From 98b6c6aca6fd4185f97dc40137707f4d8df8aa7c Mon Sep 17 00:00:00 2001
From: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Date: Wed, 8 Apr 2020 03:08:21 +0700
Subject: [PATCH] cmd/compile: do not allocate bucket for non-escaping map
MIME-Version: 1.0
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For map with hint larger than BUCKETSIZE, makemap ignore allocated
bucket and allocate buckets itself. So do not allocate bucket in
this case, save us the cost of zeroing+assignment to the bucket.

name                            old time/op    new time/op    delta
NewEmptyMap-12                    3.89ns ± 4%    3.88ns ± 2%    ~     (p=0.939 n=19+20)
NewSmallMap-12                    23.3ns ± 3%    23.1ns ± 2%    ~     (p=0.307 n=18+17)
NewEmptyMapHintLessThan8-12       6.43ns ± 3%    6.31ns ± 2%  -1.72%  (p=0.000 n=19+18)
NewEmptyMapHintGreaterThan8-12     159ns ± 2%     150ns ± 1%  -5.79%  (p=0.000 n=20+18)

Benchmark run with commit ab7c174 reverted, see #38314.

Fixes #20184

Change-Id: Ic021f57454c3a0dd50601d73bbd77b8faf8d93b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/227458
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
---
 src/cmd/compile/internal/gc/walk.go | 24 +++++++++++++++++++-----
 src/runtime/map_benchmark_test.go   | 19 +++++++++++++++++++
 2 files changed, 38 insertions(+), 5 deletions(-)

diff --git a/src/cmd/compile/internal/gc/walk.go b/src/cmd/compile/internal/gc/walk.go
index 21658db21d..57d71bf3d4 100644
--- a/src/cmd/compile/internal/gc/walk.go
+++ b/src/cmd/compile/internal/gc/walk.go
@@ -1247,12 +1247,23 @@ opswitch:
 			// are stored with an indirection. So max bucket size is 2048+eps.
 			if !Isconst(hint, CTINT) ||
 				hint.Val().U.(*Mpint).CmpInt64(BUCKETSIZE) <= 0 {
+
+				// In case hint is larger than BUCKETSIZE runtime.makemap
+				// will allocate the buckets on the heap, see #20184
+				//
+				// if hint <= BUCKETSIZE {
+				//     var bv bmap
+				//     b = &bv
+				//     h.buckets = b
+				// }
+
+				nif := nod(OIF, nod(OLE, hint, nodintconst(BUCKETSIZE)), nil)
+				nif.SetLikely(true)
+
 				// var bv bmap
 				bv := temp(bmap(t))
-
 				zero = nod(OAS, bv, nil)
-				zero = typecheck(zero, ctxStmt)
-				init.Append(zero)
+				nif.Nbody.Append(zero)
 
 				// b = &bv
 				b := nod(OADDR, bv, nil)
@@ -1260,8 +1271,11 @@ opswitch:
 				// h.buckets = b
 				bsym := hmapType.Field(5).Sym // hmap.buckets see reflect.go:hmap
 				na := nod(OAS, nodSym(ODOT, h, bsym), b)
-				na = typecheck(na, ctxStmt)
-				init.Append(na)
+				nif.Nbody.Append(na)
+
+				nif = typecheck(nif, ctxStmt)
+				nif = walkstmt(nif)
+				init.Append(nif)
 			}
 		}
 
diff --git a/src/runtime/map_benchmark_test.go b/src/runtime/map_benchmark_test.go
index bae1aa0dbd..893cb6c5b6 100644
--- a/src/runtime/map_benchmark_test.go
+++ b/src/runtime/map_benchmark_test.go
@@ -513,3 +513,22 @@ func BenchmarkMapInterfacePtr(b *testing.B) {
 		BoolSink = m[key]
 	}
 }
+
+var (
+	hintLessThan8    = 7
+	hintGreaterThan8 = 32
+)
+
+func BenchmarkNewEmptyMapHintLessThan8(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		_ = make(map[int]int, hintLessThan8)
+	}
+}
+
+func BenchmarkNewEmptyMapHintGreaterThan8(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		_ = make(map[int]int, hintGreaterThan8)
+	}
+}
-- 
2.50.0