From aee71dd70b3779c66950ce6a952deca13d48e55e Mon Sep 17 00:00:00 2001
From: Martin Möhrmann <moehrmann@google.com>
Date: Fri, 27 Apr 2018 21:58:59 +0200
Subject: cmd/compile: optimize map-clearing range idiom
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

replace map clears of the form:

        for k := range m {
                delete(m, k)
        }

(where m is map with key type that is reflexive for ==)
with a new runtime function that clears the maps backing
array with a memclr and reinitializes the hmap struct.

Map key types that for example contain floats are not
replaced by this optimization since NaN keys cannot
be deleted from maps using delete.

name                           old time/op  new time/op  delta
GoMapClear/Reflexive/1         92.2ns ± 1%  47.1ns ± 2%  -48.89%  (p=0.000 n=9+9)
GoMapClear/Reflexive/10         108ns ± 1%    48ns ± 2%  -55.68%  (p=0.000 n=10+10)
GoMapClear/Reflexive/100        303ns ± 2%   110ns ± 3%  -63.56%  (p=0.000 n=10+10)
GoMapClear/Reflexive/1000      3.58µs ± 3%  1.23µs ± 2%  -65.49%  (p=0.000 n=9+10)
GoMapClear/Reflexive/10000     28.2µs ± 3%  10.3µs ± 2%  -63.55%  (p=0.000 n=9+10)
GoMapClear/NonReflexive/1       121ns ± 2%   124ns ± 7%     ~     (p=0.097 n=10+10)
GoMapClear/NonReflexive/10      137ns ± 2%   139ns ± 3%   +1.53%  (p=0.033 n=10+10)
GoMapClear/NonReflexive/100     331ns ± 3%   334ns ± 2%     ~     (p=0.342 n=10+10)
GoMapClear/NonReflexive/1000   3.64µs ± 3%  3.64µs ± 2%     ~     (p=0.887 n=9+10)
GoMapClear/NonReflexive/10000  28.1µs ± 2%  28.4µs ± 3%     ~     (p=0.247 n=10+10)

Fixes #20138

Change-Id: I181332a8ef434a4f0d89659f492d8711db3f3213
Reviewed-on: https://go-review.googlesource.com/110055
Reviewed-by: Keith Randall <khr@golang.org>
---
 test/codegen/maps.go | 57 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

(limited to 'test/codegen')

diff --git a/test/codegen/maps.go b/test/codegen/maps.go
index 57e219ca06..d167715898 100644
--- a/test/codegen/maps.go
+++ b/test/codegen/maps.go
@@ -36,3 +36,60 @@ func AccessString2(m map[string]int) bool {
 	_, ok := m["abc"]
 	return ok
 }
+
+// ------------------- //
+//     Map Clear       //
+// ------------------- //
+
+// Optimization of map clear idiom (Issue #20138).
+
+func MapClearReflexive(m map[int]int) {
+	// amd64:`.*runtime\.mapclear`
+	// amd64:-`.*runtime\.mapiterinit`
+	for k := range m {
+		delete(m, k)
+	}
+}
+
+func MapClearIndirect(m map[int]int) {
+	s := struct{ m map[int]int }{m: m}
+	// amd64:`.*runtime\.mapclear`
+	// amd64:-`.*runtime\.mapiterinit`
+	for k := range s.m {
+		delete(s.m, k)
+	}
+}
+
+func MapClearPointer(m map[*byte]int) {
+	// amd64:`.*runtime\.mapclear`
+	// amd64:-`.*runtime\.mapiterinit`
+	for k := range m {
+		delete(m, k)
+	}
+}
+
+func MapClearNotReflexive(m map[float64]int) {
+	// amd64:`.*runtime\.mapiterinit`
+	// amd64:-`.*runtime\.mapclear`
+	for k := range m {
+		delete(m, k)
+	}
+}
+
+func MapClearInterface(m map[interface{}]int) {
+	// amd64:`.*runtime\.mapiterinit`
+	// amd64:-`.*runtime\.mapclear`
+	for k := range m {
+		delete(m, k)
+	}
+}
+
+func MapClearSideEffect(m map[int]int) int {
+	k := 0
+	// amd64:`.*runtime\.mapiterinit`
+	// amd64:-`.*runtime\.mapclear`
+	for k = range m {
+		delete(m, k)
+	}
+	return k
+}
-- 
cgit v1.3