cmd/internal/gc: improve flow of input params to output params

This includes the following information in the per-function summary:

outK = paramJ   encoded in outK bits for paramJ
outK = *paramJ  encoded in outK bits for paramJ
heap = paramJ   EscHeap
heap = *paramJ  EscContentEscapes

Note that (currently) if the address of a parameter is taken and
returned, necessarily a heap allocation occurred to contain that
reference, and the heap can never refer to stack, therefore the
parameter and everything downstream from it escapes to the heap.

The per-function summary information now has a tuneable number of bits
(2 is probably noticeably better than 1, 3 is likely overkill, but it
is now easy to check and the -m debugging output includes information
that allows you to figure out if more would be better.)

A new test was  added to check pointer flow through struct-typed and
*struct-typed parameters and returns; some of these are sensitive to
the number of summary bits, and ought to yield better results with a
more competent escape analysis algorithm.  Another new test checks
(some) correctness with array parameters, results, and operations.

The old analysis inferred a piece of plan9 runtime was non-escaping by
counteracting overconservative analysis with buggy analysis; with the
bug fixed, the result was too conservative (and it's not easy to fix
in this framework) so the source code was tweaked to get the desired
result.  A test was added against the discovered bug.

The escape analysis was further improved splitting the "level" into
3 parts, one tracking the conventional "level" and the other two
computing the highest-level-suffix-from-copy, which is used to
generally model the cancelling effect of indirection applied to
address-of.

With the improved escape analysis enabled, it was necessary to
modify one of the runtime tests because it now attempts to allocate
too much on the (small, fixed-size) G0 (system) stack and this
failed the test.

Compiling src/std after touching src/runtime/*.go with -m logging
turned on shows 420 fewer heap allocation sites (10538 vs 10968).

Profiling allocations in src/html/template with
for i in {1..5} ;
  do go tool 6g -memprofile=mastx.${i}.prof  -memprofilerate=1 *.go;
  go tool pprof -alloc_objects -text  mastx.${i}.prof ;
done

showed a 15% reduction in allocations performed by the compiler.

Update #3753
Update #4720
Fixes #10466

Change-Id: I0fd97d5f5ac527b45f49e2218d158a6e89951432
Reviewed-on: https://go-review.googlesource.com/8202
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

1 files changed, 19 insertions, 20 deletions

diff --git a/test/escape_field.go b/test/escape_field.go
index dcf8a31d2b..16d1e741b8 100644
--- a/test/escape_field.go
+++ b/test/escape_field.go
@@ -23,7 +23,7 @@ type Y struct {
 func field0() {
 	i := 0 // ERROR "moved to heap: i$"
 	var x X
-	x.p1 = &i // ERROR "&i escapes to heap$"
+	x.p1 = &i   // ERROR "&i escapes to heap$"
 	sink = x.p1 // ERROR "x\.p1 escapes to heap"
 }
 
@@ -31,7 +31,7 @@ func field1() {
 	i := 0 // ERROR "moved to heap: i$"
 	var x X
 	// BAD: &i should not escape
-	x.p1 = &i // ERROR "&i escapes to heap$"
+	x.p1 = &i   // ERROR "&i escapes to heap$"
 	sink = x.p2 // ERROR "x\.p2 escapes to heap"
 }
 
@@ -39,7 +39,7 @@ func field3() {
 	i := 0 // ERROR "moved to heap: i$"
 	var x X
 	x.p1 = &i // ERROR "&i escapes to heap$"
-	sink = x // ERROR "x escapes to heap"
+	sink = x  // ERROR "x escapes to heap"
 }
 
 func field4() {
@@ -54,22 +54,21 @@ func field5() {
 	i := 0 // ERROR "moved to heap: i$"
 	var x X
 	// BAD: &i should not escape here
-	x.a[0] = &i // ERROR "&i escapes to heap$"
+	x.a[0] = &i   // ERROR "&i escapes to heap$"
 	sink = x.a[1] // ERROR "x\.a\[1\] escapes to heap"
 }
 
