cmd/gc: faster code, mainly for rotate

* Eliminate bounds check on known small shifts.
* Rewrite x<<s | x>>(32-s) as a rotate (constant s).
* More aggressive (but still minimal) range analysis.

R=ken, dave, iant
CC=golang-dev
https://golang.org/cl/6209077

src/cmd/5g/cgen.c

@@ -156,6 +156,7 @@ cgen(Node *n, Node *res)
 		case OADD:
 		case OSUB:
 		case OMUL:
+		case OLROT:
 		case OLSH:
 		case ORSH:
 		case OAND:
@@ -241,9 +242,10 @@ cgen(Node *n, Node *res)
 		a = optoas(n->op, nl->type);
 		goto abop;
 
+	case OLROT:
 	case OLSH:
 	case ORSH:
-		cgen_shift(n->op, nl, nr, res);
+		cgen_shift(n->op, n->bounded, nl, nr, res);
 		break;
 
 	case OCONV:
@@ -620,7 +622,7 @@ agen(Node *n, Node *res)
 				fatal("constant string constant index");
 			v = mpgetfix(nr->val.u.xval);
 			if(isslice(nl->type) || nl->type->etype == TSTRING) {
-				if(!debug['B'] && !n->etype) {
+				if(!debug['B'] && !n->bounded) {
 					n1 = n3;
 					n1.op = OINDREG;
 					n1.type = types[tptr];
@@ -660,7 +662,7 @@ agen(Node *n, Node *res)
 		gmove(&n1, &n2);
 		regfree(&n1);
 
-		if(!debug['B'] && !n->etype) {
+		if(!debug['B'] && !n->bounded) {
 			// check bounds
 			regalloc(&n4, types[TUINT32], N);
 			if(isconst(nl, CTSTR)) {

src/cmd/5g/cgen64.c

@@ -94,6 +94,7 @@ cgen64(Node *n, Node *res)
 	case OAND:
 	case OOR:
 	case OXOR:
+	case OLROT:
 		// binary operators.
 		// common setup below.
 		break;
@@ -197,6 +198,47 @@ cgen64(Node *n, Node *res)
 
 		break;
 
+	case OLROT:
+		// We only rotate by a constant c in [0,64).
+		// if c >= 32:
+		//	lo, hi = hi, lo
+		//	c -= 32
+		// if c == 0:
+		//	no-op
+		// else:
+		//	t = hi
+		//	shld hi:lo, c
+		//	shld lo:t, c
+		v = mpgetfix(r->val.u.xval);
+		regalloc(&bl, lo1.type, N);
+		regalloc(&bh, hi1.type, N);
+		if(v >= 32) {
+			// reverse during load to do the first 32 bits of rotate
+			v -= 32;
+			gins(AMOVW, &hi1, &bl);
+			gins(AMOVW, &lo1, &bh);
+		} else {
+			gins(AMOVW, &hi1, &bh);
+			gins(AMOVW, &lo1, &bl);
+		}
+		if(v == 0) {
+			gins(AMOVW, &bh, &ah);
+			gins(AMOVW, &bl, &al);
+		} else {
+			// rotate by 1 <= v <= 31
+			//	MOVW	bl<<v, al
+			//	MOVW	bh<<v, ah
+			//	OR		bl>>(32-v), ah
+			//	OR		bh>>(32-v), al
+			gshift(AMOVW, &bl, SHIFT_LL, v, &al);
+			gshift(AMOVW, &bh, SHIFT_LL, v, &ah);
+			gshift(AORR, &bl, SHIFT_LR, 32-v, &ah);
+			gshift(AORR, &bh, SHIFT_LR, 32-v, &al);
+		}
+		regfree(&bl);
+		regfree(&bh);
+		break;
+
 	case OLSH:
 		regalloc(&bl, lo1.type, N);
 		regalloc(&bh, hi1.type, N);

src/cmd/5g/gg.h

@@ -104,7 +104,7 @@ Prog*	gshift(int as, Node *lhs, int32 stype, int32 sval, Node *rhs);
 Prog *	gregshift(int as, Node *lhs, int32 stype, Node *reg, Node *rhs);
 void	naddr(Node*, Addr*, int);
 void	cgen_aret(Node*, Node*);
-void	cgen_shift(int, Node*, Node*, Node*);
+void	cgen_shift(int, int, Node*, Node*, Node*);
 
 /*
  * cgen64.c

src/cmd/5g/ggen.c

@@ -469,10 +469,10 @@ samereg(Node *a, Node *b)
  *	res = nl >> nr
  */
 void
-cgen_shift(int op, Node *nl, Node *nr, Node *res)
+cgen_shift(int op, int bounded, Node *nl, Node *nr, Node *res)
 {
 	Node n1, n2, n3, nt, t, lo, hi;
-	int w;
+	int w, v;
 	Prog *p1, *p2, *p3;
 	Type *tr;
 	uvlong sc;
@@ -482,6 +482,24 @@ cgen_shift(int op, Node *nl, Node *nr, Node *res)
 
 	w = nl->type->width * 8;
 
+	if(op == OLROT) {
+		v = mpgetfix(nr->val.u.xval);
+		regalloc(&n1, nl->type, res);
+		if(w == 32) {
+			cgen(nl, &n1);
+			gshift(AMOVW, &n1, SHIFT_RR, w-v, &n1);
+		} else {
+			regalloc(&n2, nl->type, N);
+			cgen(nl, &n2);
+			gshift(AMOVW, &n2, SHIFT_LL, v, &n1);
+			gshift(AORR, &n2, SHIFT_LR, w-v, &n1);
+			regfree(&n2);
+		}
+		gmove(&n1, res);
+		regfree(&n1);
+		return;
+	}
+
 	if(nr->op == OLITERAL) {
 		regalloc(&n1, nl->type, res);
 		cgen(nl, &n1);
@@ -549,6 +567,7 @@ cgen_shift(int op, Node *nl, Node *nr, Node *res)
 	p3 = gbranch(ABEQ, T);
 
 	// test and fix up large shifts
+	// TODO: if(!bounded), don't emit some of this.
 	regalloc(&n3, tr, N);
 	nodconst(&t, types[TUINT32], w);
 	gmove(&t, &n3);

src/cmd/5g/gsubr.c

@@ -2032,7 +2032,7 @@ oindex_const:
 	v = mpgetfix(r->val.u.xval);
 	if(o & ODynam) {
 
