cmd/compile: optimize append(x, make([]T, y)...) slice extension

Changes the compiler to recognize the slice extension pattern

  append(x, make([]T, y)...)

and replace it with growslice and an optional memclr to avoid an allocation for make([]T, y).

Memclr is not called in case growslice already allocated a new cleared backing array
when T contains pointers.

amd64:
name                      old time/op    new time/op    delta
ExtendSlice/IntSlice         103ns ± 4%      57ns ± 4%   -44.55%  (p=0.000 n=18+18)
ExtendSlice/PointerSlice     155ns ± 3%      77ns ± 3%   -49.93%  (p=0.000 n=20+20)
ExtendSlice/NoGrow          50.2ns ± 3%     5.2ns ± 2%   -89.67%  (p=0.000 n=18+18)

name                      old alloc/op   new alloc/op   delta
ExtendSlice/IntSlice         64.0B ± 0%     32.0B ± 0%   -50.00%  (p=0.000 n=20+20)
ExtendSlice/PointerSlice     64.0B ± 0%     32.0B ± 0%   -50.00%  (p=0.000 n=20+20)
ExtendSlice/NoGrow           32.0B ± 0%      0.0B       -100.00%  (p=0.000 n=20+20)

name                      old allocs/op  new allocs/op  delta
ExtendSlice/IntSlice          2.00 ± 0%      1.00 ± 0%   -50.00%  (p=0.000 n=20+20)
ExtendSlice/PointerSlice      2.00 ± 0%      1.00 ± 0%   -50.00%  (p=0.000 n=20+20)
ExtendSlice/NoGrow            1.00 ± 0%      0.00       -100.00%  (p=0.000 n=20+20)

Fixes #21266

Change-Id: Idc3077665f63cbe89762b590c5967a864fd1c07f
Reviewed-on: https://go-review.googlesource.com/109517
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

1 files changed, 12 insertions, 4 deletions

diff --git a/src/runtime/slice.go b/src/runtime/slice.go
index 40c5995153..fd5d08b52c 100644
--- a/src/runtime/slice.go
+++ b/src/runtime/slice.go
@@ -44,6 +44,14 @@ func maxSliceCap(elemsize uintptr) uintptr {
 	return maxAlloc / elemsize
 }
 
+func panicmakeslicelen() {
+	panic(errorString("makeslice: len out of range"))
+}
+
+func panicmakeslicecap() {
+	panic(errorString("makeslice: cap out of range"))
+}
+
 func makeslice(et *_type, len, cap int) slice {
 	// NOTE: The len > maxElements check here is not strictly necessary,
 	// but it produces a 'len out of range' error instead of a 'cap out of range' error
@@ -52,11 +60,11 @@ func makeslice(et *_type, len, cap int) slice {
 	// See issue 4085.
 	maxElements := maxSliceCap(et.size)
 	if len < 0 || uintptr(len) > maxElements {
-		panic(errorString("makeslice: len out of range"))
+		panicmakeslicelen()
 	}
 
 	if cap < len || uintptr(cap) > maxElements {
-		panic(errorString("makeslice: cap out of range"))
+		panicmakeslicecap()
 	}
 
 	p := mallocgc(et.size*uintptr(cap), et, true)
@@ -66,12 +74,12 @@ func makeslice(et *_type, len, cap int) slice {
 func makeslice64(et *_type, len64, cap64 int64) slice {
 	len := int(len64)
 	if int64(len) != len64 {
-		panic(errorString("makeslice: len out of range"))
+		panicmakeslicelen()
 	}
 
 	cap := int(cap64)
 	if int64(cap) != cap64 {
-		panic(errorString("makeslice: cap out of range"))
+		panicmakeslicecap()
 	}
 
 	return makeslice(et, len, cap)