[dev.typeparams] all: always enable regabig on AMD64

Always enable regabig on AMD64, which enables the G register and
the X15 zero register. Remove the fallback path.

Also remove the regabig GOEXPERIMENT. On AMD64 it is always
enabled (this CL). Other architectures already have a G register,
except for 386, where there are too few registers and it is
unlikely that we will reserve one. (If we really do, we can just
add a new experiment).

Change-Id: I229cac0060f48fe58c9fdaabd38d6fa16b8a0855
Reviewed-on: https://go-review.googlesource.com/c/go/+/327272
Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

16 files changed, 13 insertions, 122 deletions

diff --git a/src/cmd/compile/internal/amd64/ggen.go b/src/cmd/compile/internal/amd64/ggen.go
index 1484ad5404..b8dce81a92 100644
--- a/src/cmd/compile/internal/amd64/ggen.go
+++ b/src/cmd/compile/internal/amd64/ggen.go
@@ -57,7 +57,6 @@ func dzDI(b int64) int64 {
 func zerorange(pp *objw.Progs, p *obj.Prog, off, cnt int64, state *uint32) *obj.Prog {
 	const (
 		r13 = 1 << iota // if R13 is already zeroed.
-		x15             // if X15 is already zeroed. Note: in new ABI, X15 is always zero.
 	)
 
