cmd/gc: capture variables by value

Language specification says that variables are captured by reference.
And that is what gc compiler does. However, in lots of cases it is
possible to capture variables by value under the hood without
affecting visible behavior of programs. For example, consider
the following typical pattern:

	func (o *Obj) requestMany(urls []string) []Result {
		wg := new(sync.WaitGroup)
		wg.Add(len(urls))
		res := make([]Result, len(urls))
		for i := range urls {
			i := i
			go func() {
				res[i] = o.requestOne(urls[i])
				wg.Done()
			}()
		}
		wg.Wait()
		return res
	}

Currently o, wg, res, and i are captured by reference causing 3+len(urls)
allocations (e.g. PPARAM o is promoted to PPARAMREF and moved to heap).
But all of them can be captured by value without changing behavior.

This change implements simple strategy for capturing by value:
if a captured variable is not addrtaken and never assigned to,
then it is captured by value (it is effectively const).
This simple strategy turned out to be very effective:
~80% of all captures in std lib are turned into value captures.
The remaining 20% are mostly in defers and non-escaping closures,
that is, they do not cause allocations anyway.

benchmark                                    old allocs     new allocs     delta
BenchmarkCompressedZipGarbage                153            126            -17.65%
BenchmarkEncodeDigitsSpeed1e4                91             69             -24.18%
BenchmarkEncodeDigitsSpeed1e5                178            129            -27.53%
BenchmarkEncodeDigitsSpeed1e6                1510           1051           -30.40%
BenchmarkEncodeDigitsDefault1e4              100            75             -25.00%
BenchmarkEncodeDigitsDefault1e5              193            139            -27.98%
BenchmarkEncodeDigitsDefault1e6              1420           985            -30.63%
BenchmarkEncodeDigitsCompress1e4             100            75             -25.00%
BenchmarkEncodeDigitsCompress1e5             193            139            -27.98%
BenchmarkEncodeDigitsCompress1e6             1420           985            -30.63%
BenchmarkEncodeTwainSpeed1e4                 109            81             -25.69%
BenchmarkEncodeTwainSpeed1e5                 211            151            -28.44%
BenchmarkEncodeTwainSpeed1e6                 1588           1097           -30.92%
BenchmarkEncodeTwainDefault1e4               103            77             -25.24%
BenchmarkEncodeTwainDefault1e5               199            143            -28.14%
BenchmarkEncodeTwainDefault1e6               1324           917            -30.74%
BenchmarkEncodeTwainCompress1e4              103            77             -25.24%
BenchmarkEncodeTwainCompress1e5              190            137            -27.89%
BenchmarkEncodeTwainCompress1e6              1327           919            -30.75%
BenchmarkConcurrentDBExec                    16223          16220          -0.02%
BenchmarkConcurrentStmtQuery                 17687          16182          -8.51%
BenchmarkConcurrentStmtExec                  5191           5186           -0.10%
BenchmarkConcurrentTxQuery                   17665          17661          -0.02%
BenchmarkConcurrentTxExec                    15154          15150          -0.03%
BenchmarkConcurrentTxStmtQuery               17661          16157          -8.52%
BenchmarkConcurrentTxStmtExec                3677           3673           -0.11%
BenchmarkConcurrentRandom                    14000          13614          -2.76%
BenchmarkManyConcurrentQueries               25             22             -12.00%
BenchmarkDecodeComplex128Slice               318            252            -20.75%
BenchmarkDecodeFloat64Slice                  318            252            -20.75%
BenchmarkDecodeInt32Slice                    318            252            -20.75%
BenchmarkDecodeStringSlice                   2318           2252           -2.85%
BenchmarkDecode                              11             8              -27.27%
BenchmarkEncodeGray                          64             56             -12.50%
BenchmarkEncodeNRGBOpaque                    64             56             -12.50%
BenchmarkEncodeNRGBA                         67             58             -13.43%
BenchmarkEncodePaletted                      68             60             -11.76%
BenchmarkEncodeRGBOpaque                     64             56             -12.50%
BenchmarkGoLookupIP                          153            139            -9.15%
BenchmarkGoLookupIPNoSuchHost                508            466            -8.27%
BenchmarkGoLookupIPWithBrokenNameServer      245            226            -7.76%
BenchmarkClientServer                        62             59             -4.84%
BenchmarkClientServerParallel4               62             59             -4.84%
BenchmarkClientServerParallel64              62             59             -4.84%
BenchmarkClientServerParallelTLS4            79             76             -3.80%
BenchmarkClientServerParallelTLS64           112            109            -2.68%
BenchmarkCreateGoroutinesCapture             10             6              -40.00%
BenchmarkAfterFunc                           1006           1005           -0.10%

Fixes #6632.

Change-Id: I0cd51e4d356331d7f3c5f447669080cd19b0d2ca
Reviewed-on: https://go-review.googlesource.com/3166
Reviewed-by: Russ Cox <rsc@golang.org>

1 files changed, 68 insertions, 16 deletions

diff --git a/test/escape2.go b/test/escape2.go
index 1523d9f1ff..8c50277e9d 100644
--- a/test/escape2.go
+++ b/test/escape2.go
@@ -203,9 +203,17 @@ func (b *Bar2) LeakSelf2() { // ERROR "leaking param: b"
 }
 
 func foo21() func() int {
+	x := 42
+	return func() int { // ERROR "func literal escapes to heap"
+		return x
+	}
+}
+
+func foo21a() func() int {
 	x := 42             // ERROR "moved to heap: x"
 	return func() int { // ERROR "func literal escapes to heap"
-		return x // ERROR "&x escapes to heap"
+		x++  // ERROR "&x escapes to heap"
+		return x
 	}
 }
 
