cmd/compile: randomize value order in block for testing

A little bit of compiler stress testing. Randomize the order
of the values in a block before every phase. This randomization
makes sure that we're not implicitly depending on that order.

Currently the random seed is a hash of the function name.
It provides determinism, but sacrifices some coverage.
Other arrangements are possible (env var, ...) but require
more setup.

Fixes #20178

Change-Id: Idae792a23264bd9a3507db6ba49b6d591a608e83
Reviewed-on: https://go-review.googlesource.com/c/33909
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/ssa/compile.go

@@ -8,7 +8,9 @@ import (
 	"cmd/internal/objabi"
 	"cmd/internal/src"
 	"fmt"
+	"hash/crc32"
 	"log"
+	"math/rand"
 	"os"
 	"regexp"
 	"runtime"
@@ -29,6 +31,11 @@ func Compile(f *Func) {
 		f.Logf("compiling %s\n", f.Name)
 	}
 
+	var rnd *rand.Rand
+	if checkEnabled {
+		rnd = rand.New(rand.NewSource(int64(crc32.ChecksumIEEE(([]byte)(f.Name)))))
+	}
+
 	// hook to print function & phase if panic happens
 	phaseName := "init"
 	defer func() {
@@ -68,6 +75,17 @@ func Compile(f *Func) {
 			runtime.ReadMemStats(&mStart)
 		}
 
+		if checkEnabled && !f.scheduled {
+			// Test that we don't depend on the value order, by randomizing
+			// the order of values in each block. See issue 18169.
+			for _, b := range f.Blocks {
+				for i := 0; i < len(b.Values)-1; i++ {
+					j := i + rnd.Intn(len(b.Values)-i)
+					b.Values[i], b.Values[j] = b.Values[j], b.Values[i]
+				}
+			}
+		}
+
 		tStart := time.Now()
 		p.fn(f)
 		tEnd := time.Now()

src/cmd/compile/internal/ssa/nilcheck.go

@@ -290,6 +290,6 @@ func nilcheckelim2(f *Func) {
 		b.Values = b.Values[:i]
 
 		// TODO: if b.Kind == BlockPlain, start the analysis in the subsequent block to find
-		// more unnecessary nil checks.  Would fix test/nilptr3_ssa.go:157.
+		// more unnecessary nil checks.  Would fix test/nilptr3.go:159.
 	}
 }

test/run.go

@@ -559,6 +559,19 @@ func (t *test) run() {
 		}
 		args = args[1:]
 	}
+	if action == "errorcheck" {
+		found := false
+		for i, f := range flags {
+			if strings.HasPrefix(f, "-d=") {
+				flags[i] = f + ",ssa/check/on"
+				found = true
+				break
+			}
+		}
+		if !found {
+			flags = append(flags, "-d=ssa/check/on")
+		}
+	}
 
 	t.makeTempDir()
 	if !*keep {