cmd/compile: fix store chain in schedule pass

Tuple ops are weird. They are essentially a pair of ops,
one which consumes a mem and one which generates a mem (the Select1).
The schedule pass didn't handle these quite right.

Fix the scheduler to include both parts of the paired op in
the store chain. That makes sure that loads are correctly ordered
with respect to the first of the pair.

Add a check for the ssacheck builder, that there is only one
live store at a time. I thought we already had such a check, but
apparently not...

Fixes #20335

Change-Id: I59eb3446a329100af38d22820b1ca2190ca46a78
Reviewed-on: https://go-review.googlesource.com/43294
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -309,6 +309,39 @@ func checkFunc(f *Func) {
 			}
 		}
 	}
+
+	// Check that if a tuple has a memory type, it is second.
+	for _, b := range f.Blocks {
+		for _, v := range b.Values {
+			if v.Type.IsTuple() && v.Type.FieldType(0).IsMemory() {
+				f.Fatalf("memory is first in a tuple: %s\n", v.LongString())
+			}
+		}
+	}
+
+	// Check that only one memory is live at any point.
+	// TODO: make this check examine interblock.
+	if f.scheduled {
+		for _, b := range f.Blocks {
+			var mem *Value // the live memory
+			for _, v := range b.Values {
+				if v.Op != OpPhi {
+					for _, a := range v.Args {
+						if a.Type.IsMemory() || a.Type.IsTuple() && a.Type.FieldType(1).IsMemory() {
+							if mem == nil {
+								mem = a
+							} else if mem != a {
+								f.Fatalf("two live mems @ %s: %s and %s", v, mem, a)
+							}
+						}
+					}
+				}
+				if v.Type.IsMemory() || v.Type.IsTuple() && v.Type.FieldType(1).IsMemory() {
+					mem = v
+				}
+			}
+		}
+	}
 }
 
 // domCheck reports whether x dominates y (including x==y).

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

@@ -132,19 +132,20 @@ func schedule(f *Func) {
 		}
 	}
 
+	// TODO: make this logic permanent in types.IsMemory?
+	isMem := func(v *Value) bool {
+		return v.Type.IsMemory() || v.Type.IsTuple() && v.Type.FieldType(1).IsMemory()
+	}
+
 	for _, b := range f.Blocks {
 		// Find store chain for block.
 		// Store chains for different blocks overwrite each other, so
 		// the calculated store chain is good only for this block.
 		for _, v := range b.Values {
-			if v.Op != OpPhi && v.Type.IsMemory() {
-				mem := v
-				if v.Op == OpSelect1 {
-					v = v.Args[0]
-				}
+			if v.Op != OpPhi && isMem(v) {
 				for _, w := range v.Args {
-					if w.Type.IsMemory() {
-						nextMem[w.ID] = mem
+					if isMem(w) {
+						nextMem[w.ID] = v
 					}
 				}
 			}
@@ -163,9 +164,8 @@ func schedule(f *Func) {
 					uses[w.ID]++
 				}
 				// Any load must come before the following store.
-				if v.Type.IsMemory() || !w.Type.IsMemory() {
-					continue // not a load
-				}
+				if !isMem(v) && isMem(w) {
+					// v is a load.
 					s := nextMem[w.ID]
 					if s == nil || s.Block != b {
 						continue
@@ -174,6 +174,7 @@ func schedule(f *Func) {
 					uses[v.ID]++
 				}
 			}
+		}
 
 		if b.Control != nil && b.Control.Op != OpPhi {
 			// Force the control value to be scheduled at the end,

test/fixedbugs/issue20335.go

+// compile
+
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 20335: don't reorder loads with stores.
+// This test should fail on the ssacheck builder
+// without the fix in the CL that added this file.
+// TODO: check the generated assembly?
+
+package a
+
+import "sync/atomic"
+
+func f(p, q *int32) bool {
+	x := *q
+	return atomic.AddInt32(p, 1) == x
+}