A little warning in GMP.h
bhurt at spnz.org
Sat Sep 25 03:28:00 CEST 2004
On 25 Sep 2004, Torbjorn Granlund wrote:
> Brian Hurt <bhurt at spnz.org> writes:
> And in the past eliminating branchs wasn't that big of a problem. It's
> only been recently that we've started getting CPUs where the branch
> mispredict cost was extreme.
> If you by recently mean the last 10 years, then I agree.
More like last five. There's some handwaving around what precisely
"extreme" means. The P3 had a 12-stage pipeline IIRC- a branch
mispredict was about half the cost on the P3 as on the P4 in term of
> Micro-optimizing GMP is a difficult task. It requires meticulous
> testing of several platforms and compilers.
I agree. And if shaving every single clock cycle off the operation is
needed, it probably needs to be done in assembly- for precisely the
reasons being stated- the difficulty of handling different compilers all
> > We're not going to go through the GMP code just to work around a
> > bogus Microsoft compiler warning. Please use GCC for compiling
> > GMP if Microsoft's compiler's bogus warnings bother you.
> This isn't a microsoft-ism.
> Really? What other compilers have issues with twos complement of
> unsigned types?
> Curious remark: If you read the ISO C standard, you'll find out
> that unary "-" is completely well-defined only for unsigned
> types. For signed types the standard allows negation to be
> implementation defined when overflow occurs. It may make demons
> fly out from your nose without being in breach of the standard.
Taking a look at the C standard that I happen to have within arm's reach,
I find section 18.104.22.168, which does define not one, but at least three
different effects negation can have on unsigned numbers. Note that this
is the C99 standard.
> This is a code quality question.
> I don't understand. Surely you don't mean that our decision not
> to work around bogus compiler warnings mean our code quality
> isn't good?
No. The fact that you're applying micro-optimizations to your C code
makes it harder to maintain. You can stop attacking that straw man now.
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