@@ -216,24 +224,31 @@ func foo22() int {
 	}()
 }
 
-func foo23(x int) func() int { // ERROR "moved to heap: x"
+func foo23(x int) func() int {
 	return func() int { // ERROR "func literal escapes to heap"
-		return x // ERROR "&x escapes to heap"
+		return x
 	}
 }
 
-func foo23a(x int) func() int { // ERROR "moved to heap: x"
+func foo23a(x int) func() int {
 	f := func() int { // ERROR "func literal escapes to heap"
-		return x // ERROR "&x escapes to heap"
+		return x
 	}
 	return f
 }
 
-func foo23b(x int) *(func() int) { // ERROR "moved to heap: x"
-	f := func() int { return x } // ERROR "moved to heap: f" "func literal escapes to heap" "&x escapes to heap"
+func foo23b(x int) *(func() int) {
+	f := func() int { return x } // ERROR "moved to heap: f" "func literal escapes to heap"
 	return &f                    // ERROR "&f escapes to heap"
 }
 
+func foo23c(x int) func() int { // ERROR "moved to heap: x"
+	return func() int { // ERROR "func literal escapes to heap"
+		x++ // ERROR "&x escapes to heap"
+		return x
+	}
+}
+
 func foo24(x int) int {
 	return func() int { // ERROR "func literal does not escape"
 		return x
@@ -525,10 +540,22 @@ func foo72b() [10]*int {
 func foo73() {
 	s := []int{3, 2, 1} // ERROR "\[\]int literal does not escape"
 	for _, v := range s {
+		vv := v
+		// actually just escapes its scope
+		defer func() { // ERROR "func literal escapes to heap"
+			println(vv)
+		}()
+	}
+}
+
+func foo731() {
+	s := []int{3, 2, 1} // ERROR "\[\]int literal does not escape"
+	for _, v := range s {
 		vv := v // ERROR "moved to heap: vv"
 		// actually just escapes its scope
 		defer func() { // ERROR "func literal escapes to heap"
-			println(vv) // ERROR "&vv escapes to heap"
+			vv = 42 // ERROR "&vv escapes to heap"
+			println(vv)
 		}()
 	}
 }
@@ -536,10 +563,23 @@ func foo73() {
 func foo74() {
 	s := []int{3, 2, 1} // ERROR "\[\]int literal does not escape"
 	for _, v := range s {
+		vv := v
+		// actually just escapes its scope
+		fn := func() { // ERROR "func literal escapes to heap"
+			println(vv)
+		}
+		defer fn()
+	}
+}
+
+func foo74a() {
+	s := []int{3, 2, 1} // ERROR "\[\]int literal does not escape"
+	for _, v := range s {
 		vv := v // ERROR "moved to heap: vv"
 		// actually just escapes its scope
 		fn := func() { // ERROR "func literal escapes to heap"
-			println(vv) // ERROR "&vv escapes to heap"
+			vv += 1 // ERROR "&vv escapes to heap"
+			println(vv)
 		}
 		defer fn()
 	}
@@ -550,10 +590,22 @@ func foo74b() {
 	var array [3]func()
 	s := []int{3, 2, 1} // ERROR "\[\]int literal does not escape"
 	for i, v := range s {
+		vv := v
+		// actually just escapes its scope
+		array[i] = func() { // ERROR "func literal escapes to heap"
+			println(vv)
+		}
+	}
+}
+
+func foo74c() {
+	var array [3]func()
+	s := []int{3, 2, 1} // ERROR "\[\]int literal does not escape"
+	for i, v := range s {
 		vv := v // ERROR "moved to heap: vv"
 		// actually just escapes its scope
 		array[i] = func() { // ERROR "func literal escapes to heap"
-			println(vv) // ERROR "&vv escapes to heap"
+			println(&vv) // ERROR "&vv escapes to heap" "&vv does not escape"
 		}
 	}
 }
@@ -1213,9 +1265,9 @@ func foo134() {
 
 func foo135() {
 	var i int   // ERROR "moved to heap: i"
-	p := &i     // ERROR "&i escapes to heap" "moved to heap: p"
+	p := &i     // ERROR "&i escapes to heap"
 	go func() { // ERROR "func literal escapes to heap"
-		q := p   // ERROR "&p escapes to heap"
+		q := p
 		func() { // ERROR "func literal does not escape"
 			r := q
 			_ = r
@@ -1225,9 +1277,9 @@ func foo135() {
 
 func foo136() {
 	var i int   // ERROR "moved to heap: i"
-	p := &i     // ERROR "&i escapes to heap" "moved to heap: p"
+	p := &i     // ERROR "&i escapes to heap"
 	go func() { // ERROR "func literal escapes to heap"
-		q := p   // ERROR "&p escapes to heap" "leaking closure reference p"
+		q := p   // ERROR "leaking closure reference p"
 		func() { // ERROR "func literal does not escape"
 			r := q // ERROR "leaking closure reference q"
 			px = r
@@ -1239,9 +1291,9 @@ func foo137() {
 	var i int // ERROR "moved to heap: i"
 	p := &i   // ERROR "&i escapes to heap"
 	func() {  // ERROR "func literal does not escape"
-		q := p      // ERROR "leaking closure reference p" "moved to heap: q"
+		q := p      // ERROR "leaking closure reference p"
 		go func() { // ERROR "func literal escapes to heap"
-			r := q // ERROR "&q escapes to heap"
+			r := q
 			_ = r
 		}()
 	}()