-		if(!debug['B'] && !n->etype) {
+		if(!debug['B'] && !n->bounded) {
 			n1 = *reg;
 			n1.op = OINDREG;
 			n1.type = types[tptr];

src/cmd/6g/cgen.c

@@ -240,6 +240,25 @@ cgen(Node *n, Node *res)
 		goto abop;
 
 	case OCONV:
+		if(n->type->width > nl->type->width) {
+			// If loading from memory, do conversion during load,
+			// so as to avoid use of 8-bit register in, say, int(*byteptr).
+			switch(nl->op) {
+			case ODOT:
+			case ODOTPTR:
+			case OINDEX:
+			case OIND:
+			case ONAME:
+				igen(nl, &n1, res);
+				regalloc(&n2, n->type, res);
+				gmove(&n1, &n2);
+				gmove(&n2, res);
+				regfree(&n2);
+				regfree(&n1);
+				goto ret;
+			}
+		}
+
 		regalloc(&n1, nl->type, res);
 		regalloc(&n2, n->type, &n1);
 		cgen(nl, &n1);
@@ -370,13 +389,14 @@ cgen(Node *n, Node *res)
 
 	case OLSH:
 	case ORSH:
-		cgen_shift(n->op, nl, nr, res);
+	case OLROT:
+		cgen_shift(n->op, n->bounded, nl, nr, res);
 		break;
 	}
 	goto ret;
 
 sbop:	// symmetric binary
-	if(nl->ullman < nr->ullman) {
+	if(nl->ullman < nr->ullman || nl->op == OLITERAL) {
 		r = nl;
 		nl = nr;
 		nr = r;
@@ -386,7 +406,13 @@ abop:	// asymmetric binary
 	if(nl->ullman >= nr->ullman) {
 		regalloc(&n1, nl->type, res);
 		cgen(nl, &n1);
-
+	/*
+	 * This generates smaller code - it avoids a MOV - but it's
+	 * easily 10% slower due to not being able to
+	 * optimize/manipulate the move.
+	 * To see, run: go test -bench . crypto/md5
+	 * with and without.
+	 *
 		if(sudoaddable(a, nr, &addr)) {
 			p1 = gins(a, N, &n1);
 			p1->from = addr;
@@ -395,18 +421,30 @@ abop:	// asymmetric binary
 			regfree(&n1);
 			goto ret;
 		}
-		regalloc(&n2, nr->type, N);
+	 *
-		cgen(nr, &n2);
+	 */
+
+		if(smallintconst(nr))
+			n2 = *nr;
+		else {
+			regalloc(&n2, nr->type, N);
+			cgen(nr, &n2);
+		}
 	} else {
-		regalloc(&n2, nr->type, res);
+		if(smallintconst(nr))
-		cgen(nr, &n2);
+			n2 = *nr;
+		else {
+			regalloc(&n2, nr->type, res);
+			cgen(nr, &n2);
+		}
 		regalloc(&n1, nl->type, N);
 		cgen(nl, &n1);
 	}
 	gins(a, &n2, &n1);
 	gmove(&n1, res);
 	regfree(&n1);
-	regfree(&n2);
+	if(n2.op != OLITERAL)
+		regfree(&n2);
 	goto ret;
 
 uop:	// unary
@@ -529,7 +567,7 @@ agen(Node *n, Node *res)
 				fatal("constant string constant index");	// front end should handle
 			v = mpgetfix(nr->val.u.xval);
 			if(isslice(nl->type) || nl->type->etype == TSTRING) {
-				if(!debug['B'] && !n->etype) {
+				if(!debug['B'] && !n->bounded) {
 					n1 = n3;
 					n1.op = OINDREG;
 					n1.type = types[tptr];
@@ -564,7 +602,7 @@ agen(Node *n, Node *res)
 		gmove(&n1, &n2);
 		regfree(&n1);
 
-		if(!debug['B'] && !n->etype) {
+		if(!debug['B'] && !n->bounded) {
 			// check bounds
 			n5.op = OXXX;
 			t = types[TUINT32];
@@ -692,6 +730,7 @@ igen(Node *n, Node *a, Node *res)
 {
 	Type *fp;
 	Iter flist;
+	Node n1, n2;
  
 	switch(n->op) {
 	case ONAME:
@@ -710,8 +749,29 @@ igen(Node *n, Node *a, Node *res)
 		a->xoffset = fp->width;
 		a->type = n->type;
 		return;
+	
+	case OINDEX:
+		// Index of fixed-size array by constant can
+		// put the offset in the addressing.
+		// Could do the same for slice except that we need
+		// to use the real index for the bounds checking.
+		if(isfixedarray(n->left->type) ||
+		   (isptr[n->left->type->etype] && isfixedarray(n->left->left->type)))
+		if(isconst(n->right, CTINT)) {
+			nodconst(&n1, types[TINT64], 0);
+			n2 = *n;
+			n2.right = &n1;
+
+			regalloc(a, types[tptr], res);
+			agen(&n2, a);
+			a->op = OINDREG;
+			a->xoffset = mpgetfix(n->right->val.u.xval)*n->type->width;
+			a->type = n->type;
+			return;
+		}
+			
 	}
- 
+
 	regalloc(a, types[tptr], res);
 	agen(n, a);
 	a->op = OINDREG;

src/cmd/6g/gg.h

@@ -71,7 +71,7 @@ void	cgen_proc(Node*, int);
 void	cgen_callret(Node*, Node*);
 void	cgen_div(int, Node*, Node*, Node*);
 void	cgen_bmul(int, Node*, Node*, Node*);
-void	cgen_shift(int, Node*, Node*, Node*);
+void	cgen_shift(int, int, Node*, Node*, Node*);
 void	cgen_dcl(Node*);
 int	needconvert(Type*, Type*);
 void	genconv(Type*, Type*);

src/cmd/6g/ggen.c

@@ -865,7 +865,7 @@ longmod:
  *	res = nl >> nr
  */
 void
-cgen_shift(int op, Node *nl, Node *nr, Node *res)
+cgen_shift(int op, int bounded, Node *nl, Node *nr, Node *res)
 {
 	Node n1, n2, n3, n4, n5, cx, oldcx;
 	int a, rcx;
@@ -880,7 +880,7 @@ cgen_shift(int op, Node *nl, Node *nr, Node *res)
 		cgen(nl, &n1);
 		sc = mpgetfix(nr->val.u.xval);
 		if(sc >= nl->type->width*8) {
-			// large shift gets 2 shifts by width
+			// large shift gets 2 shifts by width-1
 			nodconst(&n3, types[TUINT32], nl->type->width*8-1);
 			gins(a, &n3, &n1);
 			gins(a, &n3, &n1);
@@ -939,17 +939,20 @@ cgen_shift(int op, Node *nl, Node *nr, Node *res)
 	regfree(&n3);
 
