build: enable framepointer mode by default

This has a minor performance cost, but far less than is being gained by SSA.
As an experiment, enable it during the Go 1.7 beta.
Having frame pointers on by default makes Linux's perf, Intel VTune,
and other profilers much more useful, because it lets them gather a
stack trace efficiently on profiling events.
(It doesn't help us that much, since when we walk the stack we usually
need to look up PC-specific information as well.)

Fixes #15840.

Change-Id: I4efd38412a0de4a9c87b1b6e5d11c301e63f1a2a
Reviewed-on: https://go-review.googlesource.com/23451
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/amd64/galign.go

@@ -25,18 +25,16 @@ func betypeinit() {
 		cmpptr = x86.ACMPL
 	}
 
-	if gc.Ctxt.Flag_dynlink {
-		gc.Thearch.ReservedRegs = append(gc.Thearch.ReservedRegs, x86.REG_R15)
+	if gc.Ctxt.Flag_dynlink || obj.Getgoos() == "nacl" {
+		resvd = append(resvd, x86.REG_R15)
 	}
+	if gc.Ctxt.Framepointer_enabled || obj.Getgoos() == "nacl" {
+		resvd = append(resvd, x86.REG_BP)
+	}
+	gc.Thearch.ReservedRegs = resvd
 }
 
 func Main() {
-	if obj.Getgoos() == "nacl" {
-		resvd = append(resvd, x86.REG_BP, x86.REG_R15)
-	} else if obj.Framepointer_enabled != 0 {
-		resvd = append(resvd, x86.REG_BP)
-	}
-
 	gc.Thearch.LinkArch = &x86.Linkamd64
 	if obj.Getgoarch() == "amd64p32" {
 		gc.Thearch.LinkArch = &x86.Linkamd64p32
@@ -51,7 +49,6 @@ func Main() {
 	gc.Thearch.FREGMIN = x86.REG_X0
 	gc.Thearch.FREGMAX = x86.REG_X15
 	gc.Thearch.MAXWIDTH = 1 << 50
-	gc.Thearch.ReservedRegs = resvd
 
 	gc.Thearch.AddIndex = addindex
 	gc.Thearch.Betypeinit = betypeinit

src/cmd/compile/internal/amd64/reg.go

@@ -32,7 +32,6 @@ package amd64
 
 import (
 	"cmd/compile/internal/gc"
-	"cmd/internal/obj"
 	"cmd/internal/obj/x86"
 )
 
@@ -121,7 +120,7 @@ func BtoR(b uint64) int {
 	b &= 0xffff
 	if gc.Nacl {
 		b &^= (1<<(x86.REG_BP-x86.REG_AX) | 1<<(x86.REG_R15-x86.REG_AX))
-	} else if obj.Framepointer_enabled != 0 {
+	} else if gc.Ctxt.Framepointer_enabled {
 		// BP is part of the calling convention if framepointer_enabled.
 		b &^= (1 << (x86.REG_BP - x86.REG_AX))
 	}

src/cmd/compile/internal/ssa/regalloc.go

@@ -106,7 +106,6 @@
 package ssa
 
 import (
-	"cmd/internal/obj"
 	"fmt"
 	"unsafe"
 )
@@ -456,7 +455,7 @@ func (s *regAllocState) init(f *Func) {
 	s.allocatable = regMask(1)<<s.numRegs - 1
 	s.allocatable &^= 1 << s.SPReg
 	s.allocatable &^= 1 << s.SBReg
-	if obj.Framepointer_enabled != 0 {
+	if s.f.Config.ctxt.Framepointer_enabled {
 		s.allocatable &^= 1 << 5 // BP
 	}
 	if s.f.Config.ctxt.Flag_dynlink {

src/cmd/internal/obj/go.go

@@ -13,7 +13,7 @@ import (
 // go-specific code shared across loaders (5l, 6l, 8l).
 
 var (
-	Framepointer_enabled int
+	framepointer_enabled int
 	Fieldtrack_enabled   int
 )
 
@@ -26,14 +26,21 @@ var exper = []struct {
 	val  *int
 }{
 	{"fieldtrack", &Fieldtrack_enabled},
-	{"framepointer", &Framepointer_enabled},
+	{"framepointer", &framepointer_enabled},
 }
 
 func addexp(s string) {
+	// Could do general integer parsing here, but the runtime copy doesn't yet.
+	v := 1
+	name := s
+	if len(name) > 2 && name[:2] == "no" {
+		v = 0
+		name = name[2:]
+	}
 	for i := 0; i < len(exper); i++ {
-		if exper[i].name == s {
+		if exper[i].name == name {
 			if exper[i].val != nil {
-				*exper[i].val = 1
+				*exper[i].val = v
 			}
 			return
 		}
@@ -44,6 +51,7 @@ func addexp(s string) {
 }
 
 func init() {
+	framepointer_enabled = 1 // default
 	for _, f := range strings.Split(goexperiment, ",") {
 		if f != "" {
 			addexp(f)
@@ -51,6 +59,10 @@ func init() {
 	}
 }
 
