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Subject: Re: [Bug c/46186] Clang creates code running 1600 times faster than gcc's
Date: Tue, 26 Oct 2010 15:56:00 -0000
Cc: "gcc-bugs@gcc.gnu.org" <gcc-bugs@gcc.gnu.org>
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On Oct 26, 2010, at 7:30 AM, "jak@jak-linux.org" <gcc-bugzilla@gcc.gnu.org 
 > wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46186
>
> --- Comment #1 from Julian Andres Klode <jak@jak-linux.org>  
> 2010-10-26 14:30:24 UTC ---
> Created attachment 22162
>  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=22162
> Clang's assember

This multiplication transformation is incorrect if the loop wraps  
(unsigned always wraps; never overflows). Gcc is correct in its speed.  
Though -O3 is faster but not because of multiplication but rather  
constant propatagtion. So this bug is invalid and some one should  
report a bug to llvm folks about this.

>
> Attaching the assembler output from clang, it should help understand  
> which
> optimizations clang does (and improve gcc to do them as well).