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>

3 files changed, 9 insertions, 5 deletions

diff --git a/src/cmd/compile/internal/ssa/decompose.go b/src/cmd/compile/internal/ssa/decompose.go
index d2f715a453..a3d0fbd406 100644
--- a/src/cmd/compile/internal/ssa/decompose.go
+++ b/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)
 	}
 }
diff --git a/src/cmd/compile/internal/ssa/expand_calls.go b/src/cmd/compile/internal/ssa/expand_calls.go
index 1a2985d5af..ba2bedc65f 100644
--- a/src/cmd/compile/internal/ssa/expand_calls.go
+++ b/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)
diff --git a/src/cmd/compile/internal/ssa/value.go b/src/cmd/compile/internal/ssa/value.go
index 5f60409901..dba73de771 100644
--- a/src/cmd/compile/internal/ssa/value.go
+++ b/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
 		}