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runtime: fix uint64 division on 386

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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://golang.org/cl/13722044
This commit is contained in:
Russ Cox 2013-09-16 15:11:32 -04:00
parent 51266761fd
commit 70138a2108
2 changed files with 6 additions and 3 deletions
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4
src/pkg/runtime/vlop_386.s
View File

@ -29,6 +29,8 @@
* C runtime for 64-bit divide.
*/
// _mul64x32(r *uint64, a uint64, b uint32)
// sets *r = low 64 bits of 96-bit product a*b; returns high 32 bits.
TEXT _mul64by32(SB), NOSPLIT, $0
MOVL r+0(FP), CX
MOVL a+4(FP), AX
@ -38,7 +40,9 @@ TEXT _mul64by32(SB), NOSPLIT, $0
MOVL a+8(FP), AX
MULL b+12(FP)
ADDL AX, BX
ADCL $0, DX
MOVL BX, 4(CX)
MOVL DX, AX
RET
TEXT _div64by32(SB), NOSPLIT, $0

5
src/pkg/runtime/vlrt_386.c
View File

@ -147,7 +147,7 @@ _v2f(Vlong x)
}
ulong _div64by32(Vlong, ulong, ulong*);
void _mul64by32(Vlong*, Vlong, ulong);
int _mul64by32(Vlong*, Vlong, ulong);
static void
slowdodiv(Vlong num, Vlong den, Vlong *q, Vlong *r)
@ -232,8 +232,7 @@ dodiv(Vlong num, Vlong den, Vlong *qp, Vlong *rp)
if(den.hi != 0){
q.hi = 0;
n = num.hi/den.hi;
_mul64by32(&x, den, n);
if(x.hi > num.hi || (x.hi == num.hi && x.lo > num.lo))
if(_mul64by32(&x, den, n) || x.hi > num.hi || (x.hi == num.hi && x.lo > num.lo))
slowdodiv(num, den, &q, &r);
else {
q.lo = n;