all: move OptimizeMaps to transforms and add tests

transform/maps.go

@@ -0,0 +1,47 @@
+package transform
+
+import (
+	"tinygo.org/x/go-llvm"
+)
+
+// OptimizeMaps eliminates created but unused maps.
+//
+// In the future, this should statically allocate created but never modified
+// maps. This has not yet been implemented, however.
+func OptimizeMaps(mod llvm.Module) {
+	hashmapMake := mod.NamedFunction("runtime.hashmapMake")
+	if hashmapMake.IsNil() {
+		// nothing to optimize
+		return
+	}
+
+	hashmapBinarySet := mod.NamedFunction("runtime.hashmapBinarySet")
+	hashmapStringSet := mod.NamedFunction("runtime.hashmapStringSet")
+
+	for _, makeInst := range getUses(hashmapMake) {
+		updateInsts := []llvm.Value{}
+		unknownUses := false // are there any uses other than setting a value?
+
+		for _, use := range getUses(makeInst) {
+			if use := use.IsACallInst(); !use.IsNil() {
+				switch use.CalledValue() {
+				case hashmapBinarySet, hashmapStringSet:
+					updateInsts = append(updateInsts, use)
+				default:
+					unknownUses = true
+				}
+			} else {
+				unknownUses = true
+			}
+		}
+
+		if !unknownUses {
+			// This map can be entirely removed, as it is only created but never
+			// used.
+			for _, inst := range updateInsts {
+				inst.EraseFromParentAsInstruction()
+			}
+			makeInst.EraseFromParentAsInstruction()
+		}
+	}
+}