From: Michael Pratt <mpratt@google.com>
Date: Wed, 21 Aug 2024 20:17:16 +0000 (-0400)
Subject: internal/runtime/maps: shift optimizations
X-Git-Tag: go1.24rc1~562
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=cd54b9bae94b36f67869ef174cbb432bc4012183;p=gostls13.git

internal/runtime/maps: shift optimizations

Masking the shift lets the compiler elide a few instructions for
handling a shift of > 63 bits.

For #54766.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap
Change-Id: I669fe01caa1de1b8521f1f56b6906f3e9066a39b
Reviewed-on: https://go-review.googlesource.com/c/go/+/611190
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
---

diff --git a/src/cmd/compile/internal/reflectdata/map_swiss.go b/src/cmd/compile/internal/reflectdata/map_swiss.go
index a76864bdff..38a23908a6 100644
--- a/src/cmd/compile/internal/reflectdata/map_swiss.go
+++ b/src/cmd/compile/internal/reflectdata/map_swiss.go
@@ -148,6 +148,7 @@ func SwissMapType() *types.Type {
 	//     dirLen int
 	//
 	//     globalDepth uint8
+	//     globalShift uint8
 	//     // N.B Padding
 	//
 	//     clearSeq uint64
@@ -160,6 +161,7 @@ func SwissMapType() *types.Type {
 		makefield("dirPtr", types.Types[types.TUNSAFEPTR]),
 		makefield("dirLen", types.Types[types.TINT]),
 		makefield("globalDepth", types.Types[types.TUINT8]),
+		makefield("globalShift", types.Types[types.TUINT8]),
 		makefield("clearSeq", types.Types[types.TUINT64]),
 	}
 
diff --git a/src/internal/runtime/maps/map.go b/src/internal/runtime/maps/map.go
index a4fa07635a..67e4bd6811 100644
--- a/src/internal/runtime/maps/map.go
+++ b/src/internal/runtime/maps/map.go
@@ -233,11 +233,22 @@ type Map struct {
 	// The number of bits to use in table directory lookups.
 	globalDepth uint8
 
+	// The number of bits to shift out of the hash for directory lookups.
+	// On 64-bit systems, this is 64 - globalDepth.
+	globalShift uint8
+
 	// clearSeq is a sequence counter of calls to Clear. It is used to
 	// detect map clears during iteration.
 	clearSeq uint64
 }
 
+func depthToShift(depth uint8) uint8 {
+	if goarch.PtrSize == 4 {
+		return 32 - depth
+	}
+	return 64 - depth
+}
+
 func NewMap(mt *abi.SwissMapType, capacity uint64) *Map {
 	if capacity < abi.SwissMapGroupSlots {
 		// TODO: temporary to simplify initial implementation.
@@ -259,6 +270,7 @@ func NewMap(mt *abi.SwissMapType, capacity uint64) *Map {
 		//directory: make([]*table, dirSize),
 
 		globalDepth: globalDepth,
+		globalShift: depthToShift(globalDepth),
 	}
 
 	if capacity > abi.SwissMapGroupSlots {
@@ -294,12 +306,7 @@ func (m *Map) directoryIndex(hash uintptr) uintptr {
 	if m.dirLen == 1 {
 		return 0
 	}
-	// TODO(prattmic): Store the shift as globalShift, as we need that more
-	// often than globalDepth.
-	if goarch.PtrSize == 4 {
-		return hash >> (32 - m.globalDepth)
-	}
-	return hash >> (64 - m.globalDepth)
+	return hash >> (m.globalShift & 63)
 }
 
 func (m *Map) directoryAt(i uintptr) *table {
@@ -338,6 +345,7 @@ func (m *Map) installTableSplit(old, left, right *table) {
 			}
 		}
 		m.globalDepth++
+		m.globalShift--
 		//m.directory = newDir
 		m.dirPtr = unsafe.Pointer(&newDir[0])
 		m.dirLen = len(newDir)