 	// test and fix up large shifts
-	nodconst(&n3, tcount, nl->type->width*8);
+	if(!bounded) {
-	gins(optoas(OCMP, tcount), &n1, &n3);
+		nodconst(&n3, tcount, nl->type->width*8);
-	p1 = gbranch(optoas(OLT, tcount), T);
+		gins(optoas(OCMP, tcount), &n1, &n3);
-	if(op == ORSH && issigned[nl->type->etype]) {
+		p1 = gbranch(optoas(OLT, tcount), T);
-		nodconst(&n3, types[TUINT32], nl->type->width*8-1);
+		if(op == ORSH && issigned[nl->type->etype]) {
-		gins(a, &n3, &n2);
+			nodconst(&n3, types[TUINT32], nl->type->width*8-1);
-	} else {
+			gins(a, &n3, &n2);
-		nodconst(&n3, nl->type, 0);
+		} else {
-		gmove(&n3, &n2);
+			nodconst(&n3, nl->type, 0);
+			gmove(&n3, &n2);
+		}
+		patch(p1, pc);
 	}
-	patch(p1, pc);
+
 	gins(a, &n1, &n2);
 
 	if(oldcx.op != 0) {

src/cmd/6g/gsubr.c

@@ -616,7 +616,7 @@ gmove(Node *f, Node *t)
 	Prog *p1, *p2;
 
 	if(debug['M'])
-		print("gmove %N -> %N\n", f, t);
+		print("gmove %lN -> %lN\n", f, t);
 
 	ft = simsimtype(f->type);
 	tt = simsimtype(t->type);
@@ -1050,7 +1050,9 @@ gins(int as, Node *f, Node *t)
 		w = 8;
 		break;
 	}
-	if(w != 0 && f != N && (af.width > w || at.width > w)) {
+	if(w != 0 && ((f != N && af.width < w) || (t != N && at.width > w))) {
+		dump("f", f);
+		dump("t", t);
 		fatal("bad width: %P (%d, %d)\n", p, af.width, at.width);
 	}
 
@@ -1087,9 +1089,15 @@ naddr(Node *n, Addr *a, int canemitcode)
 	a->type = D_NONE;
 	a->gotype = S;
 	a->node = N;
+	a->width = 0;
 	if(n == N)
 		return;
 
+	if(n->type != T && n->type->etype != TIDEAL) {
+		dowidth(n->type);
+		a->width = n->type->width;
+	}
+
 	switch(n->op) {
 	default:
 		fatal("naddr: bad %O %D", n->op, a);
@@ -1140,10 +1148,8 @@ naddr(Node *n, Addr *a, int canemitcode)
 
 	case ONAME:
 		a->etype = 0;
-		a->width = 0;
 		if(n->type != T) {
 			a->etype = simtype[n->type->etype];
-			a->width = n->type->width;
 			a->gotype = ngotype(n);
 		}
 		a->offset = n->xoffset;
@@ -1176,6 +1182,7 @@ naddr(Node *n, Addr *a, int canemitcode)
 		case PFUNC:
 			a->index = D_EXTERN;
 			a->type = D_ADDR;
+			a->width = widthptr;
 			break;
 		}
 		break;
@@ -1213,6 +1220,7 @@ naddr(Node *n, Addr *a, int canemitcode)
 
 	case OADDR:
 		naddr(n->left, a, canemitcode);
+		a->width = widthptr;
 		if(a->type >= D_INDIR) {
 			a->type -= D_INDIR;
 			break;
@@ -1648,6 +1656,28 @@ optoas(int op, Type *t)
 		a = AXORQ;
 		break;
 
+	case CASE(OLROT, TINT8):
+	case CASE(OLROT, TUINT8):
+		a = AROLB;
+		break;
+
+	case CASE(OLROT, TINT16):
+	case CASE(OLROT, TUINT16):
+		a = AROLW;
+		break;
+
+	case CASE(OLROT, TINT32):
+	case CASE(OLROT, TUINT32):
+	case CASE(OLROT, TPTR32):
+		a = AROLL;
+		break;
+
+	case CASE(OLROT, TINT64):
+	case CASE(OLROT, TUINT64):
+	case CASE(OLROT, TPTR64):
+		a = AROLQ;
+		break;
+
 	case CASE(OLSH, TINT8):
 	case CASE(OLSH, TUINT8):
 		a = ASHLB;
@@ -2056,7 +2086,7 @@ oindex:
 	}
 
 	// check bounds
-	if(!debug['B'] && !n->etype) {
+	if(!debug['B'] && !n->bounded) {
 		// check bounds
 		n4.op = OXXX;
 		t = types[TUINT32];
@@ -2143,11 +2173,11 @@ oindex_const:
 	reg->op = OEMPTY;
 	reg1->op = OEMPTY;
 
-	regalloc(reg, types[tptr], N);
-	agen(l, reg);
-
 	if(o & ODynam) {
-		if(!debug['B'] && !n->etype) {
+		regalloc(reg, types[tptr], N);
+		agen(l, reg);
+	
+		if(!debug['B'] && !n->bounded) {
 			n1 = *reg;
 			n1.op = OINDREG;
 			n1.type = types[tptr];
@@ -2165,14 +2195,24 @@ oindex_const:
 		n1.xoffset = Array_array;
 		gmove(&n1, reg);
 
+		n2 = *reg;
+		n2.op = OINDREG;
+		n2.xoffset = v*w;
+		a->type = D_NONE;
+		a->index = D_NONE;
+		naddr(&n2, a, 1);
+		goto yes;
 	}
-
+	
-	n2 = *reg;
+	igen(l, &n1, N);
-	n2.op = OINDREG;
+	if(n1.op == OINDREG) {
-	n2.xoffset = v*w;
+		*reg = n1;
+		reg->op = OREGISTER;
+	}
+	n1.xoffset += v*w;
 	a->type = D_NONE;
-	a->index = D_NONE;
+	a->index= D_NONE;
-	naddr(&n2, a, 1);
+	naddr(&n1, a, 1);
 	goto yes;
 
 oindex_const_sudo:
@@ -2183,7 +2223,7 @@ oindex_const_sudo:
 	}
 
 	// slice indexed by a constant
-	if(!debug['B'] && !n->etype) {
+	if(!debug['B'] && !n->bounded) {
 		a->offset += Array_nel;
 		nodconst(&n2, types[TUINT64], v);
 		p1 = gins(optoas(OCMP, types[TUINT32]), N, &n2);

src/cmd/8g/cgen.c

@@ -160,6 +160,7 @@ cgen(Node *n, Node *res)
 		case OADD:
 		case OSUB:
 		case OMUL:
+		case OLROT:
 		case OLSH:
 		case ORSH:
 		case OAND:
@@ -360,20 +361,27 @@ cgen(Node *n, Node *res)
 
 	case OLSH:
 	case ORSH:
-		cgen_shift(n->op, nl, nr, res);
+	case OLROT:
+		cgen_shift(n->op, n->bounded, nl, nr, res);
 		break;
 	}
 	return;
 
 sbop:	// symmetric binary
-	if(nl->ullman < nr->ullman) {
+	if(nl->ullman < nr->ullman || nl->op == OLITERAL) {
 		r = nl;
 		nl = nr;
 		nr = r;
 	}
 
 abop:	// asymmetric binary
-	if(nl->ullman >= nr->ullman) {
+	if(smallintconst(nr)) {
+		regalloc(&n1, nr->type, res);
+		cgen(nl, &n1);
+		gins(a, nr, &n1);
+		gmove(&n1, res);
+		regfree(&n1);
+	} else if(nl->ullman >= nr->ullman) {
 		tempname(&nt, nl->type);
 		cgen(nl, &nt);
 		mgen(nr, &n2, N);
@@ -580,7 +588,7 @@ agen(Node *n, Node *res)
 				fatal("constant string constant index");
 			v = mpgetfix(nr->val.u.xval);
 			if(isslice(nl->type) || nl->type->etype == TSTRING) {
-				if(!debug['B'] && !n->etype) {
+				if(!debug['B'] && !n->bounded) {
 					n1 = n3;
 					n1.op = OINDREG;
 					n1.type = types[tptr];
@@ -612,7 +620,7 @@ agen(Node *n, Node *res)
 		gmove(&n1, &n2);
 		regfree(&n1);
 
