gcc will become the best optimizing x86 compiler

[Tim Prince]

Agner Fog wrote:
> Michael Matz wrote:
>> You must be doing something wrong.  If the compiler decides to inline 
>> the string ops it either knows the size or you told it to do it anyway 
>> (-minline-all-stringops or -minline-stringops-dynamically).  In both 
>> cases will it use wider than byte moves when possible.
>>   
> g++ (v. 4.2.3) without any options converts memcpy with unknown size to  
> rep movsb
> g++ with option -fno-builtin calls memcpy in libc
> 
> The rep movs, stos, scas, cmps instructions are slower than function 
> calls except in rare cases. The compiler should never use the string 
> instructions. It is OK to use mov instructions if the size is known, but 
> not string instructions.
I assume Agner is talking about the i386 target defaults, while Michael 
was talking about other target defaults.  People who code for i386 must 
often use memcpy for short or medium length unaligned strings, and should 
be aware of the issues of long memcpy strings.  Even for i386, the 
compiler should recognize where, for example, a single int move is 
explicitly the right thing.  The rep string issue has been prominent 
enough for newer CPUs to be designed to recover some of the performance.