[llvm-commits] [llvm] r126964 - in /llvm/trunk: lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp lib/CodeGen/SelectionDAG/LegalizeTypes.h test/CodeGen/X86/umulo-64.ll

[Eli Friedman]

On Thu, Mar 3, 2011 at 2:39 PM, Eric Christopher <echristo at apple.com> wrote:
>
> On Mar 3, 2011, at 2:33 PM, Eli Friedman wrote:
>
>> Author: efriedma
>> Date: Thu Mar  3 16:33:23 2011
>> New Revision: 126964
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=126964&view=rev
>> Log:
>> Revert r123908; the code in question is completely untested and wrong.
>
> How very anti-social of you.
>
> Don't do that again without sending email first or filing a bug.

Sorry, I agree that was inappropriate.

> What's up?

On x86-64, trying to compile an i128 umulo crashes.  As for smulo,
it's returning completely wrong results because it's trying to use the
bottom 128 bits of an i128-bit multiply to determine whether it
overflowed, which is impossible to do.  For example, compiling the
following at -O0 with -ftrapv traps, while it is trivial to conclude
it shouldn't because a 64x64->128 multiply can't overflow.

#include <stdio.h>
__int128_t x(__int128_t a, __int128_t b) {
  return a*b;
}

int main() {
  __int128_t r = x((long long)0x8000000000000001, (long
long)0x8000000000000000);
  printf("%llx %llx\n", (long long)(r >> 64), (long long)r);
  return 0;
}

-Eli