-		if(!debug['B'] && !n->etype) {
+		if(!debug['B'] && !n->bounded) {
 			// check bounds
 			if(isconst(nl, CTSTR))
 				nodconst(&n1, types[TUINT32], nl->val.u.sval->len);

src/cmd/8g/cgen64.c

@@ -49,6 +49,7 @@ cgen64(Node *n, Node *res)
 	case OADD:
 	case OSUB:
 	case OMUL:
+	case OLROT:
 	case OLSH:
 	case ORSH:
 	case OAND:
@@ -131,6 +132,40 @@ cgen64(Node *n, Node *res)
 		regfree(&ex);
 		regfree(&fx);
 		break;
+	
+	case OLROT:
+		// We only rotate by a constant c in [0,64).
+		// if c >= 32:
+		//	lo, hi = hi, lo
+		//	c -= 32
+		// if c == 0:
+		//	no-op
+		// else:
+		//	t = hi
+		//	shld hi:lo, c
+		//	shld lo:t, c
+		v = mpgetfix(r->val.u.xval);
+		if(v >= 32) {
+			// reverse during load to do the first 32 bits of rotate
+			v -= 32;
+			gins(AMOVL, &lo1, &dx);
+			gins(AMOVL, &hi1, &ax);
+		} else {
+			gins(AMOVL, &lo1, &ax);
+			gins(AMOVL, &hi1, &dx);
+		}
+		if(v == 0) {
+			// done
+		} else {
+			gins(AMOVL, &dx, &cx);
+			p1 = gins(ASHLL, ncon(v), &dx);
+			p1->from.index = D_AX;	// double-width shift
+			p1->from.scale = 0;
+			p1 = gins(ASHLL, ncon(v), &ax);
+			p1->from.index = D_CX;	// double-width shift
+			p1->from.scale = 0;
+		}
+		break;
 
 	case OLSH:
 		if(r->op == OLITERAL) {

src/cmd/8g/gg.h

@@ -83,7 +83,7 @@ void	cgen_proc(Node*, int);
 void	cgen_callret(Node*, Node*);
 void	cgen_div(int, Node*, Node*, Node*);
 void	cgen_bmul(int, Node*, Node*, Node*);
-void	cgen_shift(int, Node*, Node*, Node*);
+void	cgen_shift(int, int, Node*, Node*, Node*);
 void	cgen_dcl(Node*);
 int	needconvert(Type*, Type*);
 void	genconv(Type*, Type*);

src/cmd/8g/ggen.c

@@ -630,7 +630,7 @@ cgen_div(int op, Node *nl, Node *nr, Node *res)
  *	res = nl >> nr
  */
 void
-cgen_shift(int op, Node *nl, Node *nr, Node *res)
+cgen_shift(int op, int bounded, Node *nl, Node *nr, Node *res)
 {
 	Node n1, n2, nt, cx, oldcx, hi, lo;
 	int a, w;
@@ -651,7 +651,7 @@ cgen_shift(int op, Node *nl, Node *nr, Node *res)
 		gmove(&n2, &n1);
 		sc = mpgetfix(nr->val.u.xval);
 		if(sc >= nl->type->width*8) {
-			// large shift gets 2 shifts by width
+			// large shift gets 2 shifts by width-1
 			gins(a, ncon(w-1), &n1);
 			gins(a, ncon(w-1), &n1);
 		} else
@@ -689,27 +689,37 @@ cgen_shift(int op, Node *nl, Node *nr, Node *res)
 	}
 
 	// test and fix up large shifts
-	if(nr->type->width > 4) {
+	if(bounded) {
-		// delayed reg alloc
+		if(nr->type->width > 4) {
-		nodreg(&n1, types[TUINT32], D_CX);
+			// delayed reg alloc
-		regalloc(&n1, types[TUINT32], &n1);		// to hold the shift type in CX
+			nodreg(&n1, types[TUINT32], D_CX);
-		split64(&nt, &lo, &hi);
+			regalloc(&n1, types[TUINT32], &n1);		// to hold the shift type in CX
-		gmove(&lo, &n1);
+			split64(&nt, &lo, &hi);
-		gins(optoas(OCMP, types[TUINT32]), &hi, ncon(0));
+			gmove(&lo, &n1);
-		p2 = gbranch(optoas(ONE, types[TUINT32]), T);
+		}
-		gins(optoas(OCMP, types[TUINT32]), &n1, ncon(w));
-		p1 = gbranch(optoas(OLT, types[TUINT32]), T);
-		patch(p2, pc);
 	} else {
-		gins(optoas(OCMP, nr->type), &n1, ncon(w));
+		if(nr->type->width > 4) {
-		p1 = gbranch(optoas(OLT, types[TUINT32]), T);
+			// delayed reg alloc
+			nodreg(&n1, types[TUINT32], D_CX);
+			regalloc(&n1, types[TUINT32], &n1);		// to hold the shift type in CX
+			split64(&nt, &lo, &hi);
+			gmove(&lo, &n1);
+			gins(optoas(OCMP, types[TUINT32]), &hi, ncon(0));
+			p2 = gbranch(optoas(ONE, types[TUINT32]), T);
+			gins(optoas(OCMP, types[TUINT32]), &n1, ncon(w));
+			p1 = gbranch(optoas(OLT, types[TUINT32]), T);
+			patch(p2, pc);
+		} else {
+			gins(optoas(OCMP, nr->type), &n1, ncon(w));
+			p1 = gbranch(optoas(OLT, types[TUINT32]), T);
+		}
+		if(op == ORSH && issigned[nl->type->etype]) {
+			gins(a, ncon(w-1), &n2);
+		} else {
+			gmove(ncon(0), &n2);
+		}
+		patch(p1, pc);
 	}
-	if(op == ORSH && issigned[nl->type->etype]) {
-		gins(a, ncon(w-1), &n2);
-	} else {
-		gmove(ncon(0), &n2);
-	}
-	patch(p1, pc);
 	gins(a, &n1, &n2);
 
 	if(oldcx.op != 0)

src/cmd/8g/gsubr.c

@@ -572,6 +572,22 @@ optoas(int op, Type *t)
 		a = AXORL;
 		break;
 
