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>

test/escape_calls.go

diff --git a/test/escape_calls.go b/test/escape_calls.go
new file mode 100644
index 0000000..f289670
--- /dev/null
+++ b/test/escape_calls.go
@@ -0,0 +1,44 @@
+// 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.
+
+// In this test almost everything is BAD except the simplest cases
+// where input directly flows to output.
+
+package foo
+
+func f(buf []byte) []byte { // ERROR "leaking param: buf to result ~r1 level=0$"
+	return buf
+}
+
+func g(*byte) string
+
+func h(e int) {
+	var x [32]byte // ERROR "moved to heap: x$"
+	g(&f(x[:])[0]) // ERROR "&f\(x\[:\]\)\[0\] escapes to heap$" "x escapes to heap$"
+}
+
+type Node struct {
+	s           string
+	left, right *Node
+}
+
+func walk(np **Node) int { // ERROR "leaking param content: np"
+	n := *np
+	w := len(n.s)
+	if n == nil {
+		return 0
+	}
+	wl := walk(&n.left)  // ERROR "walk &n.left does not escape"
+	wr := walk(&n.right) // ERROR "walk &n.right does not escape"
+	if wl < wr {
+		n.left, n.right = n.right, n.left
+		wl, wr = wr, wl
+	}
+	*np = n
+	return w + wl + wr
+}