runtime: commit to spinbitmutex GOEXPERIMENT

Use the "spinbit" mutex implementation always (including on platforms
that need to emulate atomic.Xchg8), and delete the prior "tristate"
implementations.

The exception is GOARCH=wasm, where the Go runtime does not use multiple
threads.

For golang#68578

Change-Id: Ifc29bbfa05071d776c23a19ae185891a03a82417
Reviewed-on: https://go-review.googlesource.com/c/go/+/658456
Auto-Submit: Rhys Hiltner <rhys.hiltner@gmail.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: rhysh

src/internal/buildcfg/exp.go

@@ -67,8 +67,6 @@ func ParseGOEXPERIMENT(goos, goarch, goexp string) (*ExperimentFlags, error) {
 		regabiSupported = true
 	}
 
-	haveThreads := goarch != "wasm"
-
 	// Older versions (anything before V16) of dsymutil don't handle
 	// the .debug_rnglists section in DWARF5. See
 	// https://github.com/golang/go/issues/26379#issuecomment-2677068742
@@ -85,7 +83,6 @@ func ParseGOEXPERIMENT(goos, goarch, goexp string) (*ExperimentFlags, error) {
 		RegabiArgs:      regabiSupported,
 		AliasTypeParams: true,
 		SwissMap:        true,
-		SpinbitMutex:    haveThreads,
 		SyncHashTrieMap: true,
 		Dwarf5:          dwarf5Supported,
 	}

src/internal/goexperiment/exp_spinbitmutex_off.go

@@ -115,10 +115,6 @@ type Flags struct {
 	// SwissMap enables the SwissTable-based map implementation.
 	SwissMap bool
 
-	// SpinbitMutex enables the new "spinbit" mutex implementation on supported
-	// platforms. See https://go.dev/issue/68578.
-	SpinbitMutex bool
-
 	// SyncHashTrieMap enables the HashTrieMap sync.Map implementation.
 	SyncHashTrieMap bool
 

src/internal/goexperiment/exp_spinbitmutex_on.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build (aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || plan9 || solaris || windows) && goexperiment.spinbitmutex
+//go:build !wasm
 
 package runtime
 

src/internal/goexperiment/flags.go

@@ -8,7 +8,6 @@ import (
 	"internal/abi"
 	"internal/chacha8rand"
 	"internal/goarch"
-	"internal/goexperiment"
 	"internal/runtime/atomic"
 	"internal/runtime/sys"
 	"unsafe"
@@ -628,9 +627,9 @@ type mPadded struct {
 
 	// Size the runtime.m structure so it fits in the 2048-byte size class, and
 	// not in the next-smallest (1792-byte) size class. That leaves the 11 low
-	// bits of muintptr values available for flags, as required for
-	// GOEXPERIMENT=spinbitmutex.
-	_ [goexperiment.SpinbitMutexInt * (2048 - mallocHeaderSize - mRedZoneSize - unsafe.Sizeof(m{}))]byte
+	// bits of muintptr values available for flags, as required by
+	// lock_spinbit.go.
+	_ [(1 - goarch.IsWasm) * (2048 - mallocHeaderSize - mRedZoneSize - unsafe.Sizeof(m{}))]byte
 }
 
 type p struct {