cmd/compile: pointer-shaped types are SSAable even if lots of 0-sized fields

Normally we don't SSA-ify variables with types that have more than
4 fields. But we really do want to SSA-ify them if they are pointer
shaped.

An odd case, but the compiler shouldn't barf on them.

Failure probably started with CL 714421.

Fixes #77534

Change-Id: I51ef87676cc31df1e51e164bbd58d58c0ab72436
Reviewed-on: https://go-review.googlesource.com/c/go/+/744280
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -363,9 +363,9 @@ func decomposeUserPhi(v *Value) {
 func decomposeStructPhi(v *Value) {
 	t := v.Type
 	n := t.NumFields()
-	var fields [MaxStruct]*Value
+	fields := make([]*Value, 0, MaxStruct)
 	for i := 0; i < n; i++ {
-		fields[i] = v.Block.NewValue0(v.Pos, OpPhi, t.FieldType(i))
+		fields = append(fields, v.Block.NewValue0(v.Pos, OpPhi, t.FieldType(i)))
 	}
 	for _, a := range v.Args {
 		for i := 0; i < n; i++ {
@@ -373,10 +373,10 @@ func decomposeStructPhi(v *Value) {
 		}
 	}
 	v.reset(OpStructMake)
-	v.AddArgs(fields[:n]...)
+	v.AddArgs(fields...)
 
 	// Recursively decompose phis for each field.
-	for _, f := range fields[:n] {
+	for _, f := range fields {
 		decomposeUserPhi(f)
 	}
 }

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

@@ -566,7 +566,7 @@ func (x *expandState) rewriteSelectOrArg(pos src.XPos, b *Block, container, a, m
 			addArg(e)
 			pos = pos.WithNotStmt()
 		}
-		if at.NumFields() > 4 {
+		if at.NumFields() > MaxStruct && !types.IsDirectIface(at) {
 			panic(fmt.Errorf("Too many fields (%d, %d bytes), container=%s", at.NumFields(), at.Size(), container.LongString()))
 		}
 		a = makeOf(a, OpStructMake, args)

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

@@ -635,6 +635,10 @@ func CanSSA(t *types.Type) bool {
 		}
 		return false
 	case types.TSTRUCT:
+		if types.IsDirectIface(t) {
+			// Note: even if t.NumFields()>MaxStruct! See issue 77534.
+			return true
+		}
 		if t.NumFields() > MaxStruct {
 			return false
 		}

test/fixedbugs/issue77534.go

@@ -0,0 +1,36 @@
+// compile
+
+// Copyright 2026 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 77534: compiler crash when >4 fields, but only one nonempty pointer field.
+
+package p
+
+type T struct {
+	a, b, c, d struct{}
+	e          *byte
+}
+
+func f1(p *any, t T) {
+	*p = t
+}
+
+func f2(p *any, t *T) {
+	*p = *t
+}
+
+func f3(p, x, y *T, b bool) {
+	var z T
+	if b {
+		z = *x
+	} else {
+		z = *y
+	}
+	*p = z
+}
+
+func f4(i any) T {
+	return i.(T)
+}