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cmd/gc: fix &result escaping into result

There is a hierarchy of location defined by loop depth:

        -1 = the heap
        0 = function results
        1 = local variables (and parameters)
        2 = local variable declared inside a loop
        3 = local variable declared inside a loop inside a loop
        etc

In general if an address from loopdepth n is assigned to
something in loop depth m < n, that indicates an extended
lifetime of some form that requires a heap allocation.

Function results can be local variables too, though, and so
they don't actually fit into the hierarchy very well.
Treat the address of a function result as level 1 so that
if it is written back into a result, the address is treated
as escaping.

Fixes #8185.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/108870044
This commit is contained in:
Russ Cox 2014-06-11 14:21:06 -04:00
parent be91bc29a4
commit f20e4d5ecb
2 changed files with 23 additions and 2 deletions

View File

@ -673,12 +673,21 @@ esc(EscState *e, Node *n, Node *up)
// for &x, use loop depth of x if known.
// it should always be known, but if not, be conservative
// and keep the current loop depth.
if(n->left->op == ONAME && (n->left->escloopdepth != 0 || n->left->class == PPARAMOUT)) {
if(n->left->op == ONAME) {
switch(n->left->class) {
case PAUTO:
if(n->left->escloopdepth != 0)
n->escloopdepth = n->left->escloopdepth;
break;
case PPARAM:
case PPARAMOUT:
n->escloopdepth = n->left->escloopdepth;
// PPARAM is loop depth 1 always.
// PPARAMOUT is loop depth 0 for writes
// but considered loop depth 1 for address-of,
// so that writing the address of one result
// to another (or the same) result makes the
// first result move to the heap.
n->escloopdepth = 1;
break;
}
}

View File

@ -1478,3 +1478,15 @@ func foo153(v interface{}) *int { // ERROR "leaking param: v"
}
panic(0)
}
// issue 8185 - &result escaping into result
func f() (x int, y *int) { // ERROR "moved to heap: x"
y = &x // ERROR "&x escapes to heap"
return
}
func g() (x interface{}) { // ERROR "moved to heap: x"
x = &x // ERROR "&x escapes to heap"
return
}