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>
diff --git a/test/escape_array.go b/test/escape_array.go
new file mode 100644
index 0000000..ac51fe7
--- /dev/null
+++ b/test/escape_array.go
@@ -0,0 +1,72 @@
+// 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
+
+var Ssink *string
+
+type U [2]*string
+
+func bar(a, b *string) U { // ERROR "leaking param: a to result ~r2 level=0$" "leaking param: b to result ~r2 level=0$"
+ return U{a, b}
+}
+
+func foo(x U) U { // ERROR "leaking param: x to result ~r1 level=0$"
+ return U{x[1], x[0]}
+}
+
+func bff(a, b *string) U { // ERROR "leaking param: a to result ~r2 level=0$" "leaking param: b to result ~r2 level=0$"
+ return foo(foo(bar(a, b)))
+}
+
+func tbff1() *string {
+ a := "cat"
+ b := "dog" // ERROR "moved to heap: b$"
+ u := bff(&a, &b) // ERROR "tbff1 &a does not escape$" "tbff1 &b does not escape$"
+ _ = u[0]
+ return &b // ERROR "&b escapes to heap$"
+}
+
+// BAD: need fine-grained analysis to track u[0] and u[1] differently.
+func tbff2() *string {
+ a := "cat" // ERROR "moved to heap: a$"
+ b := "dog" // ERROR "moved to heap: b$"
+ u := bff(&a, &b) // ERROR "&a escapes to heap$" "&b escapes to heap$"
+ _ = u[0]
+ return u[1]
+}
+
+func car(x U) *string { // ERROR "leaking param: x to result ~r1 level=0$"
+ return x[0]
+}
+
+// BAD: need fine-grained analysis to track x[0] and x[1] differently.
+func fun(x U, y *string) *string { // ERROR "leaking param: x to result ~r2 level=0$" "leaking param: y to result ~r2 level=0$"
+ x[0] = y
+ return x[1]
+}
+
+func fup(x *U, y *string) *string { // ERROR "leaking param: x to result ~r2 level=1$" "leaking param: y$"
+ x[0] = y // leaking y to heap is intended
+ return x[1]
+}
+
+// BAD: would be nice to record that *y (content) is what leaks, not y itself
+func fum(x *U, y **string) *string { // ERROR "leaking param: x to result ~r2 level=1$" "leaking param content: y$"
+ x[0] = *y
+ return x[1]
+}
+
+// BAD: would be nice to record that y[0] (content) is what leaks, not y itself
+func fuo(x *U, y *U) *string { // ERROR "leaking param: x to result ~r2 level=1$" "leaking param content: y$"
+ x[0] = y[0]
+ return x[1]
+}
