runtime: reorder race detector calls in slicecopy

In rare circumstances, this helps report a race which would
otherwise go undetected.

Fixes #36794

Change-Id: I8a3c9bd6fc34efa51516393f7ee72531c34fb073
Reviewed-on: https://go-review.googlesource.com/c/go/+/220685
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Dmitry Vyukov <[email protected]>

Author: randall77

src/runtime/race/testdata/slice_test.go

@@ -5,6 +5,7 @@
 package race_test
 
 import (
+	"sync"
 	"testing"
 )
 
@@ -590,3 +591,18 @@ func TestRaceSlice3(t *testing.T) {
 	_ = x[:1:i]
 	<-done
 }
+
+var saved string
+
+func TestRaceSlice4(t *testing.T) {
+	// See issue 36794.
+	data := []byte("hello there")
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		_ = string(data)
+		wg.Done()
+	}()
+	copy(data, data[2:])
+	wg.Wait()
+}

src/runtime/slice.go

@@ -211,12 +211,12 @@ func slicecopy(to, fm slice, width uintptr) int {
 	if raceenabled {
 		callerpc := getcallerpc()
 		pc := funcPC(slicecopy)
-		racewriterangepc(to.array, uintptr(n*int(width)), callerpc, pc)
 		racereadrangepc(fm.array, uintptr(n*int(width)), callerpc, pc)
+		racewriterangepc(to.array, uintptr(n*int(width)), callerpc, pc)
 	}
 	if msanenabled {
-		msanwrite(to.array, uintptr(n*int(width)))
 		msanread(fm.array, uintptr(n*int(width)))
+		msanwrite(to.array, uintptr(n*int(width)))
 	}
 
 	size := uintptr(n) * width