all: replace runtime SSE2 detection with GO386 setting

When GO386=sse2 we can assume sse2 to be present without
a runtime check. If GO386=softfloat is set we can avoid
the usage of SSE2 even if detected.

This might cause a memcpy, memclr and bytealg slowdown of Go
binaries compiled with softfloat on machines that support
SSE2. Such setups are rare and should use GO386=sse2 instead
if performance matters.

On targets that support SSE2 we avoid the runtime overhead of
dynamic cpu feature dispatch.

The removal of runtime sse2 checks also allows to simplify
internal/cpu further by removing handling of the required
feature option as a followup after this CL.

Change-Id: I90a853a8853a405cb665497c6d1a86556947ba17
Reviewed-on: https://go-review.googlesource.com/c/go/+/344350
Trust: Martin Möhrmann <martin@golang.org>
Run-TryBot: Martin Möhrmann <martin@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/go/internal/work/gc.go

@@ -374,6 +374,11 @@ func asmArgs(a *Action, p *load.Package) []interface{} {
 		args = append(args, "-compiling-runtime")
 	}
 
+	if cfg.Goarch == "386" {
+		// Define GO386_value from cfg.GO386.
+		args = append(args, "-D", "GO386_"+cfg.GO386)
+	}
+
 	if cfg.Goarch == "mips" || cfg.Goarch == "mipsle" {
 		// Define GOMIPS_value from cfg.GOMIPS.
 		args = append(args, "-D", "GOMIPS_"+cfg.GOMIPS)

src/internal/bytealg/bytealg.go

@@ -11,7 +11,6 @@ import (
 
 // Offsets into internal/cpu records for use in assembly.
 const (
-	offsetX86HasSSE2   = unsafe.Offsetof(cpu.X86.HasSSE2)
 	offsetX86HasSSE42  = unsafe.Offsetof(cpu.X86.HasSSE42)
 	offsetX86HasAVX2   = unsafe.Offsetof(cpu.X86.HasAVX2)
 	offsetX86HasPOPCNT = unsafe.Offsetof(cpu.X86.HasPOPCNT)

src/internal/bytealg/compare_386.s

@@ -36,8 +36,9 @@ TEXT cmpbody<>(SB),NOSPLIT,$0-0
 	JEQ	allsame
 	CMPL	BP, $4
 	JB	small
-	CMPB	internal∕cpu·X86+const_offsetX86HasSSE2(SB), $1
+#ifdef GO386_softfloat
-	JNE	mediumloop
+	JMP	mediumloop
+#endif
 largeloop:
 	CMPL	BP, $16
 	JB	mediumloop

src/internal/bytealg/equal_386.s

@@ -43,8 +43,9 @@ TEXT memeqbody<>(SB),NOSPLIT,$0-0
 hugeloop:
 	CMPL	BX, $64
 	JB	bigloop
-	CMPB	internal∕cpu·X86+const_offsetX86HasSSE2(SB), $1
+#ifdef GO386_softfloat
-	JNE	bigloop
+	JMP	bigloop
+#endif
 	MOVOU	(SI), X0
 	MOVOU	(DI), X1
 	MOVOU	16(SI), X2

src/internal/cpu/cpu.go

@@ -37,7 +37,6 @@ var X86 struct {
 	HasPCLMULQDQ bool
 	HasPOPCNT    bool
 	HasRDTSCP    bool
-	HasSSE2      bool
 	HasSSE3      bool
 	HasSSSE3     bool
 	HasSSE41     bool

src/internal/cpu/cpu_386.go

@@ -1,7 +0,0 @@
-// Copyright 2018 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package cpu
-
-const GOARCH = "386"

src/internal/cpu/cpu_amd64.go

@@ -1,7 +0,0 @@
-// Copyright 2018 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package cpu
-
-const GOARCH = "amd64"

src/internal/cpu/cpu_x86.go

@@ -61,9 +61,6 @@ func doinit() {
 		{Name: "sse41", Feature: &X86.HasSSE41},
 		{Name: "sse42", Feature: &X86.HasSSE42},
 		{Name: "ssse3", Feature: &X86.HasSSSE3},
-
-		// These capabilities should always be enabled on amd64:
-		{Name: "sse2", Feature: &X86.HasSSE2, Required: GOARCH == "amd64"},
 	}
 
 	maxID, _, _, _ := cpuid(0, 0)
@@ -74,8 +71,7 @@ func doinit() {
 
 	maxExtendedFunctionInformation, _, _, _ = cpuid(0x80000000, 0)
 
-	_, _, ecx1, edx1 := cpuid(1, 0)
+	_, _, ecx1, _ := cpuid(1, 0)
-	X86.HasSSE2 = isSet(edx1, cpuid_SSE2)
 
 	X86.HasSSE3 = isSet(ecx1, cpuid_SSE3)
 	X86.HasPCLMULQDQ = isSet(ecx1, cpuid_PCLMULQDQ)

src/internal/cpu/cpu_x86_test.go

@@ -10,7 +10,6 @@ package cpu_test
 import (
 	. "internal/cpu"
 	"os"
-	"runtime"
 	"testing"
 )
 