 	if cnt == 0 {
@@ -85,11 +84,6 @@ func zerorange(pp *objw.Progs, p *obj.Prog, off, cnt int64, state *uint32) *obj.
 		}
 		p = pp.Append(p, x86.AMOVQ, obj.TYPE_REG, x86.REG_R13, 0, obj.TYPE_MEM, x86.REG_SP, off)
 	} else if !isPlan9 && cnt <= int64(8*types.RegSize) {
-		if !buildcfg.Experiment.RegabiG && *state&x15 == 0 {
-			p = pp.Append(p, x86.AXORPS, obj.TYPE_REG, x86.REG_X15, 0, obj.TYPE_REG, x86.REG_X15, 0)
-			*state |= x15
-		}
-
 		for i := int64(0); i < cnt/16; i++ {
 			p = pp.Append(p, x86.AMOVUPS, obj.TYPE_REG, x86.REG_X15, 0, obj.TYPE_MEM, x86.REG_SP, off+i*16)
 		}
@@ -98,10 +92,6 @@ func zerorange(pp *objw.Progs, p *obj.Prog, off, cnt int64, state *uint32) *obj.
 			p = pp.Append(p, x86.AMOVUPS, obj.TYPE_REG, x86.REG_X15, 0, obj.TYPE_MEM, x86.REG_SP, off+cnt-int64(16))
 		}
 	} else if !isPlan9 && (cnt <= int64(128*types.RegSize)) {
-		if !buildcfg.Experiment.RegabiG && *state&x15 == 0 {
-			p = pp.Append(p, x86.AXORPS, obj.TYPE_REG, x86.REG_X15, 0, obj.TYPE_REG, x86.REG_X15, 0)
-			*state |= x15
-		}
 		// Save DI to r12. With the amd64 Go register abi, DI can contain
 		// an incoming parameter, whereas R12 is always scratch.
 		p = pp.Append(p, x86.AMOVQ, obj.TYPE_REG, x86.REG_DI, 0, obj.TYPE_REG, x86.REG_R12, 0)
diff --git a/src/cmd/compile/internal/amd64/ssa.go b/src/cmd/compile/internal/amd64/ssa.go
index c27a5fe5b5..30dba057d0 100644
--- a/src/cmd/compile/internal/amd64/ssa.go
+++ b/src/cmd/compile/internal/amd64/ssa.go
@@ -823,7 +823,7 @@ func ssaGenValue(s *ssagen.State, v *ssa.Value) {
 		p.To.Reg = v.Args[0].Reg()
 		ssagen.AddAux2(&p.To, v, sc.Off64())
 	case ssa.OpAMD64MOVOstorezero:
-		if !buildcfg.Experiment.RegabiG || s.ABI != obj.ABIInternal {
+		if s.ABI != obj.ABIInternal {
 			// zero X15 manually
 			opregreg(s, x86.AXORPS, x86.REG_X15, x86.REG_X15)
 		}
@@ -914,7 +914,7 @@ func ssaGenValue(s *ssagen.State, v *ssa.Value) {
 		p.To.Type = obj.TYPE_REG
 		p.To.Reg = v.Reg()
 	case ssa.OpAMD64DUFFZERO:
-		if !buildcfg.Experiment.RegabiG || s.ABI != obj.ABIInternal {
+		if s.ABI != obj.ABIInternal {
 			// zero X15 manually
 			opregreg(s, x86.AXORPS, x86.REG_X15, x86.REG_X15)
 		}
@@ -997,13 +997,13 @@ func ssaGenValue(s *ssagen.State, v *ssa.Value) {
 		// Closure pointer is DX.
 		ssagen.CheckLoweredGetClosurePtr(v)
 	case ssa.OpAMD64LoweredGetG:
-		if buildcfg.Experiment.RegabiG && s.ABI == obj.ABIInternal {
+		if s.ABI == obj.ABIInternal {
 			v.Fatalf("LoweredGetG should not appear in ABIInternal")
 		}
 		r := v.Reg()
 		getgFromTLS(s, r)
 	case ssa.OpAMD64CALLstatic:
-		if buildcfg.Experiment.RegabiG && s.ABI == obj.ABI0 && v.Aux.(*ssa.AuxCall).Fn.ABI() == obj.ABIInternal {
+		if s.ABI == obj.ABI0 && v.Aux.(*ssa.AuxCall).Fn.ABI() == obj.ABIInternal {
 			// zeroing X15 when entering ABIInternal from ABI0
