multiplication kernel. It multiplies 1 column *
1 row of a matrix.
