do not emit USED for global x when compiling "_ = x;"

R=ken@golang.org
CC=r
http://go/go-review/1018019

diff --git a/src/cmd/gc/gen.c b/src/cmd/gc/gen.c
index bb1b19bfc9774727003078676115c1a76bfdfa11..475058fa28e0434e61ee728a084b5986c27df0d0 100644 (file)
--- a/src/cmd/gc/gen.c
+++ b/src/cmd/gc/gen.c
@@ -427,7 +427,7 @@ cgen_discard(Node *nr)
 
        switch(nr->op) {
        case ONAME:
-               if(!(nr->class & PHEAP))
+               if(!(nr->class & PHEAP) && nr->class != PEXTERN && nr->class != PFUNC)
                        gused(nr);
                break;