G++ could optimize ASM code more

[Daniel Marschall]

Am 09.05.2012 20:30, schrieb Ian Lance Taylor:
> Daniel Marschall <daniel-marschall@viathinksoft.de> writes:
>
>> I did understand that the compiler used "signed" multiplication
>> instead of an unsigned one because char*char needs to be extended.
>>
>> Maybe I am wrong, but couldn't the compiler "know" that the result
>> will be at least unsigned because unsigned * unsigned = unsigned ?
>
> Well, but the rules of C say that the unsigned char values are
> zero-extended to int, and then they are multiplied using a signed
> multiplication.  So the result is not unsigned.  The compiler really
> would have to do some sort of type or value based reasoning here to
> determine that an unsigned multiplication would work also.
>
Hello,

I could sucessfully do a benchmark of my code. I found out that the 
no-typecast-version (imull+movslq) needed 47 secs for 12 working 
packages, while the typecast-version (imulq) needed only 38 secs per 12 
working packages. That is incredible!

Maybe you should still consider preferring imulq instead of 
imull+movslq ?

I wonder if GCC has an optimization which optimizes the machine code 
itself, without knowledge of the underlaying C code, e.g. it could 
eliminate unnecessary mov commands if a register is not used resp. using 
operations which do have lower latency. I think such an "assembler-only" 
optimization still can get additional performance since the rules of the 
underlaying programming language (e.g. the expansion to signed int) can 
be ignored if the end-result is the same. But I fear that this is rather 
a hard task and maybe not possible.

Daniel