From: Josh Bleecher Snyder <josharian@gmail.com>
Date: Mon, 22 Dec 2014 19:33:47 +0000 (-0800)
Subject: cmd/gc: optimize existence-only map lookups
X-Git-Tag: go1.5beta1~2414
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=43e6923131c4c83e02f5263a9632d81819f15a62;p=gostls13.git

cmd/gc: optimize existence-only map lookups

The compiler converts 'val, ok = m[key]' to

        tmp, ok = <runtime call>
        val = *tmp

For lookups of the form '_, ok = m[key]',
the second statement is unnecessary.
By not generating it we save a nil check.

Change-Id: I21346cc195cb3c62e041af8b18770c0940358695
Reviewed-on: https://go-review.googlesource.com/1975
Reviewed-by: Russ Cox <rsc@golang.org>
---

diff --git a/src/cmd/gc/walk.c b/src/cmd/gc/walk.c
index f54302dc28..48dd17a6af 100644
--- a/src/cmd/gc/walk.c
+++ b/src/cmd/gc/walk.c
@@ -794,8 +794,6 @@ walkexpr(Node **np, NodeList **init)
 		//   var,b = mapaccess2*(t, m, i)
 		//   a = *var
 		a = n->list->n;
-		var = temp(ptrto(t->type));
-		var->typecheck = 1;
 		fn = mapfn(p, t);
 		r = mkcall1(fn, getoutargx(fn->type), init, typename(t), r->left, key);
 
@@ -806,10 +804,17 @@ walkexpr(Node **np, NodeList **init)
 			r->type->type->down->type = n->list->next->n->type;
 		n->rlist = list1(r);
 		n->op = OAS2FUNC;
-		n->list->n = var;
-		walkexpr(&n, init);
-		*init = list(*init, n);
-		n = nod(OAS, a, nod(OIND, var, N));
+
+		// don't generate a = *var if a is _
+		if(!isblank(a)) {
+			var = temp(ptrto(t->type));
+			var->typecheck = 1;
+			n->list->n = var;
+			walkexpr(&n, init);
+			*init = list(*init, n);
+			n = nod(OAS, a, nod(OIND, var, N));
+		}
+
 		typecheck(&n, Etop);
 		walkexpr(&n, init);
 		// mapaccess needs a zero value to be at least this big.
diff --git a/test/nilcheck.go b/test/nilcheck.go
index fe05d05c92..99c3c5fdb6 100644
--- a/test/nilcheck.go
+++ b/test/nilcheck.go
@@ -182,3 +182,8 @@ func f4(x *[10]int) {
 	_ = &x[9] // ERROR "nil check"
 }
 
+func f5(m map[string]struct{}) bool {
+	// Existence-only map lookups should not generate a nil check
+	_, ok := m[""]
+	return ok
+}