Re: Current int & float overflow checking is slow.

Hi,

On 2017-10-30 22:29:42 +0530, Robert Haas wrote:
> On Mon, Oct 30, 2017 at 4:57 PM, Andres Freund <andres@anarazel.de> wrote:
> > 0001) Introduces pg_{add,sub,mul}{16,32,64}_overflow(a, b, *result)
> >       These use compiler intrinsics on gcc/clang. If that's not
> >       available, they cast to a wider type and to overflow checks. For
> >       64bit there's a fallback for the case 128bit math is not
> >       available (here I stole from an old patch of Greg's).
> >
> >       These fallbacks are, as far as I can tell, C free of overflow
> >       related undefined behaviour.
> 
> Looks nice.

Thanks.


> >       Perhaps it should rather be pg_add_s32_overflow, or a similar
> >       naming scheme?
> 
> Not sure what the s is supposed to be?  Signed?

Yes, signed. So we could add a u32 or something complementing the
functions already in the patch. Even though overflow checks are a heck
of a lot easier to write for unsigned ints, the intrinsics are still
faster.  I don't have any sort of strong feelings on the naming.


> > 0002) Converts int.c, int8.c and a smattering of other functions to use
> >       the new facilities. This removes a fair amount of code.
> >
> >       It might make sense to split this up further, but right now that's
> >       the set of functions that either are affected performancewise by
> >       previous overflow checks, and/or relied on wraparound
> >       overflow. There's probably more places, but this is what I found
> >       by visual inspection and compiler warnings.
> 
> I lack the patience to review this tonight.

Understandable ;)


> > 0003) Removes -fwrapv. I'm *NOT* suggesting we apply this right now, but
> >       it seems like an important test for the new facilities. Without
> >       0002, tests would fail after this, after it all tests run
> >       successfully.
> 
> I suggest that if we think we don't need -fwrapv any more, we ought to
> remove it.  Otherwise, we won't find out if we're wrong.

I agree that we should do so at some point not too far away in the
future. Not the least because we don't specify this kind of C dialect in
a lot of other compilers. Additionally the flag causes some slowdown
(because e.g. for loop variables are optimized less). But I'm fairly
certain it needs a bit more care that I've invested as of now - should
probably at least compile with -Wstrict-overflow=some-higher-level, and
with ubsan. I'm fairly certain there's more bogus overflow checks
around...

Greetings,

Andres Freund