From 21ea5103a4dab347baeb497675e0786424d6c974 Mon Sep 17 00:00:00 2001
From: Carl Shapiro <cshapiro@google.com>
Date: Mon, 19 Aug 2013 10:19:59 -0700
Subject: [PATCH] cmd/gc, runtime: use type information to scan interface
 values

R=golang-dev, rsc, dvyukov
CC=golang-dev
https://golang.org/cl/12785045
---
 src/cmd/gc/pgen.c      |  1 -
 src/pkg/runtime/mgc0.c | 54 +++++++++++++++++++++++++++++++++---------
 2 files changed, 43 insertions(+), 12 deletions(-)

diff --git a/src/cmd/gc/pgen.c b/src/cmd/gc/pgen.c
index f3916340678..d649fc49daf 100644
--- a/src/cmd/gc/pgen.c
+++ b/src/cmd/gc/pgen.c
@@ -257,7 +257,6 @@ walktype1(Type *t, vlong *xoffset, Bvec *bv)
 		bvset(bv, ((*xoffset / widthptr) * BitsPerPointer) + 1);
 		if(isnilinter(t))
 			bvset(bv, ((*xoffset / widthptr) * BitsPerPointer));
-		bvset(bv, ((*xoffset + widthptr) / widthptr) * BitsPerPointer);
 		*xoffset += t->width;
 		break;
 
diff --git a/src/pkg/runtime/mgc0.c b/src/pkg/runtime/mgc0.c
index 6af75ae4dfc..f7162da1f0c 100644
--- a/src/pkg/runtime/mgc0.c
+++ b/src/pkg/runtime/mgc0.c
@@ -36,6 +36,10 @@ enum {
 
 	// Pointer map
 	BitsPerPointer = 2,
+	BitsNoPointer = 0,
+	BitsPointer = 1,
+	BitsIface = 2,
+	BitsEface = 3,
 };
 
 // Bits in per-word bitmap.
@@ -1398,26 +1402,52 @@ struct BitVector
 	uint32 data[];
 };
 
+// Scans an interface data value when the interface type indicates
+// that it is a pointer.
+static void
+scaninterfacedata(uintptr bits, byte *scanp, bool inprologue)
+{
+	Itab *tab;
+	Type *type;
+
+	if(!inprologue) {
+		if(bits == BitsIface) {
+			tab = *(Itab**)scanp;
+			if(tab->type->size <= sizeof(void*) && (tab->type->kind & KindNoPointers))
+				return;
+		} else { // bits == BitsEface
+			type = *(Type**)scanp;
+			if(type->size <= sizeof(void*) && (type->kind & KindNoPointers))
+				return;
+		}
+	}
+	addroot((Obj){scanp+PtrSize, PtrSize, 0});
+}
+
 // Starting from scanp, scans words corresponding to set bits.
 static void
-scanbitvector(byte *scanp, BitVector *bv)
+scanbitvector(byte *scanp, BitVector *bv, bool inprologue)
 {
-	uint32 *wp;
-	uint32 w;
+	uintptr word, bits;
+	uint32 *wordp;
 	int32 i, remptrs;
 
-	wp = bv->data;
+	wordp = bv->data;
 	for(remptrs = bv->n; remptrs > 0; remptrs -= 32) {
-		w = *wp++;
+		word = *wordp++;
 		if(remptrs < 32)
 			i = remptrs;
 		else
 			i = 32;
 		i /= BitsPerPointer;
 		for(; i > 0; i--) {
-			if(w & 3)
-				addroot((Obj){scanp, PtrSize, 0});
-			w >>= BitsPerPointer;
+			bits = word & 3;
+			if(bits != BitsNoPointer && *(void**)scanp != nil)
+				if(bits == BitsPointer)
+					addroot((Obj){scanp, PtrSize, 0});
+				else
+					scaninterfacedata(bits, scanp, inprologue);
+			word >>= BitsPerPointer;
 			scanp += PtrSize;
 		}
 	}
@@ -1430,12 +1460,14 @@ addframeroots(Stkframe *frame, void*)
 	Func *f;
 	BitVector *args, *locals;
 	uintptr size;
+	bool afterprologue;
 
 	f = frame->fn;
 
 	// Scan local variables if stack frame has been allocated.
 	// Use pointer information if known.
-	if(frame->varp > (byte*)frame->sp) {
+	afterprologue = (frame->varp > (byte*)frame->sp);
+	if(afterprologue) {
 		locals = runtime·funcdata(f, FUNCDATA_GCLocals);
 		if(locals == nil) {
 			// No locals information, scan everything.
@@ -1450,7 +1482,7 @@ addframeroots(Stkframe *frame, void*)
 			// Locals bitmap information, scan just the
 			// pointers in locals.
 			size = (locals->n*PtrSize) / BitsPerPointer;
-			scanbitvector(frame->varp - size, locals);
+			scanbitvector(frame->varp - size, locals, false);
 		}
 	}
 
@@ -1458,7 +1490,7 @@ addframeroots(Stkframe *frame, void*)
 	// Use pointer information if known.
 	args = runtime·funcdata(f, FUNCDATA_GCArgs);
 	if(args != nil && args->n > 0)
-		scanbitvector(frame->argp, args);
+		scanbitvector(frame->argp, args, !afterprologue);
 	else
 		addroot((Obj){frame->argp, frame->arglen, 0});
 }