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, 56 insertions, 7 deletions

diff --git a/src/runtime/defer_test.go b/src/runtime/defer_test.go
index 0d3e8e9d63..d830fc591f 100644
--- a/src/runtime/defer_test.go
+++ b/src/runtime/defer_test.go
@@ -15,11 +15,11 @@ import (
 // unconditional panic (hence no return from the function)
 func TestUnconditionalPanic(t *testing.T) {
 	defer func() {
-		if recover() == nil {
+		if recover() != "testUnconditional" {
 			t.Fatal("expected unconditional panic")
 		}
 	}()
-	panic("panic should be recovered")
+	panic("testUnconditional")
 }
 
 var glob int = 3
@@ -30,7 +30,7 @@ func TestOpenAndNonOpenDefers(t *testing.T) {
 	for {
 		// Non-open defer because in a loop
 		defer func(n int) {
-			if recover() == nil {
+			if recover() != "testNonOpenDefer" {
 				t.Fatal("expected testNonOpen panic")
 			}
 		}(3)
@@ -45,7 +45,7 @@ func TestOpenAndNonOpenDefers(t *testing.T) {
 //go:noinline
 func testOpen(t *testing.T, arg int) {
 	defer func(n int) {
-		if recover() == nil {
+		if recover() != "testOpenDefer" {
 			t.Fatal("expected testOpen panic")
 		}
 	}(4)
@@ -61,7 +61,7 @@ func TestNonOpenAndOpenDefers(t *testing.T) {
 	for {
 		// Non-open defer because in a loop
 		defer func(n int) {
-			if recover() == nil {
+			if recover() != "testNonOpenDefer" {
 				t.Fatal("expected testNonOpen panic")
 			}
 		}(3)
@@ -80,7 +80,7 @@ func TestConditionalDefers(t *testing.T) {
 	list = make([]int, 0, 10)
 
 	defer func() {
-		if recover() == nil {
+		if recover() != "testConditional" {
 			t.Fatal("expected panic")
 		}
 		want := []int{4, 2, 1}
@@ -106,7 +106,7 @@ func testConditionalDefers(n int) {
 			defer doappend(4)
 		}
 	}
-	panic("test")
+	panic("testConditional")
 }
 
 // Test that there is no compile-time or run-time error if an open-coded defer
@@ -174,3 +174,52 @@ func TestRecoverMatching(t *testing.T) {
 	}()
 	panic("panic1")
 }
+
+type nonSSAable [128]byte
+
+type bigStruct struct {
+	x, y, z, w, p, q int64
+}
+
+func mknonSSAable() nonSSAable {
+	globint1++
+	return nonSSAable{0, 0, 0, 0, 5}
+}
+
+var globint1, globint2 int
+
+//go:noinline
+func sideeffect(n int64) int64 {
+	globint2++
+	return n
+}
+
+// Test that nonSSAable arguments to defer are handled correctly and only evaluated once.
+func TestNonSSAableArgs(t *testing.T) {
+	globint1 = 0
+	globint2 = 0
+	var save1 byte
+	var save2 int64
+
+	defer func() {
+		if globint1 != 1 {
+			t.Fatal(fmt.Sprintf("globint1:  wanted: 1, got %v", globint1))
+		}
+		if save1 != 5 {
+			t.Fatal(fmt.Sprintf("save1:  wanted: 5, got %v", save1))
+		}
+		if globint2 != 1 {
+			t.Fatal(fmt.Sprintf("globint2:  wanted: 1, got %v", globint2))
+		}
+		if save2 != 2 {
+			t.Fatal(fmt.Sprintf("save2:  wanted: 2, got %v", save2))
+		}
+	}()
+
+	defer func(n nonSSAable) {
+		save1 = n[4]
+	}(mknonSSAable())
+	defer func(b bigStruct) {
+		save2 = b.y
+	}(bigStruct{1, 2, 3, 4, 5, sideeffect(6)})
+}