+func Framepointer_enabled(goos, goarch string) bool {
+	return framepointer_enabled != 0 && goarch == "amd64" && goos != "nacl"
+}
+
 func Nopout(p *Prog) {
 	p.As = ANOP
 	p.Scond = 0

src/cmd/internal/obj/link.go

@@ -664,6 +664,8 @@ type Link struct {
 	Etextp        *LSym
 	Errors        int
 
+	Framepointer_enabled bool
+
 	// state for writing objects
 	Text []*LSym
 	Data []*LSym

src/cmd/internal/obj/sym.go

@@ -106,6 +106,7 @@ func Linknew(arch *LinkArch) *Link {
 	}
 
 	ctxt.Flag_optimize = true
+	ctxt.Framepointer_enabled = Framepointer_enabled(Getgoos(), arch.Name)
 	return ctxt
 }
 

src/cmd/internal/obj/x86/asm6.go

@@ -3765,7 +3765,7 @@ func doasm(ctxt *obj.Link, p *obj.Prog) {
 					ctxt.Diag("directly calling duff when dynamically linking Go")
 				}
 
-				if obj.Framepointer_enabled != 0 && yt.zcase == Zcallduff && p.Mode == 64 {
+				if ctxt.Framepointer_enabled && yt.zcase == Zcallduff && p.Mode == 64 {
 					// Maintain BP around call, since duffcopy/duffzero can't do it
 					// (the call jumps into the middle of the function).
 					// This makes it possible to see call sites for duffcopy/duffzero in
@@ -3784,7 +3784,7 @@ func doasm(ctxt *obj.Link, p *obj.Prog) {
 				r.Siz = 4
 				ctxt.AsmBuf.PutInt32(0)
 
-				if obj.Framepointer_enabled != 0 && yt.zcase == Zcallduff && p.Mode == 64 {
+				if ctxt.Framepointer_enabled && yt.zcase == Zcallduff && p.Mode == 64 {
 					// Pop BP pushed above.
 					// MOVQ 0(BP), BP
 					ctxt.AsmBuf.Put(bpduff2)

src/cmd/internal/obj/x86/obj6.go

@@ -610,7 +610,7 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym) {
 	}
 
 	var bpsize int
-	if p.Mode == 64 && obj.Framepointer_enabled != 0 && autoffset > 0 && p.From3.Offset&obj.NOFRAME == 0 {
+	if p.Mode == 64 && ctxt.Framepointer_enabled && autoffset > 0 && p.From3.Offset&obj.NOFRAME == 0 {
 		// Make room for to save a base pointer. If autoffset == 0,
 		// this might do something special like a tail jump to
 		// another function, so in that case we omit this.

src/cmd/link/internal/ld/dwarf.go

@@ -1558,7 +1558,7 @@ func writelines(prev *LSym) *LSym {
 				if !haslinkregister() {
 					offs -= int64(SysArch.PtrSize)
 				}
-				if obj.Framepointer_enabled != 0 {
+				if obj.Framepointer_enabled(obj.Getgoos(), obj.Getgoarch()) {
 					// The frame pointer is saved
 					// between the CFA and the
 					// autos.

src/cmd/link/internal/ld/lib.go

@@ -639,11 +639,17 @@ func loadlib() {
 		// recording the value of GOARM.
 		if SysArch.Family == sys.ARM {
 			s := Linklookup(Ctxt, "runtime.goarm", 0)
-
 			s.Type = obj.SRODATA
 			s.Size = 0
 			Adduint8(Ctxt, s, uint8(Ctxt.Goarm))
 		}
+
+		if obj.Framepointer_enabled(obj.Getgoos(), obj.Getgoarch()) {
+			s := Linklookup(Ctxt, "runtime.framepointer_enabled", 0)
+			s.Type = obj.SRODATA
+			s.Size = 0
+			Adduint8(Ctxt, s, 1)
+		}
 	} else {
 		// If OTOH the module does not contain the runtime package,
 		// create a local symbol for the moduledata.

src/runtime/proc.go

@@ -434,9 +434,6 @@ func schedinit() {
 
 	sched.maxmcount = 10000
 
-	// Cache the framepointer experiment. This affects stack unwinding.
-	framepointer_enabled = haveexperiment("framepointer")
-
 	tracebackinit()
 	moduledataverify()
 	stackinit()
@@ -4163,6 +4160,9 @@ func setMaxThreads(in int) (out int) {
 }
 
 func haveexperiment(name string) bool {
+	if name == "framepointer" {
+		return framepointer_enabled // set by linker
+	}
 	x := sys.Goexperiment
 	for x != "" {
 		xname := ""
@@ -4175,6 +4175,9 @@ func haveexperiment(name string) bool {
 		if xname == name {
 			return true
 		}
+		if len(xname) > 2 && xname[:2] == "no" && xname[2:] == name {
+			return false
+		}
 	}
 	return false
 }

src/runtime/runtime2.go

@@ -725,7 +725,8 @@ var (
 	support_avx       bool
 	support_avx2      bool
 
-	goarm uint8 // set by cmd/link on arm systems
+	goarm                uint8 // set by cmd/link on arm systems
+	framepointer_enabled bool  // set by cmd/link
 )
 
 // Set by the linker so the runtime can determine the buildmode.

src/runtime/stack.go

@@ -155,9 +155,6 @@ var stackLarge struct {
 	free [_MHeapMap_Bits]mSpanList // free lists by log_2(s.npages)
 }
 
-// Cached value of haveexperiment("framepointer")
-var framepointer_enabled bool
-
 func stackinit() {
 	if _StackCacheSize&_PageMask != 0 {
 		throw("cache size must be a multiple of page size")