Re: G++ could optimize ASM code more

[Daniel Marschall <daniel-marschall at viathinksoft dot de>]

Hello and thanks for your quick reply!

Am 09.05.2012 15:59, schrieb Ian Lance Taylor:
> Note that the current GCC release is 4.7.0.

The problem with Debian Squeeze is always that I have to use "medieval" 
software... ;-) Maybe I should develop the server software on a local 
box using "unstable" software. On the other hand, if I develop directly 
at the production machine, I can directly optimize the program for the 
machine itself and not for my local box/CPU.

> This cast changes the meaning of the code, so it's not surprising 
> that
> you see different assembler instructions.  The first case above will 
> do
> the multiplication in the type "unsigned long long".  In the second 
> case
> the "unsigned char" values are zero-extended to int, and the
> multiplication is done in the type "int".  Then the "int" result is
> sign-extended to "unsigned long long" for the addition.
>
> In this case it's true that the compiler could convert the code as 
> you
> suggest, based on the knowledge that the int values are always in the
> range 0 to 255.

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 ?

So it could have extended the multiplication to the unsigned long-long 
datatype of c or at least just "unsigned int" instead of "signed int"?

> However, it's not clear to me that using imulq would be
> better.  My copy of the Intel optimization manual suggests that imull
> has slightly lower latency than imulq, so I think that in many cases
> imull would be preferred.

Mh... good point. I do not know much about Assembler so I just thought 
the shorter the code the better. If imull is faster than imulq, then the 
question is, if imull+movslq is still faster than a single imulq. Do you 
know where I can find these informations for my CPU (Intel Xeon X3440)? 
I was searching for a table which shows how many CPU-ticks the imull, 
imulq and movslq need, but yet I have not found one.

My Linux is 2.6.32-5-amd64 #1 SMP Mon Jan 16 16:22:28 UTC 2012 x86_64 
GNU/Linux .

And the CPU is "Intel(R) Xeon(R) CPU X3440  @ 2.53GHz". (I hope the 
"amd64" version of Debian is the correct one, or should our admin have 
installed the "ia64" variant since it is an Intel CPU?)

Best regards
Daniel Marschall