diff --git a/src/cmd/compile/internal/gc/main.go b/src/cmd/compile/internal/gc/main.go
index 74590ccc39..e9d9e5566e 100644
--- a/src/cmd/compile/internal/gc/main.go
+++ b/src/cmd/compile/internal/gc/main.go
@@ -659,14 +659,6 @@ func Main(archInit func(*Arch)) {
 			Ctxt.DwFixups = nil
 			genDwarfInline = 0
 		}
-
-		// Check whether any of the functions we have compiled have gigantic stack frames.
-		obj.SortSlice(largeStackFrames, func(i, j int) bool {
-			return largeStackFrames[i].Before(largeStackFrames[j])
-		})
-		for _, largePos := range largeStackFrames {
-			yyerrorl(largePos, "stack frame too large (>1GB)")
-		}
 	}
 
 	// Phase 9: Check external declarations.
@@ -688,6 +680,14 @@ func Main(archInit func(*Arch)) {
 		dumpasmhdr()
 	}
 
+	// Check whether any of the functions we have compiled have gigantic stack frames.
+	obj.SortSlice(largeStackFrames, func(i, j int) bool {
+		return largeStackFrames[i].Before(largeStackFrames[j])
+	})
+	for _, largePos := range largeStackFrames {
+		yyerrorl(largePos, "stack frame too large (>1GB)")
+	}
+
 	if len(compilequeue) != 0 {
 		Fatalf("%d uncompiled functions", len(compilequeue))
 	}
diff --git a/src/cmd/compile/internal/gc/pgen.go b/src/cmd/compile/internal/gc/pgen.go
index 8f3947b0a6..cf1164772b 100644
--- a/src/cmd/compile/internal/gc/pgen.go
+++ b/src/cmd/compile/internal/gc/pgen.go
@@ -265,11 +265,24 @@ func compileSSA(fn *Node, worker int) {
 		return
 	}
 	pp := newProgs(fn, worker)
+	defer pp.Free()
 	genssa(f, pp)
+	// Check frame size again.
+	// The check above included only the space needed for local variables.
+	// After genssa, the space needed includes local variables and the callee arg region.
+	// We must do this check prior to calling pp.Flush.
+	// If there are any oversized stack frames,
+	// the assembler may emit inscrutable complaints about invalid instructions.
+	if pp.Text.To.Offset >= maxStackSize {
+		largeStackFramesMu.Lock()
+		largeStackFrames = append(largeStackFrames, fn.Pos)
+		largeStackFramesMu.Unlock()
+		return
+	}
+
 	pp.Flush() // assemble, fill in boilerplate, etc.
 	// fieldtrack must be called after pp.Flush. See issue 20014.
 	fieldtrack(pp.Text.From.Sym, fn.Func.FieldTrack)
-	pp.Free()
 }
 
 func init() {
diff --git a/src/go/types/stdlib_test.go b/src/go/types/stdlib_test.go
index dd1510d37e..229d203099 100644
--- a/src/go/types/stdlib_test.go
+++ b/src/go/types/stdlib_test.go
@@ -175,6 +175,7 @@ func TestStdFixed(t *testing.T) {
 		"issue22200.go",  // go/types does not have constraints on stack size
 		"issue22200b.go", // go/types does not have constraints on stack size
 		"issue25507.go",  // go/types does not have constraints on stack size
+		"issue20780.go",  // go/types does not have constraints on stack size
 	)
 }
 
diff --git a/test/fixedbugs/issue20780.go b/test/fixedbugs/issue20780.go
new file mode 100644
index 0000000000..a31e031b78
--- /dev/null
+++ b/test/fixedbugs/issue20780.go
@@ -0,0 +1,20 @@
+// errorcheck
+
+// Copyright 2018 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.
+
+// We have a limit of 1GB for stack frames.
+// Make sure we include the callee args section.
+// (The dispatch wrapper which implements (*S).f
+// copies the return value from f to a stack temp, then
+// from that stack temp to the return value of (*S).f.
+// It uses ~800MB for each section.)
+
+package main
+
+type S struct {
+	i interface {
+		f() [800e6]byte
+	}
+}