reflect: cache IsVariadic calls in Call

These calls are cachable, so do that in order to avoid doing extra work.

This opportunity was discovered while taking a look at a CPU profile while
investigating #7818.

I added a BenchmarkCallMethod which is similar to BechmarkCall but for a
method receiver.

Benchmark results, including the new BenchmarkCallMethod:

name                       old time/op    new time/op    delta
Call-16                      22.0ns ±19%    20.2ns ±17%  -8.08%  (p=0.000 n=40+40)
CallMethod-16                 100ns ± 3%      91ns ± 2%  -9.13%  (p=0.000 n=40+39)
CallArgCopy/size=128-16      15.7ns ± 1%    14.3ns ± 4%  -8.98%  (p=0.000 n=38+37)
CallArgCopy/size=256-16      15.9ns ± 3%    15.0ns ± 5%  -6.12%  (p=0.000 n=39+39)
CallArgCopy/size=1024-16     18.8ns ± 6%    17.1ns ± 6%  -9.03%  (p=0.000 n=38+38)
CallArgCopy/size=4096-16     26.6ns ± 3%    25.2ns ± 4%  -5.19%  (p=0.000 n=39+40)
CallArgCopy/size=65536-16     379ns ± 3%     371ns ± 5%  -2.11%  (p=0.000 n=39+40)

name                       old alloc/op   new alloc/op   delta
Call-16                       0.00B          0.00B         ~     (all equal)
CallMethod-16                 0.00B          0.00B         ~     (all equal)

name                       old allocs/op  new allocs/op  delta
Call-16                        0.00           0.00         ~     (all equal)
CallMethod-16                  0.00           0.00         ~     (all equal)

name                       old speed      new speed      delta
CallArgCopy/size=128-16    8.13GB/s ± 1%  8.92GB/s ± 4%  +9.77%  (p=0.000 n=38+38)
CallArgCopy/size=256-16    16.1GB/s ± 3%  17.1GB/s ± 5%  +6.56%  (p=0.000 n=39+39)
CallArgCopy/size=1024-16   54.6GB/s ± 6%  60.1GB/s ± 5%  +9.93%  (p=0.000 n=38+38)
CallArgCopy/size=4096-16    154GB/s ± 5%   163GB/s ± 4%  +5.63%  (p=0.000 n=40+40)
CallArgCopy/size=65536-16   173GB/s ± 3%   177GB/s ± 5%  +2.18%  (p=0.000 n=39+40)

Updates #7818.

Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com>

Author: jsign

src/reflect/value.go

@@ -375,8 +375,9 @@ func (v Value) call(op string, in []Value) []Value {
 
 	isSlice := op == "CallSlice"
 	n := t.NumIn()
+	isVariadic := t.IsVariadic()
 	if isSlice {
-		if !t.IsVariadic() {
+		if !isVariadic {
 			panic("reflect: CallSlice of non-variadic function")
 		}
 		if len(in) < n {
@@ -386,13 +387,13 @@ func (v Value) call(op string, in []Value) []Value {
 			panic("reflect: CallSlice with too many input arguments")
 		}
 	} else {
-		if t.IsVariadic() {
+		if isVariadic {
 			n--
 		}
 		if len(in) < n {
 			panic("reflect: Call with too few input arguments")
 		}
-		if !t.IsVariadic() && len(in) > n {
+		if !isVariadic && len(in) > n {
 			panic("reflect: Call with too many input arguments")
 		}
 	}
@@ -406,7 +407,7 @@ func (v Value) call(op string, in []Value) []Value {
 			panic("reflect: " + op + " using " + xt.String() + " as type " + targ.String())
 		}
 	}
-	if !isSlice && t.IsVariadic() {
+	if !isSlice && isVariadic {
 		// prepare slice for remaining values
 		m := len(in) - n
 		slice := MakeSlice(t.In(n), m, m)