 			if buildcfg.GOOS != "plan9" { // do not use SSE on Plan 9
 				opregreg(s, x86.AXORPS, x86.REG_X15, x86.REG_X15)
@@ -1012,7 +1012,7 @@ func ssaGenValue(s *ssagen.State, v *ssa.Value) {
 			getgFromTLS(s, x86.REG_R14)
 		}
 		s.Call(v)
-		if buildcfg.Experiment.RegabiG && s.ABI == obj.ABIInternal && v.Aux.(*ssa.AuxCall).Fn.ABI() == obj.ABI0 {
+		if s.ABI == obj.ABIInternal && v.Aux.(*ssa.AuxCall).Fn.ABI() == obj.ABI0 {
 			// zeroing X15 when entering ABIInternal from ABI0
 			if buildcfg.GOOS != "plan9" { // do not use SSE on Plan 9
 				opregreg(s, x86.AXORPS, x86.REG_X15, x86.REG_X15)
@@ -1308,7 +1308,7 @@ func ssaGenBlock(s *ssagen.State, b, next *ssa.Block) {
 	case ssa.BlockRet:
 		s.Prog(obj.ARET)
 	case ssa.BlockRetJmp:
-		if buildcfg.Experiment.RegabiG && s.ABI == obj.ABI0 && b.Aux.(*obj.LSym).ABI() == obj.ABIInternal {
+		if s.ABI == obj.ABI0 && b.Aux.(*obj.LSym).ABI() == obj.ABIInternal {
 			// zeroing X15 when entering ABIInternal from ABI0
 			if buildcfg.GOOS != "plan9" { // do not use SSE on Plan 9
 				opregreg(s, x86.AXORPS, x86.REG_X15, x86.REG_X15)
diff --git a/src/cmd/compile/internal/ssa/config.go b/src/cmd/compile/internal/ssa/config.go
index 7d680304c9..61c65f9e54 100644
--- a/src/cmd/compile/internal/ssa/config.go
+++ b/src/cmd/compile/internal/ssa/config.go
@@ -196,7 +196,7 @@ func NewConfig(arch string, types Types, ctxt *obj.Link, optimize bool) *Config
 		c.floatParamRegs = paramFloatRegAMD64
 		c.FPReg = framepointerRegAMD64
 		c.LinkReg = linkRegAMD64
-		c.hasGReg = buildcfg.Experiment.RegabiG
+		c.hasGReg = true
 	case "386":
 		c.PtrSize = 4
 		c.RegSize = 4
diff --git a/src/cmd/compile/internal/ssa/gen/AMD64.rules b/src/cmd/compile/internal/ssa/gen/AMD64.rules
index 4cd00732fc..45c0238317 100644
--- a/src/cmd/compile/internal/ssa/gen/AMD64.rules
+++ b/src/cmd/compile/internal/ssa/gen/AMD64.rules
@@ -460,7 +460,7 @@
 (IsInBounds idx len) => (SETB (CMPQ idx len))
 (IsSliceInBounds idx len) => (SETBE (CMPQ idx len))
 (NilCheck ...) => (LoweredNilCheck ...)
-(GetG mem) && !(buildcfg.Experiment.RegabiG && v.Block.Func.OwnAux.Fn.ABI() == obj.ABIInternal) => (LoweredGetG mem) // only lower in old ABI. in new ABI we have a G register.
+(GetG mem) && v.Block.Func.OwnAux.Fn.ABI() != obj.ABIInternal => (LoweredGetG mem) // only lower in old ABI. in new ABI we have a G register.
 (GetClosurePtr ...) => (LoweredGetClosurePtr ...)
 (GetCallerPC ...) => (LoweredGetCallerPC ...)
 (GetCallerSP ...) => (LoweredGetCallerSP ...)
diff --git a/src/cmd/compile/internal/ssa/rewriteAMD64.go b/src/cmd/compile/internal/ssa/rewriteAMD64.go
index 5045ba7351..89d32c0657 100644
--- a/src/cmd/compile/internal/ssa/rewriteAMD64.go
+++ b/src/cmd/compile/internal/ssa/rewriteAMD64.go
@@ -3,7 +3,6 @@
 
