cmd/compile/internal: use sparseSet, optimize isSynthetic

changedVars was functionally a set, but couldn't be iterated over
efficiently. In functions with many variables, the wasted iteration was
costly. Use a sparseSet instead.

(*gc.Node).String() is very expensive: it calls Sprintf, which does
reflection, etc, etc. Instead, just look at .Sym.Name, which is all we
care about.

Change-Id: Ib61cd7b5c796e1813b8859135e85da5bfe2ac686
Reviewed-on: https://go-review.googlesource.com/92402
Reviewed-by: David Chase <drchase@google.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/ssa/export_test.go b/src/cmd/compile/internal/ssa/export_test.go
index d1d6831eb3..ac7a1b00e0 100644
--- a/src/cmd/compile/internal/ssa/export_test.go
+++ b/src/cmd/compile/internal/ssa/export_test.go
@@ -79,6 +79,10 @@ func (d *DummyAuto) StorageClass() StorageClass {
 	return ClassAuto
 }
 
+func (d *DummyAuto) IsSynthetic() bool {
+	return false
+}
+
 func (DummyFrontend) StringData(s string) interface{} {
 	return nil
 }