+	case CASE(OLROT, TINT8):
+	case CASE(OLROT, TUINT8):
+		a = AROLB;
+		break;
+
+	case CASE(OLROT, TINT16):
+	case CASE(OLROT, TUINT16):
+		a = AROLW;
+		break;
+
+	case CASE(OLROT, TINT32):
+	case CASE(OLROT, TUINT32):
+	case CASE(OLROT, TPTR32):
+		a = AROLL;
+		break;
+
 	case CASE(OLSH, TINT8):
 	case CASE(OLSH, TUINT8):
 		a = ASHLB;

src/cmd/gc/go.h

@@ -234,6 +234,7 @@ struct	Node
 	uchar	addable;	// type of addressability - 0 is not addressable
 	uchar	trecur;		// to detect loops
 	uchar	etype;		// op for OASOP, etype for OTYPE, exclam for export
+	uchar	bounded;	// bounds check unnecessary
 	uchar	class;		// PPARAM, PAUTO, PEXTERN, etc
 	uchar	method;		// OCALLMETH name
 	uchar	embedded;	// ODCLFIELD embedded type
@@ -488,6 +489,7 @@ enum
 
 	// for back ends
 	OCMP, ODEC, OEXTEND, OINC, OREGISTER, OINDREG,
+	OLROT,
 
 	OEND,
 };

src/cmd/gc/range.c

@@ -145,7 +145,7 @@ walkrange(Node *n)
 		if(v2) {
 			hp = temp(ptrto(n->type->type));
 			tmp = nod(OINDEX, ha, nodintconst(0));
-			tmp->etype = 1;	// no bounds check
+			tmp->bounded = 1;
 			init = list(init, nod(OAS, hp, nod(OADDR, tmp, N)));
 		}
 

src/cmd/gc/sinit.c

@@ -747,7 +747,7 @@ slicelit(int ctxt, Node *n, Node *var, NodeList **init)
 		index = r->left;
 		value = r->right;
 		a = nod(OINDEX, var, index);
-		a->etype = 1;	// no bounds checking
+		a->bounded = 1;
 		// TODO need to check bounds?
 
 		switch(value->op) {
@@ -879,11 +879,11 @@ ctxt = 0;
 		index = temp(types[TINT]);
 
 		a = nod(OINDEX, vstat, index);
-		a->etype = 1;	// no bounds checking
+		a->bounded = 1;
 		a = nod(ODOT, a, newname(symb));
 
 		r = nod(OINDEX, vstat, index);
-		r->etype = 1;	// no bounds checking
+		r->bounded = 1;
 		r = nod(ODOT, r, newname(syma));
 		r = nod(OINDEX, var, r);
 

src/cmd/gc/subr.c

@@ -2663,7 +2663,7 @@ genhash(Sym *sym, Type *t)
 		call->list = list(call->list, nh);
 		call->list = list(call->list, nodintconst(t->type->width));
 		nx = nod(OINDEX, np, ni);
-		nx->etype = 1;  // no bounds check
+		nx->bounded = 1;
 		na = nod(OADDR, nx, N);
 		na->etype = 1;  // no escape to heap
 		call->list = list(call->list, na);
@@ -2874,9 +2874,9 @@ geneq(Sym *sym, Type *t)
 		
 		// if p[i] != q[i] { *eq = false; return }
 		nx = nod(OINDEX, np, ni);
-		nx->etype = 1;  // no bounds check
+		nx->bounded = 1;
 		ny = nod(OINDEX, nq, ni);
-		ny->etype = 1;  // no bounds check
+		ny->bounded = 1;
 
 		nif = nod(OIF, N, N);
 		nif->ntest = nod(ONE, nx, ny);

src/cmd/gc/walk.c

@@ -22,6 +22,9 @@ static	Node*	addstr(Node*, NodeList**);
 static	Node*	appendslice(Node*, NodeList**);
 static	Node*	append(Node*, NodeList**);
 static	void	walkcompare(Node**, NodeList**);
+static	void	walkrotate(Node**);
+static	int	bounded(Node*, int64);
+static	Mpint	mpzero;
 
 // can this code branch reach the end
 // without an unconditional RETURN
@@ -454,8 +457,16 @@ walkexpr(Node **np, NodeList **init)
 
 	case OLSH:
 	case ORSH:
+		walkexpr(&n->left, init);
+		walkexpr(&n->right, init);
+	shiftwalked:
+		t = n->left->type;
+		n->bounded = bounded(n->right, 8*t->width);
+		if(debug['m'] && n->etype && !isconst(n->right, CTINT))
+			warn("shift bounds check elided");
+		goto ret;
+
 	case OAND:
-	case OOR:
 	case OXOR:
 	case OSUB:
 	case OMUL:
@@ -465,10 +476,17 @@ walkexpr(Node **np, NodeList **init)
 	case OGT:
 	case OADD:
 	case OCOMPLEX:
+	case OLROT:
 		walkexpr(&n->left, init);
 		walkexpr(&n->right, init);
 		goto ret;
 
+	case OOR:
+		walkexpr(&n->left, init);
+		walkexpr(&n->right, init);
+		walkrotate(&n);
+		goto ret;
+
 	case OEQ:
 	case ONE:
 		walkexpr(&n->left, init);
@@ -794,7 +812,10 @@ walkexpr(Node **np, NodeList **init)
 			typecheck(&r, Etop);
 			walkexpr(&r, init);
 			n = r;
+			goto ret;
 		}
+		if(n->etype == OLSH || n->etype == ORSH)
+			goto shiftwalked;
 		goto ret;
 
 	case OANDNOT:
@@ -844,40 +865,40 @@ walkexpr(Node **np, NodeList **init)
 		walkexpr(&n->right, init);
 
