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cmd/6g, cmd/8g: simplify integer division code.

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Change suggested by iant. The compiler generates
special code for a/b when a is -0x80...0 and b = -1.
A single instruction can cover the case where b is -1,
so only one comparison is needed.

Fixes #3551.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6922049
This commit is contained in:
Rémy Oudompheng 2012-12-12 08:35:08 +01:00
parent 4766a35e7c
commit a617d06252
2 changed files with 21 additions and 30 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

30
src/cmd/6g/ggen.c
View File

@ -454,10 +454,10 @@ void
dodiv(int op, Node *nl, Node *nr, Node *res)
{
int a, check;
Node n3, n4, n5;
Node n3, n4;
Type *t, *t0;
Node ax, dx, ax1, n31, oldax, olddx;
Prog *p1, *p2, *p3;
Prog *p1, *p2;
// Have to be careful about handling
// most negative int divided by -1 correctly.
@ -508,30 +508,22 @@ dodiv(int op, Node *nl, Node *nr, Node *res)
gmove(&n31, &n3);
}
p3 = P;
p2 = P;
if(check) {
nodconst(&n4, t, -1);
gins(optoas(OCMP, t), &n3, &n4);
p1 = gbranch(optoas(ONE, t), T, +1);
nodconst(&n4, t, -1LL<<(t->width*8-1));
if(t->width == 8) {
n5 = n4;
regalloc(&n4, t, N);
gins(AMOVQ, &n5, &n4);
}
gins(optoas(OCMP, t), &ax, &n4);
p2 = gbranch(optoas(ONE, t), T, +1);
if(op == ODIV)
gmove(&n4, res);
if(t->width == 8)
regfree(&n4);
if(op == OMOD) {
if(op == ODIV) {
// a / (-1) is -a.
gins(optoas(OMINUS, t), N, &ax);
gmove(&ax, res);
} else {
// a % (-1) is 0.
nodconst(&n4, t, 0);
gmove(&n4, res);
}
p3 = gbranch(AJMP, T, 0);
p2 = gbranch(AJMP, T, 0);
patch(p1, pc);
patch(p2, pc);
}
savex(D_DX, &dx, &olddx, res, t);
if(!issigned[t->etype]) {
@ -547,7 +539,7 @@ dodiv(int op, Node *nl, Node *nr, Node *res)
gmove(&dx, res);
restx(&dx, &olddx);
if(check)
patch(p3, pc);
patch(p2, pc);
restx(&ax, &oldax);
}

21
src/cmd/8g/ggen.c
View File

@ -495,7 +495,7 @@ dodiv(int op, Node *nl, Node *nr, Node *res, Node *ax, Node *dx)
int check;
Node n1, t1, t2, t3, t4, n4, nz;
Type *t, *t0;
Prog *p1, *p2, *p3;
Prog *p1, *p2;
// Have to be careful about handling
// most negative int divided by -1 correctly.
@ -544,23 +544,22 @@ dodiv(int op, Node *nl, Node *nr, Node *res, Node *ax, Node *dx)
regalloc(&n1, t, N);
gmove(&t2, &n1);
gmove(&t1, ax);
p3 = P;
p2 = P;
if(check) {
nodconst(&n4, t, -1);
gins(optoas(OCMP, t), &n1, &n4);
p1 = gbranch(optoas(ONE, t), T, +1);
nodconst(&n4, t, -1LL<<(t->width*8-1));
gins(optoas(OCMP, t), ax, &n4);
p2 = gbranch(optoas(ONE, t), T, +1);
if(op == ODIV)
gmove(&n4, res);
if(op == OMOD) {
if(op == ODIV) {
// a / (-1) is -a.
gins(optoas(OMINUS, t), N, ax);
gmove(ax, res);
} else {
// a % (-1) is 0.
nodconst(&n4, t, 0);
gmove(&n4, res);
}
p3 = gbranch(AJMP, T, 0);
p2 = gbranch(AJMP, T, 0);
patch(p1, pc);
patch(p2, pc);
}
if(!issigned[t->etype]) {
nodconst(&nz, t, 0);
@ -575,7 +574,7 @@ dodiv(int op, Node *nl, Node *nr, Node *res, Node *ax, Node *dx)
else
gmove(dx, res);
if(check)
patch(p3, pc);
patch(p2, pc);
}
static void