transform: don't escape append arguments if the return value doesn't

Author: eliasnaur

transform/allocs.go

@@ -154,9 +154,18 @@ func valueEscapesAt(value llvm.Value) llvm.Value {
 				return use
 			}
 		case llvm.Call:
-			if !hasFlag(use, value, "nocapture") {
-				return use
+			if hasFlag(use, value, "nocapture") {
+				break
+			}
+			// If built-in append function escapes its first argument if and
+			// only if the return value escapes.
+			if fn := use.CalledValue(); !fn.IsAFunction().IsNil() && fn.Name() == "runtime.sliceAppend" {
+				if at := valueEscapesAt(use); !at.IsNil() {
+					return at
+				}
+				break
 			}
+			return use
 		case llvm.ICmp:
 			// Comparing pointers don't let the pointer escape.
 			// This is often a compiler-inserted nil check.

transform/testdata/allocs2.go

@@ -23,7 +23,7 @@ func main() {
 	s4 := make([]byte, 300) // OUT: object allocated on the heap: object size 300 exceeds maximum stack allocation size 256
 	readByteSlice(s4)
 
-	s5 := make([]int, 4) // OUT: object allocated on the heap: escapes at line 27
+	s5 := make([]int, 4)
 	_ = append(s5, 5)
 
 	s6 := make([]int, 3)