cmd/compile: plumb prologueEnd into DWARF

This marks the first instruction after the prologue for
consumption by debuggers, specifically Delve, who asked
for it.  gdb appears to ignore it, lldb appears to use it.

The bits for end-of-prologue and beginning-of-epilogue
are added to Pos (reducing maximum line number by 4x, to
1048575).  They're added in cmd/internal/obj/<ARCH>.go
(currently x86 only), so the compiler-proper need not
deal with them.

The linker currently does nothing with beginning-of-epilogue,
but the plumbing exists to make it easier in the future.

This also upgrades the line number table to DWARF version 3.

This CL includes a regression in the coverage for
testdata/i22558.gdb-dbg.nexts, this appears to be a gdb
artifact but the fix would be in the preceding CL in the
stack.

Change-Id: I3bda5f46a0ed232d137ad48f65a14835c742c506
Reviewed-on: https://go-review.googlesource.com/110416
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/internal/obj/pcln.go

@@ -9,6 +9,11 @@ import (
 	"log"
 )
 
+const (
+	PrologueEnd   = 2 + iota // overload "is_stmt" to include prologue_end
+	EpilogueBegin            // overload "is_stmt" to include epilogue_end
+)
+
 func addvarint(d *Pcdata, v uint32) {
 	for ; v >= 0x80; v >>= 7 {
 		d.P = append(d.P, uint8(v|0x80))
@@ -235,6 +240,7 @@ func pctospadj(ctxt *Link, sym *LSym, oldval int32, p *Prog, phase int32, arg in
 
 // pctostmt returns either,
 // if phase==0, then whether the current instruction is a step-target (Dwarf is_stmt)
+//     bit-or'd with whether the current statement is a prologue end or epilogue begin
 // else (phase == 1), zero.
 //
 func pctostmt(ctxt *Link, sym *LSym, oldval int32, p *Prog, phase int32, arg interface{}) int32 {
@@ -242,14 +248,22 @@ func pctostmt(ctxt *Link, sym *LSym, oldval int32, p *Prog, phase int32, arg int
 		return 0 // Ignored; also different from initial value of -1, if that ever matters.
 	}
 	s := p.Pos.IsStmt()
-	if s == src.PosIsStmt {
-		return 1
+	l := p.Pos.Xlogue()
+
+	var is_stmt int32
+
+	// PrologueEnd, at least, is passed to the next instruction
+	switch l {
+	case src.PosPrologueEnd:
+		is_stmt = PrologueEnd
+	case src.PosEpilogueBegin:
+		is_stmt = EpilogueBegin
 	}
-	if s == src.PosNotStmt { // includes NoSrcPos case
-		return 0
+
+	if s != src.PosNotStmt {
+		is_stmt |= 1 // either PosDefaultStmt from asm, or PosIsStmt from go
 	}
-	// Line numbers in .s files will have no special setting, therefore default to is_stmt=1.
-	return 1
+	return is_stmt
 }
 
 // pctopcdata computes the pcdata value in effect at p.

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

@@ -33,6 +33,7 @@ package x86
 import (
 	"cmd/internal/obj"
 	"cmd/internal/objabi"
+	"cmd/internal/src"
 	"cmd/internal/sys"
 	"math"
 	"strings"
@@ -676,6 +677,10 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 		p = stacksplit(ctxt, cursym, p, newprog, autoffset, int32(textarg)) // emit split check
 	}
 
+	// Delve debugger would like the next instruction to be noted as the end of the function prologue.
+	// TODO: are there other cases (e.g., wrapper functions) that need marking?
+	markedPrologue := false
+
 	if autoffset != 0 {
 		if autoffset%int32(ctxt.Arch.RegSize) != 0 {
 			ctxt.Diag("unaligned stack size %d", autoffset)
@@ -685,6 +690,8 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 		p.From.Type = obj.TYPE_CONST
 		p.From.Offset = int64(autoffset)
 		p.Spadj = autoffset
+		p.Pos = p.Pos.WithXlogue(src.PosPrologueEnd)
+		markedPrologue = true
 	}
 
 	deltasp := autoffset
@@ -700,6 +707,9 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 		p.To.Reg = REG_SP
 		p.To.Scale = 1
 		p.To.Offset = int64(autoffset) - int64(bpsize)
+		if !markedPrologue {
+			p.Pos = p.Pos.WithXlogue(src.PosPrologueEnd)
+		}
 
