[llvm-dev] multiprecision add/sub

[David Majnemer via llvm-dev]

On Wed, Feb 15, 2017 at 2:28 PM, Stephen Canon via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On Feb 15, 2017, at 2:22 PM, Bagel via llvm-dev <llvm-dev at lists.llvm.org>
> wrote:
>
> > I suggest that LLVM needs intrinsics for add/sub with carry, e.g.
> >
> >  declare {T, i1} @llvm.addc.T(T %a, T %b, i1 c)
> >
> > The current multiprecision clang intrinsics example:
> >  void foo(unsigned *x, unsigned *y, unsigned *z)
> >  { unsigned carryin = 0;
> >    unsigned carryout;
> >    z[0] = __builtin_addc(x[0], y[0], carryin, &carryout);
> >    carryin = carryout;
> >    z[1] = __builtin_addc(x[1], y[1], carryin, &carryout);
> >    carryin = carryout;
> >    z[2] = __builtin_addc(x[2], y[2], carryin, &carryout);
> >    carryin = carryout;
> >    z[3] = __builtin_addc(x[3], y[3], carryin, &carryout);
> >  }
> > uses the LLVM intrinsic "llvm.uadd.with.overflow" and generates
> > horrible code that doesn't use the "adc" x86 instruction.
> >
> > What is the current thinking on improving multiprecision arithmetic?
>
> Why do you think this requires new intrinsics instead of teaching the
> optimizer what to do with the existing intrinsics?
>

In general, it is harder to reason about memory. Also, you are forced to
allocate memory for the carryout even if you are not interested in using it.


>
> – Steve
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