code review 13722044: runtime: fix uint64 division on 386

runtime: fix uint64 division on 386

The uint64 divide function calls _mul64x32 to do a 64x32-bit multiply
and then compares the result against the 64-bit numerator.
If the result is bigger than the numerator, must use the slow path.

Unfortunately, the 64x32 produces a 96-bit product, and only the
low 64 bits were being used in the comparison. Return all 96 bits,
the bottom 64 via the original uint64* pointer, and the top 32
as the function's return value.

Fixes 386 build (broken by ARM division tests).

[rsc, 2013-09-16 19:03:14]

Hello golang-dev@googlegroups.com,

I'd like you to review this change to
https://code.google.com/p/go/

[iant, 2013-09-16 19:09:46]

LGTM

https://codereview.appspot.com/13722044/diff/6001/src/pkg/runtime/vlrt_386.c
File src/pkg/runtime/vlrt_386.c (right):

https://codereview.appspot.com/13722044/diff/6001/src/pkg/runtime/vlrt_386.c#...
src/pkg/runtime/vlrt_386.c:214: extern void runtime·printf(char*, ...);
Remove this declaration before submitting.

[rsc, 2013-09-16 19:11:36]

*** Submitted as https://code.google.com/p/go/source/detail?r=7c63f0eebb6a ***

runtime: fix uint64 division on 386

The uint64 divide function calls _mul64x32 to do a 64x32-bit multiply
and then compares the result against the 64-bit numerator.
If the result is bigger than the numerator, must use the slow path.

Unfortunately, the 64x32 produces a 96-bit product, and only the
low 64 bits were being used in the comparison. Return all 96 bits,
the bottom 64 via the original uint64* pointer, and the top 32
as the function's return value.

Fixes 386 build (broken by ARM division tests).

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/13722044