http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60962
Bug ID: 60962
Summary: b+(-2.f)*a generates multiplication instruction while
b-2.f*a simplifies to addition&subtraction
Product: gcc
Version: 4.8.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: b7.10110111 at gmail dot com
Created attachment 32681
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=32681&action=edit
A procedure compilable into assembly to reproduce the bug
I've tried the following with -O3 -ffast-math -fassociative-math options (here
all operands are floats):
float lap0= point[-1]+point[1] + (-2.f)*point[0];
This part of code (compilable into assembly version attached) generates
mulss/addss code (adding a constant in .rodata and reading it before) , which
leads to 6% slowdown compared to this version:
float lap0= point[-1]+point[1] - 2.f*point[0];
, which generates addss/subss code.
My g++ version is g++ (Ubuntu 4.8.1-2ubuntu1~12.04) 4.8.1. The full command
line is:
g++ -O3 -ffast-math -fassociative-math -o test1.s -S -masm=intel test.cpp
The problem also reproduces with g++ 4.5.