 package ssa
 
-import "internal/buildcfg"
 import "math"
 import "cmd/internal/obj"
 import "cmd/compile/internal/types"
@@ -29339,11 +29338,11 @@ func rewriteValueAMD64_OpFloor(v *Value) bool {
 func rewriteValueAMD64_OpGetG(v *Value) bool {
 	v_0 := v.Args[0]
 	// match: (GetG mem)
-	// cond: !(buildcfg.Experiment.RegabiG && v.Block.Func.OwnAux.Fn.ABI() == obj.ABIInternal)
+	// cond: v.Block.Func.OwnAux.Fn.ABI() != obj.ABIInternal
 	// result: (LoweredGetG mem)
 	for {
 		mem := v_0
-		if !(!(buildcfg.Experiment.RegabiG && v.Block.Func.OwnAux.Fn.ABI() == obj.ABIInternal)) {
+		if !(v.Block.Func.OwnAux.Fn.ABI() != obj.ABIInternal) {
 			break
 		}
 		v.reset(OpAMD64LoweredGetG)
diff --git a/src/cmd/internal/obj/x86/obj6.go b/src/cmd/internal/obj/x86/obj6.go
index e2732d53e3..183ca2ebe9 100644
--- a/src/cmd/internal/obj/x86/obj6.go
+++ b/src/cmd/internal/obj/x86/obj6.go
@@ -35,7 +35,6 @@ import (
 	"cmd/internal/objabi"
 	"cmd/internal/src"
 	"cmd/internal/sys"
-	"internal/buildcfg"
 	"log"
 	"math"
 	"path"
@@ -647,13 +646,12 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 
 	var regg int16
 	if !p.From.Sym.NoSplit() || p.From.Sym.Wrapper() {
-		if ctxt.Arch.Family == sys.AMD64 && buildcfg.Experiment.RegabiG && cursym.ABI() == obj.ABIInternal {
+		if ctxt.Arch.Family == sys.AMD64 && cursym.ABI() == obj.ABIInternal {
 			regg = REGG // use the g register directly in ABIInternal
 		} else {
 			p = obj.Appendp(p, newprog)
 			regg = REG_CX
 			if ctxt.Arch.Family == sys.AMD64 {
-				// Using this register means that stacksplit works w/ //go:registerparams even when !buildcfg.Experiment.RegabiG
 				regg = REGG // == REG_R14
 			}
 			p = load_g(ctxt, p, newprog, regg) // load g into regg
diff --git a/src/internal/buildcfg/exp.go b/src/internal/buildcfg/exp.go
index b3f3de62a8..e78f987999 100644
--- a/src/internal/buildcfg/exp.go
+++ b/src/internal/buildcfg/exp.go
@@ -28,7 +28,6 @@ var regabiDeveloping = false
 // configuration and any variation from this is an experiment.
 var experimentBaseline = goexperiment.Flags{
 	RegabiWrappers: regabiSupported,
-	RegabiG:        regabiSupported,
 	RegabiReflect:  regabiSupported,
 	RegabiArgs:     regabiSupported,
 }
@@ -67,7 +66,6 @@ func parseExperiments() goexperiment.Flags {
 		// do the right thing.
 		names["regabi"] = func(v bool) {
 			flags.RegabiWrappers = v
-			flags.RegabiG = v
 			flags.RegabiReflect = v
 			flags.RegabiArgs = v
 		}
@@ -104,16 +102,12 @@ func parseExperiments() goexperiment.Flags {
 	// regabi is only supported on amd64 and arm64.
 	if GOARCH != "amd64" && GOARCH != "arm64" {
 		flags.RegabiWrappers = false
-		flags.RegabiG = false
 		flags.RegabiReflect = false
 		flags.RegabiArgs = false
 	}
 	// Check regabi dependencies.
-	if flags.RegabiG && !flags.RegabiWrappers {
-		Error = fmt.Errorf("GOEXPERIMENT regabig requires regabiwrappers")
-	}
-	if flags.RegabiArgs && !(flags.RegabiWrappers && flags.RegabiG && flags.RegabiReflect) {
-		Error = fmt.Errorf("GOEXPERIMENT regabiargs requires regabiwrappers,regabig,regabireflect")
+	if flags.RegabiArgs && !(flags.RegabiWrappers && flags.RegabiReflect) {
+		Error = fmt.Errorf("GOEXPERIMENT regabiargs requires regabiwrappers,regabireflect")
 	}
 	return flags
 }
diff --git a/src/internal/goexperiment/exp_regabig_off.go b/src/internal/goexperiment/exp_regabig_off.go
deleted file mode 100644
index 1b37d45186..0000000000
--- a/src/internal/goexperiment/exp_regabig_off.go
+++ /dev/null
@@ -1,9 +0,0 @@
-// Code generated by mkconsts.go. DO NOT EDIT.
-
-//go:build !goexperiment.regabig
-// +build !goexperiment.regabig
-
-package goexperiment
-
-const RegabiG = false
-const RegabiGInt = 0
diff --git a/src/internal/goexperiment/exp_regabig_on.go b/src/internal/goexperiment/exp_regabig_on.go
deleted file mode 100644
index 7e5b162e0b..0000000000
--- a/src/internal/goexperiment/exp_regabig_on.go
+++ /dev/null
@@ -1,9 +0,0 @@
-// Code generated by mkconsts.go. DO NOT EDIT.
-
-//go:build goexperiment.regabig
-// +build goexperiment.regabig
-
-package goexperiment
-
-const RegabiG = true
-const RegabiGInt = 1
diff --git a/src/internal/goexperiment/flags.go b/src/internal/goexperiment/flags.go
index c20dbcd9f5..71e38cd047 100644
--- a/src/internal/goexperiment/flags.go
+++ b/src/internal/goexperiment/flags.go
@@ -68,11 +68,6 @@ type Flags struct {
 	// ABI0 and ABIInternal functions. Without this, the ABIs are
 	// assumed to be identical so cross-ABI calls are direct.
 	RegabiWrappers bool
-	// RegabiG enables dedicated G and zero registers in
-	// ABIInternal.
-	//
-	// Requires wrappers because it makes the ABIs incompatible.
-	RegabiG bool
 	// RegabiReflect enables the register-passing paths in
 	// reflection calls. This is also gated by intArgRegs in
 	// reflect and runtime (which are disabled by default) so it
diff --git a/src/runtime/asm_amd64.s b/src/runtime/asm_amd64.s
index f8f5fc62e6..50ffa30ac5 100644
--- a/src/runtime/asm_amd64.s
+++ b/src/runtime/asm_amd64.s
@@ -683,10 +683,6 @@ TEXT runtime·jmpdefer(SB), NOSPLIT, $0-16
 // or else unwinding from systemstack_switch is incorrect.
 // Smashes R9.
 TEXT gosave_systemstack_switch<>(SB),NOSPLIT,$0
-#ifndef GOEXPERIMENT_regabig
-	get_tls(R14)
-	MOVQ	g(R14), R14
-#endif
 	MOVQ	$runtime·systemstack_switch(SB), R9
 	MOVQ	R9, (g_sched+gobuf_pc)(R14)
 	LEAQ	8(SP), R9
@@ -1284,10 +1280,8 @@ aes65to128:
 	PXOR	X10, X8
 	PXOR	X11, X9
 	PXOR	X9, X8
-#ifdef GOEXPERIMENT_regabig
 	// X15 must be zero on return
 	PXOR	X15, X15
-#endif
 #ifdef GOEXPERIMENT_regabiargs
 	MOVQ	X8, AX	// return X8
 #else
@@ -1408,10 +1402,8 @@ aesloop:
 	PXOR	X10, X8
 	PXOR	X11, X9
 	PXOR	X9, X8
-#ifdef GOEXPERIMENT_regabig
 	// X15 must be zero on return
 	PXOR	X15, X15
-#endif
 #ifdef GOEXPERIMENT_regabiargs
 	MOVQ	X8, AX	// return X8
 #else
@@ -1595,13 +1587,11 @@ TEXT runtime·addmoduledata(SB),NOSPLIT,$0-0
 // signals. It is quite painful to set X15 in the signal context,
 // so we do it here.
 TEXT ·sigpanic0(SB),NOSPLIT,$0-0
-#ifdef GOEXPERIMENT_regabig
 	get_tls(R14)
 	MOVQ	g(R14), R14
 #ifndef GOOS_plan9
 	XORPS	X15, X15
 #endif
-#endif
 	JMP	·sigpanic<ABIInternal>(SB)
 
