transform: allow updating tests with -update flag

This should make it much easier to update existing tests.

transform/transform_test.go

@@ -3,6 +3,7 @@ package transform
 // This file defines some helper functions for testing transforms.
 
 import (
+	"flag"
 	"io/ioutil"
 	"os"
 	"strings"
@@ -11,6 +12,8 @@ import (
 	"tinygo.org/x/go-llvm"
 )
 
+var update = flag.Bool("update", false, "update transform package tests")
+
 // testTransform runs a transformation pass on an input file (pathPrefix+".ll")
 // and checks whether it matches the expected output (pathPrefix+".out.ll"). The
 // output is compared with a fuzzy match that ignores some irrelevant lines such
@@ -31,18 +34,28 @@ func testTransform(t *testing.T, pathPrefix string, transform func(mod llvm.Modu
 	// Perform the transform.
 	transform(mod)
 
-	// Read the expected output IR.
-	out, err := ioutil.ReadFile(pathPrefix + ".out.ll")
-	if err != nil {
-		t.Fatalf("could not read output file %s: %v", pathPrefix+".out.ll", err)
-	}
-
-	// See whether the transform output matches with the expected output IR.
-	expected := string(out)
+	// Get the output from the test and filter some irrelevant lines.
 	actual := mod.String()
-	if !fuzzyEqualIR(expected, actual) {
-		t.Logf("output does not match expected output:\n%s", actual)
-		t.Fail()
+	actual = actual[strings.Index(actual, "\ntarget datalayout = ")+1:]
+
+	if *update {
+		err := ioutil.WriteFile(pathPrefix+".out.ll", []byte(actual), 0666)
+		if err != nil {
+			t.Error("failed to write out new output:", err)
+		}
+	} else {
+		// Read the expected output IR.
+		out, err := ioutil.ReadFile(pathPrefix + ".out.ll")
+		if err != nil {
+			t.Fatalf("could not read output file %s: %v", pathPrefix+".out.ll", err)
+		}
+
+		// See whether the transform output matches with the expected output IR.
+		expected := string(out)
+		if !fuzzyEqualIR(expected, actual) {
+			t.Logf("output does not match expected output:\n%s", actual)
+			t.Fail()
+		}
 	}
 }