 // BAD: we are not leaking param x, only x.p2
-func field6(x *X) { // ERROR "leaking param: x$"
+func field6(x *X) { // ERROR "leaking param content: x$"
 	sink = x.p2 // ERROR "x\.p2 escapes to heap"
 }
 
 func field6a() {
-	i := 0  // ERROR "moved to heap: i$"
-	var x X // ERROR "moved to heap: x$"
+	i := 0 // ERROR "moved to heap: i$"
+	var x X
 	// BAD: &i should not escape
-	x.p1 = &i // ERROR "&i escapes to heap$"
-	// BAD: &x should not escape
-	field6(&x) // ERROR "&x escapes to heap$"
+	x.p1 = &i  // ERROR "&i escapes to heap$"
+	field6(&x) // ERROR "field6a &x does not escape"
 }
 
 func field7() {
@@ -116,40 +115,40 @@ func field10() {
 func field11() {
 	i := 0         // ERROR "moved to heap: i$"
 	x := X{p1: &i} // ERROR "&i escapes to heap$"
-	sink = x.p1 // ERROR "x\.p1 escapes to heap"
+	sink = x.p1    // ERROR "x\.p1 escapes to heap"
 }
 
 func field12() {
 	i := 0 // ERROR "moved to heap: i$"
 	// BAD: &i should not escape
 	x := X{p1: &i} // ERROR "&i escapes to heap$"
-	sink = x.p2 // ERROR "x\.p2 escapes to heap"
+	sink = x.p2    // ERROR "x\.p2 escapes to heap"
 }
 
 func field13() {
 	i := 0          // ERROR "moved to heap: i$"
 	x := &X{p1: &i} // ERROR "&i escapes to heap$" "field13 &X literal does not escape$"
-	sink = x.p1 // ERROR "x\.p1 escapes to heap"
+	sink = x.p1     // ERROR "x\.p1 escapes to heap"
 }
 
 func field14() {
 	i := 0 // ERROR "moved to heap: i$"
 	// BAD: &i should not escape
 	x := &X{p1: &i} // ERROR "&i escapes to heap$" "field14 &X literal does not escape$"
-	sink = x.p2 // ERROR "x\.p2 escapes to heap"
+	sink = x.p2     // ERROR "x\.p2 escapes to heap"
 }
 
 func field15() {
 	i := 0          // ERROR "moved to heap: i$"
 	x := &X{p1: &i} // ERROR "&X literal escapes to heap$" "&i escapes to heap$"
-	sink = x // ERROR "x escapes to heap"
+	sink = x        // ERROR "x escapes to heap"
 }
 
 func field16() {
 	i := 0 // ERROR "moved to heap: i$"
 	var x X
 	// BAD: &i should not escape
-	x.p1 = &i // ERROR "&i escapes to heap$"
+	x.p1 = &i                 // ERROR "&i escapes to heap$"
 	var iface interface{} = x // ERROR "x escapes to heap"
 	x1 := iface.(X)
 	sink = x1.p2 // ERROR "x1\.p2 escapes to heap"
@@ -158,7 +157,7 @@ func field16() {
 func field17() {
 	i := 0 // ERROR "moved to heap: i$"
 	var x X
-	x.p1 = &i // ERROR "&i escapes to heap$"
+	x.p1 = &i                 // ERROR "&i escapes to heap$"
 	var iface interface{} = x // ERROR "x escapes to heap"
 	x1 := iface.(X)
 	sink = x1.p1 // ERROR "x1\.p1 escapes to heap"
@@ -168,8 +167,8 @@ func field18() {
 	i := 0 // ERROR "moved to heap: i$"
 	var x X
 	// BAD: &i should not escape
-	x.p1 = &i // ERROR "&i escapes to heap$"
+	x.p1 = &i                 // ERROR "&i escapes to heap$"
 	var iface interface{} = x // ERROR "x escapes to heap"
-	y, _ := iface.(Y) // Put X, but extracted Y. The cast will fail, so y is zero initialized.
-	sink = y // ERROR "y escapes to heap"
+	y, _ := iface.(Y)         // Put X, but extracted Y. The cast will fail, so y is zero initialized.
+	sink = y                  // ERROR "y escapes to heap"
 }