 // gcWriteBarrier performs a heap pointer write and informs the GC.
@@ -1619,13 +1609,7 @@ TEXT runtime·gcWriteBarrier<ABIInternal>(SB),NOSPLIT,$112
 	MOVQ	R13, 104(SP)
 	// TODO: Consider passing g.m.p in as an argument so they can be shared
 	// across a sequence of write barriers.
-#ifdef GOEXPERIMENT_regabig
 	MOVQ	g_m(R14), R13
-#else
-	get_tls(R13)
-	MOVQ	g(R13), R13
-	MOVQ	g_m(R13), R13
-#endif
 	MOVQ	m_p(R13), R13
 	MOVQ	(p_wbBuf+wbBuf_next)(R13), R12
 	// Increment wbBuf.next position.
diff --git a/src/runtime/memclr_amd64.s b/src/runtime/memclr_amd64.s
index a10f57bd8c..6c78869f4c 100644
--- a/src/runtime/memclr_amd64.s
+++ b/src/runtime/memclr_amd64.s
@@ -37,9 +37,6 @@ tail:
 	JE	_8
 	CMPQ	BX, $16
 	JBE	_9through16
-#ifndef GOEXPERIMENT_regabig
-	PXOR	X15, X15
-#endif
 	CMPQ	BX, $32
 	JBE	_17through32
 	CMPQ	BX, $64
diff --git a/src/runtime/memmove_amd64.s b/src/runtime/memmove_amd64.s
index 24c6529f58..af538d4bce 100644
--- a/src/runtime/memmove_amd64.s
+++ b/src/runtime/memmove_amd64.s
@@ -254,10 +254,8 @@ move_129through256:
 	MOVOU	X13, -48(DI)(BX*1)
 	MOVOU	X14, -32(DI)(BX*1)
 	MOVOU	X15, -16(DI)(BX*1)
-#ifdef GOEXPERIMENT_regabig
 	// X15 must be zero on return
 	PXOR	X15, X15
-#endif
 	RET
 move_256through2048:
 	SUBQ	$256, BX
@@ -297,10 +295,8 @@ move_256through2048:
 	LEAQ	256(SI), SI
 	LEAQ	256(DI), DI
 	JGE	move_256through2048
-#ifdef GOEXPERIMENT_regabig
 	// X15 must be zero on return
 	PXOR	X15, X15
-#endif
 	JMP	tail
 
