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, 13 insertions, 13 deletions

diff --git a/test/escape_level.go b/test/escape_level.go
index 581e4a95cb..867c81ae52 100644
--- a/test/escape_level.go
+++ b/test/escape_level.go
@@ -27,18 +27,18 @@ func level1() {
 }
 
 func level2() {
-	i := 0    // ERROR "moved to heap: i"
-	p0 := &i  // ERROR "moved to heap: p0" "&i escapes to heap"
-	p1 := &p0 // ERROR "&p0 escapes to heap"
-	p2 := &p1 // ERROR "&p1 does not escape"
+	i := 0     // ERROR "moved to heap: i"
+	p0 := &i   // ERROR "moved to heap: p0" "&i escapes to heap"
+	p1 := &p0  // ERROR "&p0 escapes to heap"
+	p2 := &p1  // ERROR "&p1 does not escape"
 	sink = *p2 // ERROR "\*p2 escapes to heap"
 }
 
 func level3() {
-	i := 0    // ERROR "moved to heap: i"
-	p0 := &i  // ERROR "&i escapes to heap"
-	p1 := &p0 // ERROR "&p0 does not escape"
-	p2 := &p1 // ERROR "&p1 does not escape"
+	i := 0      // ERROR "moved to heap: i"
+	p0 := &i    // ERROR "&i escapes to heap"
+	p1 := &p0   // ERROR "&p0 does not escape"
+	p2 := &p1   // ERROR "&p1 does not escape"
 	sink = **p2 // ERROR "\* \(\*p2\) escapes to heap"
 }
 
@@ -67,10 +67,10 @@ func level6() {
 }
 
 func level7() {
-	i := 0     // ERROR "moved to heap: i"
-	p0 := &i   // ERROR "moved to heap: p0" "&i escapes to heap"
-	// BAD: p0 should not escape here
-	p1 := &p0  // ERROR "&p0 escapes to heap"
+	i := 0    // ERROR "moved to heap: i"
+	p0 := &i  // ERROR "&i escapes to heap"
+	p1 := &p0 // ERROR "&p0 does not escape"
+	// note *p1 == &i
 	p2 := *p1  // ERROR "moved to heap: p2"
 	sink = &p2 // ERROR "&p2 escapes to heap"
 }
@@ -95,7 +95,7 @@ func level10() {
 	i := 0
 	p0 := &i // ERROR "&i does not escape"
 	p1 := *p0
-	p2 := &p1 // ERROR "&p1 does not escape"
+	p2 := &p1  // ERROR "&p1 does not escape"
 	sink = *p2 // ERROR "\*p2 escapes to heap"
 }