cmd/compile: remove check that Zero's arg has the correct base type

It doesn't have to. The type in the aux field is authoritative.
There are cases involving casting from interface{} where pointers
have a placeholder pointer type (because the type is not known when
the IData op is generated).

The check was introduced in CL 13447.

Fixes #39459

Change-Id: Id77a57577806a271aeebd20bea5d92d08ee7aa6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/239817
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

diff --git a/src/cmd/compile/internal/ssa/deadstore.go b/src/cmd/compile/internal/ssa/deadstore.go
index 88af7a6f4afe1aa624750e6c34a674984bcd4ad9..0664013b397ed0668552fe2174d0351cb21623a5 100644 (file)
--- a/src/cmd/compile/internal/ssa/deadstore.go
+++ b/src/cmd/compile/internal/ssa/deadstore.go
@@ -73,9 +73,11 @@ func dse(f *Func) {
                }
 
                // Walk backwards looking for dead stores. Keep track of shadowed addresses.
-               // An "address" is an SSA Value which encodes both the address and size of
-               // the write. This code will not remove dead stores to the same address
-               // of different types.
+               // A "shadowed address" is a pointer and a size describing a memory region that
+               // is known to be written. We keep track of shadowed addresses in the shadowed
+               // map, mapping the ID of the address to the size of the shadowed region.
+               // Since we're walking backwards, writes to a shadowed region are useless,
+               // as they will be immediately overwritten.
                shadowed.clear()
                v := last
 
@@ -93,17 +95,13 @@ func dse(f *Func) {
                                sz = v.AuxInt
                        }
                        if shadowedSize := int64(shadowed.get(v.Args[0].ID)); shadowedSize != -1 && shadowedSize >= sz {
-                               // Modify store into a copy
+                               // Modify the store/zero into a copy of the memory state,
+                               // effectively eliding the store operation.
                                if v.Op == OpStore {
                                        // store addr value mem
                                        v.SetArgs1(v.Args[2])
                                } else {
                                        // zero addr mem
-                                       typesz := v.Args[0].Type.Elem().Size()
-                                       if sz != typesz {
-                                               f.Fatalf("mismatched zero/store sizes: %d and %d [%s]",
-                                                       sz, typesz, v.LongString())
-                                       }
                                        v.SetArgs1(v.Args[1])
                                }
                                v.Aux = nil

diff --git a/test/fixedbugs/issue39459.go b/test/fixedbugs/issue39459.go
new file mode 100644 (file)
index 0000000..de78a17
--- /dev/null
+++ b/test/fixedbugs/issue39459.go
@@ -0,0 +1,22 @@
+// compile
+
+// Copyright 2020 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.
+
+package p
+
+type T struct { // big enough to be an unSSAable type
+       a, b, c, d, e, f int
+}
+
+func f(x interface{}, p *int) {
+       _ = *p // trigger nil check here, removing it from below
+       switch x := x.(type) {
+       case *T:
+               // Zero twice, so one of them will be removed by the deadstore pass
+               *x = T{}
+               *p = 0 // store op to prevent Zero ops from being optimized by the earlier opt pass rewrite rules
+               *x = T{}
+       }
+}