[release-branch.go1.21] cmd/compile: don't combine loads in generated equality functions

... if the architecture can't do unaligned loads.
We already handle this in a few places, but this particular place
was added in CL 399542 and missed this additional restriction.

Fixes #67164

(Reroll of CL 583303)

Change-Id: Id067cc2e39d7d199672f2017f7f53991fb23f8d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/583799
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: randall77

src/cmd/compile/internal/compare/compare.go

@@ -148,7 +148,7 @@ func calculateCostForType(t *types.Type) int64 {
 		return EqStructCost(t)
 	case types.TSLICE:
 		// Slices are not comparable.
-		base.Fatalf("eqStructFieldCost: unexpected slice type")
+		base.Fatalf("calculateCostForType: unexpected slice type")
 	case types.TARRAY:
 		elemCost := calculateCostForType(t.Elem())
 		cost = t.NumElem() * elemCost
@@ -374,6 +374,11 @@ func eqmem(p ir.Node, q ir.Node, field *types.Sym, size int64) ir.Node {
 }
 
 func eqmemfunc(size int64, t *types.Type) (fn *ir.Name, needsize bool) {
+	if !base.Ctxt.Arch.CanMergeLoads && t.Alignment() < int64(base.Ctxt.Arch.Alignment) && t.Alignment() < t.Size() {
+		// We can't use larger comparisons if the value might not be aligned
+		// enough for the larger comparison. See issues 46283 and 67160.
+		size = 0
+	}
 	switch size {
 	default:
 		fn = typecheck.LookupRuntime("memequal")

test/fixedbugs/issue67160.go

@@ -0,0 +1,32 @@
+// run
+
+// Copyright 2024 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.
+
+// Test to make sure that we don't try using larger loads for
+// generated equality functions on architectures that can't do
+// unaligned loads.
+
+package main
+
+// T has a big field that wants to be compared with larger loads/stores.
+// T is "special" because of the unnamed field, so it needs a generated equality function.
+// T is an odd number of bytes in size and has alignment 1.
+type T struct {
+	src [8]byte
+	_   byte
+}
+
+// U contains 8 copies of T, each at a different %8 alignment.
+type U [8]T
+
+//go:noinline
+func f(x, y *U) bool {
+	return *x == *y
+}
+
+func main() {
+	var a U
+	_ = f(&a, &a)
+}