https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85283
Bug ID: 85283
Summary: Generates 20 lines of assembly while only one assembly
instruction is enough.
Product: gcc
Version: 8.0.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: mcccs at gmx dot com
Target Milestone: ---
GCC version: trunk/20180407 (also older versions)
Target: x86_64-linux-gnu
Compile options: -Ofast -mavx2 -mfma -Wall -Wextra -Wpedantic
Build options: --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 --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=GCC-Explorer-Build
The exact code (no #include s):
typedef struct {
float x, y;
} Vec2;
Vec2 vec2_add(Vec2 a, Vec2 b) {
Vec2 out = {a.x + b.x,
a.y + b.y};
return out;
}
Produced assembly with line numbers:
1 vec2_add:
2 vmovq rcx, xmm0
3 vmovq rsi, xmm1
...
21 vmovq xmm0, QWORD PTR [rsp-24]
22 ret
Expected assembly (as compiled by Clang 6.0 with -Ofast -mavx2 -mfma):
1 vec2_add: # @vec2_add
2 vaddps xmm0, xmm1, xmm0
3 ret
(Yes, only three lines)
^^^^^^
(These can be experimented here: https://godbolt.org/g/tTwusV)
See also (for other inefficiencies): https://godbolt.org/g/AtWNgf
