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, 74 insertions, 0 deletions

diff --git a/test/escape_struct_return.go b/test/escape_struct_return.go
new file mode 100644
index 0000000000..b423ebd92a
--- /dev/null
+++ b/test/escape_struct_return.go
@@ -0,0 +1,74 @@
+// errorcheck -0 -m -l
+
+// Copyright 2015 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test escape analysis for function parameters.
+
+package foo
+
+var Ssink *string
+
+type U struct {
+	_sp  *string
+	_spp **string
+}
+
+func A(sp *string, spp **string) U { // ERROR "leaking param: sp to result ~r2 level=0$" "leaking param: spp to result ~r2 level=0$"
+	return U{sp, spp}
+}
+
+func B(spp **string) U { // ERROR "leaking param: spp to result ~r1 level=0$" "leaking param: spp to result ~r1 level=1$"
+	return U{*spp, spp}
+}
+
+func tA1() {
+	s := "cat"
+	sp := &s   // ERROR "tA1 &s does not escape$"
+	spp := &sp // ERROR "tA1 &sp does not escape$"
+	u := A(sp, spp)
+	_ = u
+	println(s)
+}
+
+func tA2() {
+	s := "cat"
+	sp := &s   // ERROR "tA2 &s does not escape$"
+	spp := &sp // ERROR "tA2 &sp does not escape$"
+	u := A(sp, spp)
+	println(*u._sp)
+}
+
+func tA3() {
+	s := "cat"
+	sp := &s   // ERROR "tA3 &s does not escape$"
+	spp := &sp // ERROR "tA3 &sp does not escape$"
+	u := A(sp, spp)
+	println(**u._spp)
+}
+
+func tB1() {
+	s := "cat"
+	sp := &s   // ERROR "tB1 &s does not escape$"
+	spp := &sp // ERROR "tB1 &sp does not escape$"
+	u := B(spp)
+	_ = u
+	println(s)
+}
+
+func tB2() {
+	s := "cat"
+	sp := &s   // ERROR "tB2 &s does not escape$"
+	spp := &sp // ERROR "tB2 &sp does not escape$"
+	u := B(spp)
+	println(*u._sp)
+}
+
+func tB3() {
+	s := "cat"
+	sp := &s   // ERROR "tB3 &s does not escape$"
+	spp := &sp // ERROR "tB3 &sp does not escape$"
+	u := B(spp)
+	println(**u._spp)
+}