 		// if range of type cannot exceed static array bound,
-		// disable bounds check
+		// disable bounds check.
-		if(isfixedarray(n->left->type))
+		if(n->bounded)
-		if(!issigned[n->right->type->etype])
+			goto ret;
-		if(n->right->type->width < 4)
+		t = n->left->type;
-		if((1<<(8*n->right->type->width)) <= n->left->type->bound)
+		if(t != T && isptr[t->etype])
-			n->etype = 1;
+			t = t->type;
-
+		if(isfixedarray(t)) {
-		if(isconst(n->left, CTSTR))
+			n->bounded = bounded(n->right, t->bound);
-		if(!issigned[n->right->type->etype])
+			if(debug['m'] && n->bounded && !isconst(n->right, CTINT))
-		if(n->right->type->width < 4)
+				warn("index bounds check elided");
-		if((1<<(8*n->right->type->width)) <= n->left->val.u.sval->len)
+			if(smallintconst(n->right) && !n->bounded)
-			n->etype = 1;
-
-		// check for static out of bounds
-		if(isconst(n->right, CTINT) && !n->etype) {
-			v = mpgetfix(n->right->val.u.xval);
-			len = 1LL<<60;
-			t = n->left->type;
-			if(isconst(n->left, CTSTR))
-				len = n->left->val.u.sval->len;
-			if(t != T && isptr[t->etype])
-				t = t->type;
-			if(isfixedarray(t))
-				len = t->bound;
-			if(v < 0 || v >= (1LL<<31) || v >= len)
 				yyerror("index out of bounds");
-			else if(isconst(n->left, CTSTR)) {
+		} else if(isconst(n->left, CTSTR)) {
-				// replace "abc"[2] with 'b'.
+			n->bounded = bounded(n->right, n->left->val.u.sval->len);
-				// delayed until now because "abc"[2] is not
+			if(debug['m'] && n->bounded && !isconst(n->right, CTINT))
-				// an ideal constant.
+				warn("index bounds check elided");
-				nodconst(n, n->type, n->left->val.u.sval->s[v]);
+			if(smallintconst(n->right)) {
-				n->typecheck = 1;
+				if(!n->bounded)
+					yyerror("index out of bounds");
+				else {
+					// replace "abc"[2] with 'b'.
+					// delayed until now because "abc"[2] is not
+					// an ideal constant.
+					v = mpgetfix(n->right->val.u.xval);
+					nodconst(n, n->type, n->left->val.u.sval->s[v]);
+					n->typecheck = 1;
+				}
 			}
 		}
+
+		if(isconst(n->right, CTINT))
+		if(mpcmpfixfix(n->right->val.u.xval, &mpzero) < 0 ||
+		   mpcmpfixfix(n->right->val.u.xval, maxintval[TINT]) > 0)
+			yyerror("index out of bounds");
 		goto ret;
 
 	case OINDEXMAP:
@@ -938,7 +959,7 @@ walkexpr(Node **np, NodeList **init)
 		// sliceslice(old []any, lb uint64, hb uint64, width uint64) (ary []any)
 		// sliceslice1(old []any, lb uint64, width uint64) (ary []any)
 		t = n->type;
-		et = n->etype;
+		et = n->bounded;
 		if(n->right->left == N)
 			l = nodintconst(0);
 		else
@@ -961,7 +982,7 @@ walkexpr(Node **np, NodeList **init)
 				l,
 				nodintconst(t->type->width));
 		}
-		n->etype = et;	// preserve no-typecheck flag from OSLICE to the slice* call.
+		n->bounded = et;	// preserve flag from OSLICE to the slice* call.
 		goto ret;
 
 	slicearray:
@@ -2395,12 +2416,12 @@ append(Node *n, NodeList **init)
 	l = list(l, nod(OAS, nn, nod(OLEN, ns, N)));	 // n = len(s)
 
 	nx = nod(OSLICE, ns, nod(OKEY, N, nod(OADD, nn, na)));	 // ...s[:n+argc]
-	nx->etype = 1;	// disable bounds check
+	nx->bounded = 1;
 	l = list(l, nod(OAS, ns, nx));			// s = s[:n+argc]
 
 	for (a = n->list->next;	 a != nil; a = a->next) {
 		nx = nod(OINDEX, ns, nn);		// s[n] ...
-		nx->etype = 1;	// disable bounds check
+		nx->bounded = 1;
 		l = list(l, nod(OAS, nx, a->n));	// s[n] = arg
 		if (a->next != nil)
 			l = list(l, nod(OAS, nn, nod(OADD, nn, nodintconst(1))));  // n = n + 1
@@ -2596,3 +2617,135 @@ hard:
 	*np = r;
 	return;
 }
+
+static int
+samecheap(Node *a, Node *b)
+{
+	if(a == N || b == N || a->op != b->op)
+		return 0;
+	
+	switch(a->op) {
+	case ONAME:
+		return a == b;
+	// TODO: Could do more here, but maybe this is enough.
+	// It's all cheapexpr does.
+	}
+	return 0;
+}
+
+static void
+walkrotate(Node **np)
+{
+	int w, sl, sr, s;
+	Node *l, *r;
+	Node *n;
+	
+	n = *np;
+
+	// Want << | >> or >> | << on unsigned value.
+	l = n->left;
+	r = n->right;
+	if(n->op != OOR ||
+	   (l->op != OLSH && l->op != ORSH) ||
+	   (r->op != OLSH && r->op != ORSH) ||
+	   n->type == T || issigned[n->type->etype] ||
+	   l->op == r->op) {
+		return;
+	}
+
+	// Want same, side effect-free expression on lhs of both shifts.
+	if(!samecheap(l->left, r->left))
+		return;
+	
+	// Constants adding to width?
+	w = l->type->width * 8;
+	if(smallintconst(l->right) && smallintconst(r->right)) {
+		if((sl=mpgetfix(l->right->val.u.xval)) >= 0 && (sr=mpgetfix(r->right->val.u.xval)) >= 0 && sl+sr == w)
+			goto yes;
+		return;
+	}
+	
+	// TODO: Could allow s and 32-s if s is bounded (maybe s&31 and 32-s&31).
+	return;
+	
+yes:
+	// Rewrite left shift half to left rotate.
+	if(l->op == OLSH)
+		n = l;
+	else
+		n = r;
+	n->op = OLROT;
+	
+	// Remove rotate 0 and rotate w.
+	s = mpgetfix(n->right->val.u.xval);
+	if(s == 0 || s == w)
+		n = n->left;
+
+	*np = n;
+	return;
+}
+
+// return 1 if integer n must be in range [0, max), 0 otherwise
+static int
+bounded(Node *n, int64 max)
+{
+	int64 v;
+	int32 bits;
+	int sign;
+
+	if(n->type == T || !isint[n->type->etype])
+		return 0;
+
+	sign = issigned[n->type->etype];
+	bits = 8*n->type->width;
+
+	if(smallintconst(n)) {
+		v = mpgetfix(n->val.u.xval);
+		return 0 <= v && v < max;
+	}
+
+	switch(n->op) {
+	case OAND:
+		v = -1;
+		if(smallintconst(n->left)) {
+			v = mpgetfix(n->left->val.u.xval);
+		} else if(smallintconst(n->right)) {
+			v = mpgetfix(n->right->val.u.xval);
+		}
+		if(0 <= v && v < max)
+			return 1;
+		break;
+
+	case OMOD:
+		if(!sign && smallintconst(n->right)) {
+			v = mpgetfix(n->right->val.u.xval);
+			if(0 <= v && v <= max)
+				return 1;
+		}
+		break;
+	
+	case ODIV:
+		if(!sign && smallintconst(n->right)) {
+			v = mpgetfix(n->right->val.u.xval);
+			while(bits > 0 && v >= 2) {
+				bits--;
+				v >>= 1;
+			}
+		}
+		break;
+	
+	case ORSH:
+		if(!sign && smallintconst(n->right)) {
+			v = mpgetfix(n->right->val.u.xval);
+			if(v > bits)
+				return 1;
+			bits -= v;
+		}
+		break;
+	}
+	
+	if(!sign && bits <= 62 && (1LL<<bits) <= max)
+		return 1;
+	
+	return 0;
+}

