[llvm-dev] multiprecision add/sub

Bagel via llvm-dev llvm-dev at lists.llvm.org
Thu Feb 16 09:15:22 PST 2017

I was suggesting #1.  I don't care about clang.

Am I supposed to file a bug against every backend?

On 02/15/2017 09:44 PM, Chandler Carruth wrote:
> I feel like there are two things being conflated here:
> 1) Does LLVM need new *LLVM* intrinsics to model this. The add-with-overflow
> intrinsics seem likely fine for as-is, don't require memory, but we don't lower
> the code to use adc for some reason. That seems like an LLVM bug that should be
> fixed in the x86 backend. If optimizing the LLVM intrinsics is hard, then maybe
> we need better ones, but that's likely about the (somewhat ridiculous) dance we
> have to do to "sum" the overflow flags rather than anything to do with memory.
> 2) Does Clang need new *C* builtins to avoid needing the carry-in / carry-out
> being in memory? I don't have a strong opinion about this. But the example in
> the original email gets optimized to have no unnecessary memory accesses
> already: https://godbolt.org/g/GFij97
> #1 seems like the most interesting problem to solve. File a bug?
> On Wed, Feb 15, 2017 at 4:24 PM Brian Smith via llvm-dev
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>     Stephen Canon via llvm-dev <llvm-dev at lists.llvm.org
>     <mailto:llvm-dev at lists.llvm.org>> wrote:
>     >
>     > Why do you think this requires new intrinsics instead of teaching the
>     optimizer what to do with the existing intrinsics?
>     IMO, as a multiprecision math library maker, the "teaching the
>     optimizer what to do with the existing intrinsics" approach is much
>     better as long as it can be made to work. If one is careful, MSVC does
>     optimize its intrinsics into ADC instructions in a reasonable way, so
>     I think it is probably doable.
>     (Below, all math is multiprecision.)
>     There are actually a few different operations besides pure addition
>     and subtraction, e.g. `a - (b >> 1)` instead of just `a - b`. Also
>     consider that we sometimes want "a - b" to be side-channel free and
>     other times we'd rather "a - b" to be as fast as possible and
>     optimized for the case where carries are unlikely to propagate (far).
>     That's already three different operations, just for subtraction.
>     Cheers,
>     Brian
>     --
>     https://briansmith.org/
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