cmd/compile/internal/escape: optimize indirect closure calls

This CL extends escape analysis in two ways.

First, we already optimize directly called closures. For example,
given:

	var x int  // already stack allocated today
	p := func() *int { return &x }()

we don't need to move x to the heap, because we can statically track
where &x flows. This CL extends the same idea to work for indirectly
called closures too, as long as we know everywhere that they're
called. For example:

	var x int  // stack allocated after this CL
	f := func() *int { return &x }
	p := f()

This will allow a subsequent CL to move the generation of go/defer
wrappers earlier.

Second, this CL adds tracking to detect when pointer values flow to
the pointee operand of an indirect assignment statement (i.e., flows
to p in "*p = x") or to builtins that modify memory (append, copy,
clear). This isn't utilized in the current CL, but a subsequent CL
will make use of it to better optimize string->[]byte conversions.

Updates #2205.

Change-Id: I610f9c531e135129c947684833e288ce64406f35
Reviewed-on: https://go-review.googlesource.com/c/go/+/520259
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/test/inline_big.go b/test/inline_big.go
index 7dd1abdb6a..f579fc0910 100644
--- a/test/inline_big.go
+++ b/test/inline_big.go
@@ -9,18 +9,18 @@
 
 package foo
 
-func small(a []int) int { // ERROR "can inline small with cost .* as:.*" "a does not escape"
+func small(a []int) int { // ERROR "can inline small with cost .* as:.*" "a does not escape" "does not mutate param: a" "does not call param: a"
 	// Cost 16 body (need cost < 20).
 	// See cmd/compile/internal/gc/inl.go:inlineBigFunction*
 	return a[0] + a[1] + a[2] + a[3]
 }
-func medium(a []int) int { // ERROR "can inline medium with cost .* as:.*" "a does not escape"
+func medium(a []int) int { // ERROR "can inline medium with cost .* as:.*" "a does not escape" "does not mutate param: a" "does not call param: a"
 	// Cost 32 body (need cost > 20 and cost < 80).
 	// See cmd/compile/internal/gc/inl.go:inlineBigFunction*
 	return a[0] + a[1] + a[2] + a[3] + a[4] + a[5] + a[6] + a[7]
 }
 
-func f(a []int) int { // ERROR "cannot inline f:.*" "a does not escape" "function f considered 'big'"
+func f(a []int) int { // ERROR "cannot inline f:.*" "a does not escape" "function f considered 'big'" "mutates param: a derefs=0" "does not call param: a"
 	// Add lots of nodes to f's body. We need >5000.
 	// See cmd/compile/internal/gc/inl.go:inlineBigFunction*
 	a[0] = 0