@@ -20,23 +19,6 @@ func TestX86ifAVX2hasAVX(t *testing.T) {
 	}
 }
 
-func TestDisableSSE2(t *testing.T) {
-	runDebugOptionsTest(t, "TestSSE2DebugOption", "cpu.sse2=off")
-}
-
-func TestSSE2DebugOption(t *testing.T) {
-	MustHaveDebugOptionsSupport(t)
-
-	if os.Getenv("GODEBUG") != "cpu.sse2=off" {
-		t.Skipf("skipping test: GODEBUG=cpu.sse2=off not set")
-	}
-
-	want := runtime.GOARCH != "386" // SSE2 can only be disabled on 386.
-	if got := X86.HasSSE2; got != want {
-		t.Errorf("X86.HasSSE2 on %s expected %v, got %v", runtime.GOARCH, want, got)
-	}
-}
-
 func TestDisableSSE3(t *testing.T) {
 	runDebugOptionsTest(t, "TestSSE3DebugOption", "cpu.sse3=off")
 }

src/runtime/asm_386.s

@@ -838,8 +838,9 @@ TEXT runtime·cputicks(SB),NOSPLIT,$0-8
 	// When no SSE2 support is present do not enforce any serialization
 	// since using CPUID to serialize the instruction stream is
 	// very costly.
-	CMPB	internal∕cpu·X86+const_offsetX86HasSSE2(SB), $1
+#ifdef GO386_softfloat
-	JNE	rdtsc
+	JMP	rdtsc  // no fence instructions available
+#endif
 	CMPB	internal∕cpu·X86+const_offsetX86HasRDTSCP(SB), $1
 	JNE	fences
 	// Instruction stream serializing RDTSCP is supported.

src/runtime/cpuflags.go

@@ -15,7 +15,6 @@ const (
 	offsetX86HasAVX2   = unsafe.Offsetof(cpu.X86.HasAVX2)
 	offsetX86HasERMS   = unsafe.Offsetof(cpu.X86.HasERMS)
 	offsetX86HasRDTSCP = unsafe.Offsetof(cpu.X86.HasRDTSCP)
-	offsetX86HasSSE2   = unsafe.Offsetof(cpu.X86.HasSSE2)
 
 	offsetARMHasIDIVA = unsafe.Offsetof(cpu.ARM.HasIDIVA)
 

src/runtime/memclr_386.s

@@ -30,8 +30,9 @@ tail:
 	JBE	_5through8
 	CMPL	BX, $16
 	JBE	_9through16
-	CMPB	internal∕cpu·X86+const_offsetX86HasSSE2(SB), $1
+#ifdef GO386_softfloat
-	JNE	nosse2
+	JMP	nosse2
+#endif
 	PXOR	X0, X0
 	CMPL	BX, $32
 	JBE	_17through32

src/runtime/memmove_386.s

@@ -55,8 +55,9 @@ tail:
 	JBE	move_5through8
 	CMPL	BX, $16
 	JBE	move_9through16
-	CMPB	internal∕cpu·X86+const_offsetX86HasSSE2(SB), $1
+#ifdef GO386_softfloat
-	JNE	nosse2
+	JMP	nosse2
+#endif
 	CMPL	BX, $32
 	JBE	move_17through32
 	CMPL	BX, $64

src/runtime/mkpreempt.go

@@ -200,6 +200,8 @@ func gen386() {
 		l.add("MOVL", reg, 4)
 	}
 
+	softfloat := "GO386_softfloat"
+
 	// Save SSE state only if supported.
 	lSSE := layout{stack: l.stack, sp: "SP"}
 	for i := 0; i < 8; i++ {
@@ -209,13 +211,13 @@ func gen386() {
 	p("ADJSP $%d", lSSE.stack)
 	p("NOP SP")
 	l.save()
-	p("CMPB internal∕cpu·X86+const_offsetX86HasSSE2(SB), $1\nJNE nosse")
+	p("#ifndef %s", softfloat)
 	lSSE.save()
-	label("nosse:")
+	p("#endif")
 	p("CALL ·asyncPreempt2(SB)")
-	p("CMPB internal∕cpu·X86+const_offsetX86HasSSE2(SB), $1\nJNE nosse2")
+	p("#ifndef %s", softfloat)
 	lSSE.restore()
-	label("nosse2:")
+	p("#endif")
 	l.restore()
 	p("ADJSP $%d", -lSSE.stack)
 

src/runtime/preempt_386.s

@@ -14,8 +14,7 @@ TEXT ·asyncPreempt(SB),NOSPLIT|NOFRAME,$0-0
 	MOVL BP, 16(SP)
 	MOVL SI, 20(SP)
 	MOVL DI, 24(SP)
-	CMPB internal∕cpu·X86+const_offsetX86HasSSE2(SB), $1
+	#ifndef GO386_softfloat
-	JNE nosse
 	MOVUPS X0, 28(SP)
 	MOVUPS X1, 44(SP)
 	MOVUPS X2, 60(SP)
@@ -24,10 +23,9 @@ TEXT ·asyncPreempt(SB),NOSPLIT|NOFRAME,$0-0
 	MOVUPS X5, 108(SP)
 	MOVUPS X6, 124(SP)
 	MOVUPS X7, 140(SP)
-nosse:
+	#endif
 	CALL ·asyncPreempt2(SB)
-	CMPB internal∕cpu·X86+const_offsetX86HasSSE2(SB), $1
+	#ifndef GO386_softfloat
-	JNE nosse2
 	MOVUPS 140(SP), X7
 	MOVUPS 124(SP), X6
 	MOVUPS 108(SP), X5
@@ -36,7 +34,7 @@ nosse:
 	MOVUPS 60(SP), X2
 	MOVUPS 44(SP), X1
 	MOVUPS 28(SP), X0
-nosse2:
+	#endif
 	MOVL 24(SP), DI
 	MOVL 20(SP), SI
 	MOVL 16(SP), BP