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, 10 insertions, 3 deletions

diff --git a/test/escape_indir.go b/test/escape_indir.go
index 7c06ceb5f8..fe03c3fb1b 100644
--- a/test/escape_indir.go
+++ b/test/escape_indir.go
@@ -54,14 +54,14 @@ func constptr1() {
 	i := 0           // ERROR "moved to heap: i"
 	x := &ConstPtr{} // ERROR "&ConstPtr literal escapes to heap"
 	x.p = &i         // ERROR "&i escapes to heap"
-	sink = x // ERROR "x escapes to heap"
+	sink = x         // ERROR "x escapes to heap"
 }
 
 func constptr2() {
 	i := 0           // ERROR "moved to heap: i"
 	x := &ConstPtr{} // ERROR "&ConstPtr literal does not escape"
 	x.p = &i         // ERROR "&i escapes to heap"
-	sink = *x// ERROR "\*x escapes to heap"
+	sink = *x        // ERROR "\*x escapes to heap"
 }
 
 func constptr4() *ConstPtr {
@@ -78,7 +78,7 @@ func constptr5() *ConstPtr {
 }
 
 // BAD: p should not escape here
-func constptr6(p *ConstPtr) { // ERROR "leaking param: p"
+func constptr6(p *ConstPtr) { // ERROR "leaking param content: p"
 	p1 := &ConstPtr{} // ERROR "&ConstPtr literal does not escape"
 	*p1 = *p
 	_ = p1
@@ -151,3 +151,10 @@ func foo2() {
 	i := 0  // ERROR "moved to heap: i"
 	*p = &i // ERROR "&i escapes to heap"
 }
+
+var global *byte
+
+func f() {
+	var x byte    // ERROR "moved to heap: x"
+	global = &*&x // ERROR "&\(\*\(&x\)\) escapes to heap" "&x escapes to heap"
+}