cmd/compile, cmd/link, runtime: make defers low-cost through inline code and extra funcdata

Generate inline code at defer time to save the args of defer calls to unique
(autotmp) stack slots, and generate inline code at exit time to check which defer
calls were made and make the associated function/method/interface calls. We
remember that a particular defer statement was reached by storing in the deferBits
variable (always stored on the stack). At exit time, we check the bits of the
deferBits variable to determine which defer function calls to make (in reverse
order). These low-cost defers are only used for functions where no defers
appear in loops. In addition, we don't do these low-cost defers if there are too
many defer statements or too many exits in a function (to limit code increase).

When a function uses open-coded defers, we produce extra
FUNCDATA_OpenCodedDeferInfo information that specifies the number of defers, and
for each defer, the stack slots where the closure and associated args have been
stored. The funcdata also includes the location of the deferBits variable.
Therefore, for panics, we can use this funcdata to determine exactly which defers
are active, and call the appropriate functions/methods/closures with the correct
arguments for each active defer.

In order to unwind the stack correctly after a recover(), we need to add an extra
code segment to functions with open-coded defers that simply calls deferreturn()
and returns. This segment is not reachable by the normal function, but is returned
to by the runtime during recovery. We set the liveness information of this
deferreturn() to be the same as the liveness at the first function call during the
last defer exit code (so all return values and all stack slots needed by the defer
calls will be live).

I needed to increase the stackguard constant from 880 to 896, because of a small
amount of new code in deferreturn().

The -N flag disables open-coded defers. '-d defer' prints out the kind of defer
being used at each defer statement (heap-allocated, stack-allocated, or
open-coded).

Cost of defer statement  [ go test -run NONE -bench BenchmarkDefer$ runtime ]
  With normal (stack-allocated) defers only:         35.4  ns/op
  With open-coded defers:                             5.6  ns/op
  Cost of function call alone (remove defer keyword): 4.4  ns/op

Text size increase (including funcdata) for go cmd without/with open-coded defers:  0.09%

The average size increase (including funcdata) for only the functions that use
open-coded defers is 1.1%.

The cost of a panic followed by a recover got noticeably slower, since panic
processing now requires a scan of the stack for open-coded defer frames. This scan
is required, even if no frames are using open-coded defers:

Cost of panic and recover [ go test -run NONE -bench BenchmarkPanicRecover runtime ]
  Without open-coded defers:        62.0 ns/op
  With open-coded defers:           255  ns/op

A CGO Go-to-C-to-Go benchmark got noticeably faster because of open-coded defers:

CGO Go-to-C-to-Go benchmark [cd misc/cgo/test; go test -run NONE -bench BenchmarkCGoCallback ]
  Without open-coded defers:        443 ns/op
  With open-coded defers:           347 ns/op

Updates #14939 (defer performance)
Updates #34481 (design doc)

Change-Id: I51a389860b9676cfa1b84722f5fb84d3c4ee9e28
Reviewed-on: https://go-review.googlesource.com/c/go/+/190098
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 11 insertions, 7 deletions

diff --git a/test/live.go b/test/live.go
index b6e6d93f5f..32c397f4a9 100644
--- a/test/live.go
+++ b/test/live.go
@@ -367,16 +367,19 @@ func f24() {
 	m2[[2]string{"x", "y"}] = nil
 }
 
-// defer should not cause spurious ambiguously live variables
-
+// Non-open-coded defers should not cause autotmps.  (Open-coded defers do create extra autotmps).
 func f25(b bool) {
-	defer g25()
+	for i := 0; i < 2; i++ {
+		// Put in loop to make sure defer is not open-coded
+		defer g25()
+	}
 	if b {
 		return
 	}
 	var x string
 	x = g14()
 	printstring(x)
+	return
 }
 
 func g25()
@@ -417,7 +420,8 @@ func f27defer(b bool) {
 		defer call27(func() { x++ }) // ERROR "stack object .autotmp_[0-9]+ struct \{"
 	}
 	defer call27(func() { x++ }) // ERROR "stack object .autotmp_[0-9]+ struct \{"
-	printnl()
+	printnl()                    // ERROR "live at call to printnl: .autotmp_[0-9]+ .autotmp_[0-9]+"
+	return                       // ERROR "live at call to call27: .autotmp_[0-9]+"
 }
 
 // and newproc (go) escapes to the heap
@@ -687,12 +691,12 @@ type R struct{ *T } // ERRORAUTO "live at entry to \(\*R\)\.Foo: \.this ptr" "li
 // In particular, at printint r must be live.
 func f41(p, q *int) (r *int) { // ERROR "live at entry to f41: p q$"
 	r = p
-	defer func() { // ERROR "live at call to deferprocStack: q r$" "live at call to deferreturn: r$"
+	defer func() {
 		recover()
 	}()
-	printint(0) // ERROR "live at call to printint: q r$"
+	printint(0) // ERROR "live at call to printint: q r .autotmp_[0-9]+$"
 	r = q
-	return // ERROR "live at call to deferreturn: r$"
+	return // ERROR "live at call to f41.func1: r .autotmp_[0-9]+$"
 }
 
 func f42() {