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

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

diff --git a/src/cmd/6g/ggen.c b/src/cmd/6g/ggen.c
index db83d576992c04c370660bdb25b0491de18b8c00..d8ac6cef639cb09ce1f3962c997cb288c41f33ca 100644 (file)
--- a/src/cmd/6g/ggen.c
+++ b/src/cmd/6g/ggen.c
@@ -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);
 }
 

diff --git a/src/cmd/8g/ggen.c b/src/cmd/8g/ggen.c
index 39521b9a38591ea10a8f5ff573bf97f1aedb7fe6..d72c2259bd5f701262671ce9bddc67ecdd708f08 100644 (file)
--- a/src/cmd/8g/ggen.c
+++ b/src/cmd/8g/ggen.c
@@ -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