 avxUnaligned:
diff --git a/src/runtime/race_amd64.s b/src/runtime/race_amd64.s
index 469623ff20..8a17113232 100644
--- a/src/runtime/race_amd64.s
+++ b/src/runtime/race_amd64.s
@@ -161,10 +161,6 @@ TEXT	runtime·racewriterangepc1(SB), NOSPLIT, $0-24
 // If addr (RARG1) is out of range, do nothing.
 // Otherwise, setup goroutine context and invoke racecall. Other arguments already set.
 TEXT	racecalladdr<>(SB), NOSPLIT, $0-0
-#ifndef GOEXPERIMENT_regabig
-	get_tls(R12)
-	MOVQ	g(R12), R14
-#endif
 	MOVQ	g_racectx(R14), RARG0	// goroutine context
 	// Check that addr is within [arenastart, arenaend) or within [racedatastart, racedataend).
 	CMPQ	RARG1, runtime·racearenastart(SB)
@@ -192,10 +188,6 @@ TEXT	runtime·racefuncenter(SB), NOSPLIT, $0-8
 // R11 = caller's return address
 TEXT	racefuncenter<>(SB), NOSPLIT, $0-0
 	MOVQ	DX, BX		// save function entry context (for closures)
-#ifndef GOEXPERIMENT_regabig
-	get_tls(R12)
-	MOVQ	g(R12), R14
-#endif
 	MOVQ	g_racectx(R14), RARG0	// goroutine context
 	MOVQ	R11, RARG1
 	// void __tsan_func_enter(ThreadState *thr, void *pc);
@@ -208,10 +200,6 @@ TEXT	racefuncenter<>(SB), NOSPLIT, $0-0
 // func runtime·racefuncexit()
 // Called from instrumented code.
 TEXT	runtime·racefuncexit(SB), NOSPLIT, $0-0
-#ifndef GOEXPERIMENT_regabig
-	get_tls(R12)
-	MOVQ	g(R12), R14
-#endif
 	MOVQ	g_racectx(R14), RARG0	// goroutine context
 	// void __tsan_func_exit(ThreadState *thr);
 	MOVQ	$__tsan_func_exit(SB), AX
@@ -370,10 +358,6 @@ racecallatomic_data:
 	JAE	racecallatomic_ignore
 racecallatomic_ok:
 	// Addr is within the good range, call the atomic function.
-#ifndef GOEXPERIMENT_regabig
-	get_tls(R12)
-	MOVQ	g(R12), R14
-#endif
 	MOVQ	g_racectx(R14), RARG0	// goroutine context
 	MOVQ	8(SP), RARG1	// caller pc
 	MOVQ	(SP), RARG2	// pc
@@ -385,10 +369,6 @@ racecallatomic_ignore:
 	// An attempt to synchronize on the address would cause crash.
 	MOVQ	AX, BX	// remember the original function
 	MOVQ	$__tsan_go_ignore_sync_begin(SB), AX
-#ifndef GOEXPERIMENT_regabig
-	get_tls(R12)
-	MOVQ	g(R12), R14
-#endif
 	MOVQ	g_racectx(R14), RARG0	// goroutine context
 	CALL	racecall<>(SB)
 	MOVQ	BX, AX	// restore the original function
@@ -416,10 +396,6 @@ TEXT	runtime·racecall(SB), NOSPLIT, $0-0
 