test/bounds.go

@@ -0,0 +1,277 @@
+// errchk -0 $G -m -l $D/$F.go
+
+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test, using compiler diagnostic flags, that bounds check elimination
+// is eliminating the correct checks.
+
+package foo
+
+var (
+	s []int
+
+	a1 [1]int
+	a1k [1000]int
+	a100k [100000]int
+
+	p1 *[1]int
+	p1k *[1000]int
+	p100k *[100000]int
+
+	i int
+	ui uint
+	i8 int8
+	ui8 uint8
+	i16 int16
+	ui16 uint16
+	i32 int32
+	ui32 uint32
+	i64 int64
+	ui64 uint64
+)
+
+func main() {
+	// Most things need checks.
+	use(s[i])
+	use(a1[i])
+	use(a1k[i])
+	use(a100k[i])
+	use(p1[i])
+	use(p1k[i])
+	use(p100k[i])
+
+	use(s[ui])
+	use(a1[ui])
+	use(a1k[ui])
+	use(a100k[ui])
+	use(p1[ui])
+	use(p1k[ui])
+	use(p100k[ui])
+
+	use(s[i8])
+	use(a1[i8])
+	use(a1k[i8])
+	use(a100k[i8])
+	use(p1[i8])
+	use(p1k[i8])
+	use(p100k[i8])
+
+	// Unsigned 8-bit numbers don't need checks for len >= 2⁸.
+	use(s[ui8])
+	use(a1[ui8])
+	use(a1k[ui8])  // ERROR "index bounds check elided"
+	use(a100k[ui8])  // ERROR "index bounds check elided"
+	use(p1[ui8])
+	use(p1k[ui8])  // ERROR "index bounds check elided"
+	use(p100k[ui8])  // ERROR "index bounds check elided"
+
+	use(s[i16])
+	use(a1[i16])
+	use(a1k[i16])
+	use(a100k[i16])
+	use(p1[i16])
+	use(p1k[i16])
+	use(p100k[i16])
+
+	// Unsigned 16-bit numbers don't need checks for len >= 2¹⁶.
+	use(s[ui16])
+	use(a1[ui16])
+	use(a1k[ui16])
+	use(a100k[ui16])  // ERROR "index bounds check elided"
+	use(p1[ui16])
+	use(p1k[ui16])
+	use(p100k[ui16])  // ERROR "index bounds check elided"
+
+	use(s[i32])
+	use(a1[i32])
+	use(a1k[i32])
+	use(a100k[i32])
+	use(p1[i32])
+	use(p1k[i32])
+	use(p100k[i32])
+
+	use(s[ui32])
+	use(a1[ui32])
+	use(a1k[ui32])
+	use(a100k[ui32])
+	use(p1[ui32])
+	use(p1k[ui32])
+	use(p100k[ui32])
+
+	use(s[i64])
+	use(a1[i64])
+	use(a1k[i64])
+	use(a100k[i64])
+	use(p1[i64])
+	use(p1k[i64])
+	use(p100k[i64])
+
+	use(s[ui64])
+	use(a1[ui64])
+	use(a1k[ui64])
+	use(a100k[ui64])
+	use(p1[ui64])
+	use(p1k[ui64])
+	use(p100k[ui64])
+
+	// Mod truncates the maximum value to one less than the argument,
+	// but signed mod can be negative, so only unsigned mod counts.
+	use(s[i%999])
+	use(a1[i%999])
+	use(a1k[i%999])
+	use(a100k[i%999])
+	use(p1[i%999])
+	use(p1k[i%999])
+	use(p100k[i%999])
+
+	use(s[ui%999])
+	use(a1[ui%999])
+	use(a1k[ui%999])  // ERROR "index bounds check elided"
+	use(a100k[ui%999])  // ERROR "index bounds check elided"
+	use(p1[ui%999])
+	use(p1k[ui%999])  // ERROR "index bounds check elided"
+	use(p100k[ui%999])  // ERROR "index bounds check elided"
+
+	use(s[i%1000])
+	use(a1[i%1000])
+	use(a1k[i%1000])
+	use(a100k[i%1000])
+	use(p1[i%1000])
+	use(p1k[i%1000])
+	use(p100k[i%1000])
+
+	use(s[ui%1000])
+	use(a1[ui%1000])
+	use(a1k[ui%1000])  // ERROR "index bounds check elided"
+	use(a100k[ui%1000])  // ERROR "index bounds check elided"
+	use(p1[ui%1000])
+	use(p1k[ui%1000])  // ERROR "index bounds check elided"
+	use(p100k[ui%1000])  // ERROR "index bounds check elided"
+
+	use(s[i%1001])
+	use(a1[i%1001])
+	use(a1k[i%1001])
+	use(a100k[i%1001])
+	use(p1[i%1001])
+	use(p1k[i%1001])
+	use(p100k[i%1001])
+
+	use(s[ui%1001])
+	use(a1[ui%1001])
+	use(a1k[ui%1001])
+	use(a100k[ui%1001])  // ERROR "index bounds check elided"
+	use(p1[ui%1001])
+	use(p1k[ui%1001])
+	use(p100k[ui%1001])  // ERROR "index bounds check elided"
+
+	// Bitwise and truncates the maximum value to the mask value.
+	// The result (for a positive mask) cannot be negative, so elision
+	// applies to both signed and unsigned indexes.
+	use(s[i&999])
+	use(a1[i&999])
+	use(a1k[i&999])  // ERROR "index bounds check elided"
+	use(a100k[i&999])  // ERROR "index bounds check elided"
+	use(p1[i&999])
+	use(p1k[i&999])  // ERROR "index bounds check elided"
+	use(p100k[i&999])  // ERROR "index bounds check elided"
+
+	use(s[ui&999])
+	use(a1[ui&999])
+	use(a1k[ui&999])  // ERROR "index bounds check elided"
+	use(a100k[ui&999])  // ERROR "index bounds check elided"
+	use(p1[ui&999])
+	use(p1k[ui&999])  // ERROR "index bounds check elided"
+	use(p100k[ui&999])  // ERROR "index bounds check elided"
+
+	use(s[i&1000])
+	use(a1[i&1000])
+	use(a1k[i&1000])
+	use(a100k[i&1000])  // ERROR "index bounds check elided"
+	use(p1[i&1000])
+	use(p1k[i&1000])
+	use(p100k[i&1000])  // ERROR "index bounds check elided"
+
+	use(s[ui&1000])
+	use(a1[ui&1000])
+	use(a1k[ui&1000])
+	use(a100k[ui&1000])  // ERROR "index bounds check elided"
+	use(p1[ui&1000])
+	use(p1k[ui&1000])
+	use(p100k[ui&1000])  // ERROR "index bounds check elided"
+
+	// Right shift cuts the effective number of bits in the index,
+	// but only for unsigned (signed stays negative).
+	use(s[i32>>22])
+	use(a1[i32>>22])
+	use(a1k[i32>>22])
+	use(a100k[i32>>22])
+	use(p1[i32>>22])
+	use(p1k[i32>>22])
+	use(p100k[i32>>22])
+
+	use(s[ui32>>22])
+	use(a1[ui32>>22])
+	use(a1k[ui32>>22])
+	use(a100k[ui32>>22])  // ERROR "index bounds check elided"
+	use(p1[ui32>>22])
+	use(p1k[ui32>>22])
+	use(p100k[ui32>>22])  // ERROR "index bounds check elided"
+
+	use(s[i32>>23])
+	use(a1[i32>>23])
+	use(a1k[i32>>23])
+	use(a100k[i32>>23])
+	use(p1[i32>>23])
+	use(p1k[i32>>23])
+	use(p100k[i32>>23])
+
+	use(s[ui32>>23])
+	use(a1[ui32>>23])
+	use(a1k[ui32>>23])  // ERROR "index bounds check elided"
+	use(a100k[ui32>>23])  // ERROR "index bounds check elided"
+	use(p1[ui32>>23])
+	use(p1k[ui32>>23])  // ERROR "index bounds check elided"
+	use(p100k[ui32>>23])  // ERROR "index bounds check elided"
+
+	// Division cuts the range like right shift does.
+	use(s[i/1e6])
+	use(a1[i/1e6])
+	use(a1k[i/1e6])
+	use(a100k[i/1e6])
+	use(p1[i/1e6])
+	use(p1k[i/1e6])
+	use(p100k[i/1e6])
+
+	use(s[ui/1e6])
+	use(a1[ui/1e6])
+	use(a1k[ui/1e6])
+	use(a100k[ui/1e6])  // ERROR "index bounds check elided"
+	use(p1[ui/1e6])
+	use(p1k[ui/1e6])
+	use(p100k[ui/1e6])  // ERROR "index bounds check elided"
+
+	use(s[i/1e7])
+	use(a1[i/1e7])
+	use(a1k[i/1e7])
+	use(a100k[i/1e7])
+	use(p1[i/1e7])
+	use(p1k[i/1e7])
+	use(p100k[i/1e7])
+
+	use(s[ui/1e7])
+	use(a1[ui/1e7])
+	use(a1k[ui/1e7])  // ERROR "index bounds check elided"
+	use(a100k[ui/1e7])  // ERROR "index bounds check elided"
+	use(p1[ui/1e7])
+	use(p1k[ui/1e7])  // ERROR "index bounds check elided"
+	use(p100k[ui/1e7])  // ERROR "index bounds check elided"
+
+}
+
+var sum int 
+
+func use(x int) {
+	sum += x
+}

