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>

2 files changed, 19 insertions, 1 deletions

diff --git a/src/cmd/compile/internal/ssa/compile.go b/src/cmd/compile/internal/ssa/compile.go
index 96bc5f03c1..29618e29c3 100644
--- a/src/cmd/compile/internal/ssa/compile.go
+++ b/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()
diff --git a/src/cmd/compile/internal/ssa/nilcheck.go b/src/cmd/compile/internal/ssa/nilcheck.go
index e0669cf80c..5f58e2d7ec 100644
--- a/src/cmd/compile/internal/ssa/nilcheck.go
+++ b/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.
 	}
 }