 // Switches SP to g0 stack and calls (AX). Arguments already set.
 TEXT	racecall<>(SB), NOSPLIT, $0-0
-#ifndef GOEXPERIMENT_regabig
-	get_tls(R12)
-	MOVQ	g(R12), R14
-#endif
 	MOVQ	g_m(R14), R13
 	// Switch to g0 stack.
 	MOVQ	SP, R12		// callee-saved, preserved across the CALL
diff --git a/src/runtime/sys_linux_amd64.s b/src/runtime/sys_linux_amd64.s
index f22b7ad928..64ddc2354e 100644
--- a/src/runtime/sys_linux_amd64.s
+++ b/src/runtime/sys_linux_amd64.s
@@ -215,13 +215,7 @@ TEXT runtime·nanotime1(SB),NOSPLIT,$16-8
 
 	MOVQ	SP, R12	// Save old SP; R12 unchanged by C code.
 
-#ifdef GOEXPERIMENT_regabig
 	MOVQ	g_m(R14), BX // BX unchanged by C code.
-#else
-	get_tls(CX)
-	MOVQ	g(CX), AX
-	MOVQ	g_m(AX), BX // BX unchanged by C code.
-#endif
 
 	// Set vdsoPC and vdsoSP for SIGPROF traceback.
 	// Save the old values on stack and restore them on exit,
@@ -236,11 +230,7 @@ TEXT runtime·nanotime1(SB),NOSPLIT,$16-8
 	MOVQ	CX, m_vdsoPC(BX)
 	MOVQ	DX, m_vdsoSP(BX)
 
-#ifdef GOEXPERIMENT_regabig
 	CMPQ	R14, m_curg(BX)	// Only switch if on curg.
-#else
-	CMPQ	AX, m_curg(BX)	// Only switch if on curg.
-#endif
 	JNE	noswitch
 
 	MOVQ	m_g0(BX), DX
diff --git a/src/runtime/time_linux_amd64.s b/src/runtime/time_linux_amd64.s
index 0dd7919896..c88e92bd0c 100644
--- a/src/runtime/time_linux_amd64.s
+++ b/src/runtime/time_linux_amd64.s
@@ -15,13 +15,7 @@
 TEXT time·now(SB),NOSPLIT,$16-24
 	MOVQ	SP, R12 // Save old SP; R12 unchanged by C code.
 
-#ifdef GOEXPERIMENT_regabig
 	MOVQ	g_m(R14), BX // BX unchanged by C code.
-#else
-	get_tls(CX)
-	MOVQ	g(CX), AX
-	MOVQ	g_m(AX), BX // BX unchanged by C code.
-#endif
 
 	// Store CLOCK_REALTIME results directly to return space.
 	LEAQ	sec+0(FP), SI
@@ -38,11 +32,7 @@ TEXT time·now(SB),NOSPLIT,$16-24
 	MOVQ	CX, m_vdsoPC(BX)
 	MOVQ	SI, m_vdsoSP(BX)
 
-#ifdef GOEXPERIMENT_regabig
 	CMPQ	R14, m_curg(BX)	// Only switch if on curg.
-#else
-	CMPQ	AX, m_curg(BX)	// Only switch if on curg.
-#endif
 	JNE	noswitch
 
 	MOVQ	m_g0(BX), DX