test/rotate.go

@@ -0,0 +1,141 @@
+// $G $D/$F.go && $L $F.$A &&
+// ./$A.out >tmp.go && $G tmp.go && $L -o $A.out1 tmp.$A && ./$A.out1
+// rm -f tmp.go $A.out1
+
+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Generate test of shift and rotate by constants.
+// The output is compiled and run.
+//
+// The output takes around a minute to compile, link, and run
+// but it is only done during ./run, not in normal builds using run.go.
+
+package main
+
+import (
+	"bufio"
+	"flag"
+	"fmt"
+	"os"
+)
+
+func main() {
+	flag.Parse()
+
+	b := bufio.NewWriter(os.Stdout)
+	defer b.Flush()
+
+	fmt.Fprintf(b, "%s\n", prolog)
+
+	for logBits := uint(3); logBits <= 6; logBits++ {
+		for mode := 0; mode < 1<<2; mode++ {
+			gentest(b, 1<<logBits, mode&1 != 0, mode&2 != 0)
+		}
+	}
+}
+
+const prolog = `
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+var (
+	i8 int8 = 0x12
+	i16 int16 = 0x1234
+	i32 int32 = 0x12345678
+	i64 int64 = 0x123456789abcdef0
+	ui8 uint8 = 0x12
+	ui16 uint16 = 0x1234
+	ui32 uint32 = 0x12345678
+	ui64 uint64 = 0x123456789abcdef0
+
+	ni8 = ^i8
+	ni16 = ^i16
+	ni32 = ^i32
+	ni64 = ^i64
+	nui8 = ^ui8
+	nui16 = ^ui16
+	nui32 = ^ui32
+	nui64 = ^ui64
+)
+
+var nfail = 0
+
+func check(desc string, have, want interface{}) {
+	if have != want {
+		nfail++
+		fmt.Printf("%s = %T(%#x), want %T(%#x)\n", desc, have, have, want, want)
+		if nfail >= 100 {
+			fmt.Printf("BUG: stopping after 100 failures\n")
+			os.Exit(0)
+		}
+	}
+}
+
+func main() {
+	if nfail > 0 {
+		fmt.Printf("BUG\n")
+	}
+}
+
+`
+
+func gentest(b *bufio.Writer, bits uint, unsigned, inverted bool) {
+	fmt.Fprintf(b, "func init() {\n")
+	defer fmt.Fprintf(b, "}\n")
+	n := 0
+
+	// Generate tests for left/right and right/left.
+	for l := uint(0); l <= bits; l++ {
+		for r := uint(0); r <= bits; r++ {
+			typ := fmt.Sprintf("int%d", bits)
+			v := fmt.Sprintf("i%d", bits)
+			if unsigned {
+				typ = "u" + typ
+				v = "u" + v
+			}
+			v0 := int64(0x123456789abcdef0)
+			if inverted {
+				v = "n" + v
+				v0 = ^v0
+			}
+			expr1 := fmt.Sprintf("%s<<%d | %s>>%d", v, l, v, r)
+			expr2 := fmt.Sprintf("%s>>%d | %s<<%d", v, r, v, l)
+			
+			var result string
+			if unsigned {
+				v := uint64(v0) >> (64 - bits)
+				v = v<<l | v>>r
+				v <<= 64 - bits
+				v >>= 64 - bits
+				result = fmt.Sprintf("%#x", v)
+			} else {
+				v := int64(v0) >> (64 - bits)
+				v = v<<l | v>>r
+				v <<= 64 - bits
+				v >>= 64 - bits
+				result = fmt.Sprintf("%#x", v)
+			}
+
+			fmt.Fprintf(b, "\tcheck(%q, %s, %s(%s))\n", expr1, expr1, typ, result)
+			fmt.Fprintf(b, "\tcheck(%q, %s, %s(%s))\n", expr2, expr2, typ, result)
+
+			// Chop test into multiple functions so that there's not one
+			// enormous function to compile/link.
+			// All the functions are named init so we don't have to do
+			// anything special to call them.  ☺
+			if n++; n >= 100 {
+				fmt.Fprintf(b, "}\n")
+				fmt.Fprintf(b, "func init() {\n")
+				n = 0
+			}
+		}
+	}
+}
+