C language > Expert questions

128-bit intrinsic math buggy?

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Akko:
The following code works in MSVC (64-bit) but I cannot make it run in PC12 (64-bit console):

// Intrinsic 128-bit multiplication/division test

#include <stdlib.h>
#include <stdint.h>
#include <stdio.h>
#include <intrin.h>

int main(void) {
   uint64_t upl,uph,uq,ur=7;
   upl=_umul128(-1,2,&uph);
   printf("\nuph=1?%d upl=-2?%d",(int)uph,(int)upl);
   uq=_udiv128(uph,upl,2,&ur);
   printf("\nuq=-1?%d ur=0?%d",(int)uq,(int)ur);
   return 0;
}

I tried /Ze compiler flags et cetera, but PC12 always struggles with the division  :-(
Multiplication is no issue.

frankie:
It seems that for the functions _umul128() and _udiv128() the compiler don't emits the intrinsic code, but emits a call to the specific function that, obviously, is missing (doesn't exist in any library).
For me is a compiler bug.

Pelle:
Confirmed.

The compiler internal intrinsic signature for __udiv128 (and _udiv128 if you insist on using the /Ze option) contains (signed) __int64 while the externally visible signature contains (the correct) unsigned __int64. The signatures will never match, so you get the default behavior of calling an external function.

AFAICT this has been broken since __udiv128 and _udiv128 were introduced in version 10 (you guys did a poor job of testing...  :P). With version 12 being done, this will be fixed in some possible future version.

frankie:

--- Quote from: Pelle on May 27, 2023, 01:12:52 PM ---Confirmed.
....

AFAICT this has been broken since __udiv128 and _udiv128 were introduced in version 10 (you guys did a poor job of testing...  :P).

--- End quote ---
LOL ;D :D ;D :D ;D :D ;D :D ;D

Of course you're absolutely right Pelle.   ;)

Akko:
AFAICT also clang has/had similar difficulties with compiling (u)int128_t intrinsics.
Here clang lags behind gcc that has working builtins since long.

I can understand why Pelles C does not support a 64-bit assembler, but for such cases
it would be nice to have one. Long division (eg shift-subtract) in C is painfully slow.   

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