cmd/compile: make TestAssembly resilient to output ordering

To preserve reproducible builds, the text entries
during compilation will be sorted before being printed.
TestAssembly currently assumes that function init
comes after all user-defined functions.
Remove that assumption.
Instead of looking for "TEXT" to tell you where
a function ends--which may now yield lots of
non-function-code junk--look for a line beginning
with non-whitespace.

Updates #15756

Change-Id: Ibc82dba6143d769ef4c391afc360e523b1a51348
Reviewed-on: https://go-review.googlesource.com/39853
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/asm_test.go

@@ -42,23 +42,32 @@ func TestAssembly(t *testing.T) {
 				asm := ats.compileToAsm(tt, dir)
 
 				for _, at := range ats.tests {
-					funcName := nameRegexp.FindString(at.function)[5:]
-					fa := funcAsm(asm, funcName)
-					at.verifyAsm(tt, fa)
+					funcName := nameRegexp.FindString(at.function)[len("func "):]
+					fa := funcAsm(tt, asm, funcName)
+					if fa != "" {
+						at.verifyAsm(tt, fa)
+					}
 				}
 			})
 		}
 	})
 }
 
+var nextTextRegexp = regexp.MustCompile(`\n\S`)
+
 // funcAsm returns the assembly listing for the given function name.
-func funcAsm(asm string, funcName string) string {
+func funcAsm(t *testing.T, asm string, funcName string) string {
 	if i := strings.Index(asm, fmt.Sprintf("TEXT\t\"\".%s(SB)", funcName)); i >= 0 {
 		asm = asm[i:]
+	} else {
+		t.Errorf("could not find assembly for function %v", funcName)
+		return ""
 	}
 
-	if i := strings.Index(asm[1:], "TEXT\t\"\"."); i >= 0 {
-		asm = asm[:i+1]
+	// Find the next line that doesn't begin with whitespace.
+	loc := nextTextRegexp.FindStringIndex(asm)
+	if loc != nil {
+		asm = asm[:loc[0]]
 	}
 
 	return asm
@@ -130,12 +139,6 @@ func (ats *asmTests) compileToAsm(t *testing.T, dir string) string {
 
 	// Now, compile the individual file for which we want to see the generated assembly.
 	asm := ats.runGo(t, "tool", "compile", "-I", testDir, "-S", "-o", filepath.Join(testDir, "out.o"), src)
-
-	// Get rid of code for "".init. Also gets rid of type algorithms & other junk.
-	if i := strings.Index(asm, "\n\"\".init "); i >= 0 {
-		asm = asm[:i+1]
-	}
-
 	return asm
 }