[llvm-dev] compiler-rt incorrect for this udivmodti4 case?

[Andrew Kelley via llvm-dev]

This is a rather embarrassing mistake that I made - I simply misunderstood
Python's output:

$ python3
>>> 152313999999999991610955792383 / 10000000000000000000
15231400000.0
>>> 152313999999999991610955792383 // 10000000000000000000
15231399999

Apologies for the noise.

On Wed, Apr 25, 2018 at 10:34 AM, Stephen Canon <scanon at apple.com> wrote:

>
>
> > On Apr 25, 2018, at 12:33 AM, Andrew Kelley via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > Here is my test case:
> >
> > #include <stdio.h>
> >
> > int main(int argc, char **argv) {
> >     tu_int a = (tu_int)0x1ec273014 << 64 | 0xff7377ffffffffffuLL;
> >     tu_int b = (tu_int)0x8ac7230489e80000uLL;
> >     tu_int r;
> >     tu_int q = __udivmodti4(a, b, &r);
> >
> >     utwords qt;
> >     qt.all = q;
> >     utwords rt;
> >     rt.all = r;
> >     fprintf(stderr, "q=0x%.16llX%.16llX\nr=0x%.16llX%.16llX\n",
> >             qt.s.high, qt.s.low, rt.s.high, rt.s.low);
> >
> >     return 0;
> > }
> >
> > This corresponds to the inputs
> > a=152313999999999991610955792383
> > b=10000000000000000000
> >
> > The correct result of division is
> > r=15231400000
> > q=9999991610955792383
> >
> > However compiler-rt gives:
> > r=15231399999
> > q=9999991610955792383
>
> Are your results mislabeled? Integer division rounds towards zero, so the
> correct division result is 15231399999, which you have as the compiler-rt
> result.
>
> – Steve
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