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runtime: redo stack map entries to avoid false retention

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Change two-bit stack map entries to encode:
0 = dead
1 = scalar
2 = pointer
3 = multiword

If multiword, the two-bit entry for the following word encodes:
0 = string
1 = slice
2 = iface
3 = eface

That way, during stack scanning we can check if a string
is zero length or a slice has zero capacity.  We can avoid
following the contained pointer in those cases.  It is safe
to do so because it can never be dereferenced, and it is
desirable to do so because it may cause false retention
of the following block in memory.

Slice feature turned off until issue 7564 is fixed.

Update #7549

LGTM=rsc
R=golang-codereviews, bradfitz, rsc
CC=golang-codereviews
https://golang.org/cl/76380043
This commit is contained in:
Keith Randall 2014-03-25 14:11:34 -07:00
parent 4ebfa83199
commit 1b45cc45e3
9 changed files with 218 additions and 37 deletions
Download patch file Download diff file Expand all files Collapse all files

58
src/pkg/runtime/stack.c
View file

@ -354,7 +354,11 @@ adjustpointers(byte **scanp, BitVector *bv, AdjustInfo *adjinfo, Func *f)
if(StackDebug >= 4)
runtime·printf(" %p:%s:%p\n", &scanp[i], mapnames[bv->data[i / (32 / BitsPerPointer)] >> (i * BitsPerPointer & 31) & 3], scanp[i]);
switch(bv->data[i / (32 / BitsPerPointer)] >> (i * BitsPerPointer & 31) & 3) {
case BitsNoPointer:
case BitsDead:
if(runtime·debug.gcdead)
scanp[i] = (byte*)0x6868686868686868LL;
break;
case BitsScalar:
break;
case BitsPointer:
p = scanp[i];
@ -370,33 +374,53 @@ adjustpointers(byte **scanp, BitVector *bv, AdjustInfo *adjinfo, Func *f)
scanp[i] = p + delta;
}
break;
case BitsEface:
t = (Type*)scanp[i];
if(t != nil && (t->size > PtrSize || (t->kind & KindNoPointers) == 0)) {
p = scanp[i+1];
case BitsMultiWord:
switch(bv->data[(i+1) / (32 / BitsPerPointer)] >> ((i+1) * BitsPerPointer & 31) & 3) {
case BitsString:
// string referents are never on the stack, never need to be adjusted
i++; // skip len
break;
case BitsSlice:
p = scanp[i];
if(minp <= p && p < maxp) {
if(StackDebug >= 3)
runtime·printf("adjust eface %p\n", p);
if(t->size > PtrSize) // currently we always allocate such objects on the heap
runtime·throw("large interface value found on stack");
scanp[i+1] = p + delta;
runtime·printf("adjust slice %p\n", p);
scanp[i] = p + delta;
}
}
break;
case BitsIface:
tab = (Itab*)scanp[i];
if(tab != nil) {
t = tab->type;
if(t->size > PtrSize || (t->kind & KindNoPointers) == 0) {
i += 2; // skip len, cap
break;
case BitsEface:
t = (Type*)scanp[i];
if(t != nil && (t->size > PtrSize || (t->kind & KindNoPointers) == 0)) {
p = scanp[i+1];
if(minp <= p && p < maxp) {
if(StackDebug >= 3)
runtime·printf("adjust iface %p\n", p);
runtime·printf("adjust eface %p\n", p);
if(t->size > PtrSize) // currently we always allocate such objects on the heap
runtime·throw("large interface value found on stack");
scanp[i+1] = p + delta;
}
}
i++;
break;
case BitsIface:
tab = (Itab*)scanp[i];
if(tab != nil) {
t = tab->type;
//runtime·printf(" type=%p\n", t);
if(t->size > PtrSize || (t->kind & KindNoPointers) == 0) {
p = scanp[i+1];
if(minp <= p && p < maxp) {
if(StackDebug >= 3)
runtime·printf("adjust iface %p\n", p);
if(t->size > PtrSize) // currently we always allocate such objects on the heap
runtime·throw("large interface value found on stack");
scanp[i+1] = p + delta;
}
}
}
i++;
break;
}
break;
}