cmd/compile: ignore some dead code during escape analysis

This is the escape analysis analog of CL 37499.

Fixes #12397
Fixes #16871

The only "moved to heap" decisions eliminated by this
CL in std+cmd are:

cmd/compile/internal/gc/const.go:1514: moved to heap: ac
cmd/compile/internal/gc/const.go:1515: moved to heap: bd
cmd/compile/internal/gc/const.go:1516: moved to heap: bc
cmd/compile/internal/gc/const.go:1517: moved to heap: ad
cmd/compile/internal/gc/const.go:1546: moved to heap: ac
cmd/compile/internal/gc/const.go:1547: moved to heap: bd
cmd/compile/internal/gc/const.go:1548: moved to heap: bc
cmd/compile/internal/gc/const.go:1549: moved to heap: ad
cmd/compile/internal/gc/const.go:1550: moved to heap: cc_plus
cmd/compile/internal/gc/export.go:162: moved to heap: copy
cmd/compile/internal/gc/mpfloat.go:66: moved to heap: b
cmd/compile/internal/gc/mpfloat.go:97: moved to heap: b

Change-Id: I0d420b69c84a41ba9968c394e8957910bab5edea
Reviewed-on: https://go-review.googlesource.com/37508
Reviewed-by: David Chase <drchase@google.com>

diff --git a/src/cmd/compile/internal/gc/esc.go b/src/cmd/compile/internal/gc/esc.go
index 1b29aebcc46a53f78ccb6e70d2b1a23950c78770..dcfab54a09b7913a5555c89e9383d0d138ae43c0 100644 (file)
--- a/src/cmd/compile/internal/gc/esc.go
+++ b/src/cmd/compile/internal/gc/esc.go
@@ -685,11 +685,20 @@ func (e *EscState) esc(n *Node, parent *Node) {
                e.escassignSinkWhy(n, n, "too large for stack") // TODO category: tooLarge
        }
 
-       e.esc(n.Left, n)
-       e.esc(n.Right, n)
-       e.esclist(n.Nbody, n)
-       e.esclist(n.List, n)
-       e.esclist(n.Rlist, n)
+       if n.Op == OIF && Isconst(n.Left, CTBOOL) {
+               // Don't examine dead code.
+               if n.Left.Bool() {
+                       e.esclist(n.Nbody, n)
+               } else {
+                       e.esclist(n.Rlist, n)
+               }
+       } else {
+               e.esc(n.Left, n)
+               e.esc(n.Right, n)
+               e.esclist(n.Nbody, n)
+               e.esclist(n.List, n)
+               e.esclist(n.Rlist, n)
+       }
 
        if n.Op == OFOR || n.Op == ORANGE {
                e.loopdepth--

diff --git a/test/escape2.go b/test/escape2.go
index 3490c29d3bf6a3b2c3718887609bd457e6368fba..e10dbc2accbd36a95335651e500c4abf8a390c10 100644 (file)
--- a/test/escape2.go
+++ b/test/escape2.go
@@ -1824,3 +1824,18 @@ func issue11387(x int) func() int {
        copy(slice2, slice1)
        return slice2[0]
 }
+
+func issue12397(x, y int) { // ERROR "moved to heap: y$"
+       // x does not escape below, because all relevant code is dead.
+       if false {
+               gxx = &x
+       } else {
+               gxx = &y // ERROR "&y escapes to heap$"
+       }
+
+       if true {
+               gxx = &y // ERROR "&y escapes to heap$"
+       } else {
+               gxx = &x
+       }
+}