 		// Move current frame to BP
 		p = obj.Appendp(p, newprog)

src/cmd/internal/src/pos.go

@@ -293,7 +293,7 @@ func (b *PosBase) InliningIndex() int {
 // A lico is a compact encoding of a LIne and COlumn number.
 type lico uint32
 
-// Layout constants: 22 bits for line, 8 bits for column, 2 for isStmt
+// Layout constants: 20 bits for line, 8 bits for column, 2 for isStmt, 2 for pro/epilogue
 // (If this is too tight, we can either make lico 64b wide,
 // or we can introduce a tiered encoding where we remove column
 // information as line numbers grow bigger; similar to what gcc
@@ -301,13 +301,18 @@ type lico uint32
 // The bitfield order is chosen to make IsStmt be the least significant
 // part of a position; its use is to communicate statement edges through
 // instruction scrambling in code generation, not to impose an order.
+// TODO: Prologue and epilogue are perhaps better handled as psuedoops for the assembler,
+// because they have almost no interaction with other uses of the position.
 const (
-	lineBits, lineMax     = 22, 1<<lineBits - 1
+	lineBits, lineMax     = 20, 1<<lineBits - 1
 	isStmtBits, isStmtMax = 2, 1<<isStmtBits - 1
-	colBits, colMax       = 32 - lineBits - isStmtBits, 1<<colBits - 1
-	isStmtShift           = 0
-	colShift              = isStmtBits + isStmtShift
-	lineShift             = colBits + colShift
+	xlogueBits, xlogueMax = 2, 1<<xlogueBits - 1
+	colBits, colMax       = 32 - lineBits - xlogueBits - isStmtBits, 1<<colBits - 1
+
+	isStmtShift = 0
+	xlogueShift = isStmtBits + isStmtShift
+	colShift    = xlogueBits + xlogueShift
+	lineShift   = colBits + colShift
 )
 const (
 	// It is expected that the front end or a phase in SSA will usually generate positions tagged with
@@ -342,6 +347,14 @@ const (
 	PosNotStmt                 // Position should not be a statement boundary, but line should be preserved for profiling and low-level debugging purposes.
 )
 
+type PosXlogue uint
+
+const (
+	PosDefaultLogue PosXlogue = iota
+	PosPrologueEnd
+	PosEpilogueBegin
+)
+
 func makeLico(line, col uint) lico {
 	if line > lineMax {
 		// cannot represent line, use max. line so we have some information
@@ -363,6 +376,9 @@ func (x lico) IsStmt() uint {
 	}
 	return uint(x) >> isStmtShift & isStmtMax
 }
+func (x lico) Xlogue() PosXlogue {
+	return PosXlogue(uint(x) >> xlogueShift & xlogueMax)
+}
 
 // withNotStmt returns a lico for the same location, but not a statement
 func (x lico) withNotStmt() lico {
@@ -379,6 +395,18 @@ func (x lico) withIsStmt() lico {
 	return x.withStmt(PosIsStmt)
 }
 
+// withLogue attaches a prologue/epilogue attribute to a lico
+func (x lico) withXlogue(xlogue PosXlogue) lico {
+	if x == 0 {
+		if xlogue == 0 {
+			return x
+		}
+		// Normalize 0 to "not a statement"
+		x = lico(PosNotStmt << isStmtShift)
+	}
+	return lico(uint(x) & ^uint(xlogueMax<<xlogueShift) | (uint(xlogue) << xlogueShift))
+}
+
 // withStmt returns a lico for the same location with specified is_stmt attribute
 func (x lico) withStmt(stmt uint) lico {
 	if x == 0 {

src/cmd/internal/src/pos_test.go

@@ -184,3 +184,43 @@ func TestIsStmt(t *testing.T) {
 		}
 	}
 }
+
+func TestLogue(t *testing.T) {
+	defp := fmt.Sprintf(":%d", PosDefaultLogue)
+	pro := fmt.Sprintf(":%d", PosPrologueEnd)
+	epi := fmt.Sprintf(":%d", PosEpilogueBegin)
+
+	defs := fmt.Sprintf(":%d", PosDefaultStmt)
+	not := fmt.Sprintf(":%d", PosNotStmt)
+
+	for i, test := range []struct {
+		x         lico
+		string    string
+		line, col uint
+	}{
+		{makeLico(0, 0).withXlogue(PosDefaultLogue), ":0" + not + defp, 0, 0},
+		{makeLico(0, 0).withXlogue(PosPrologueEnd), ":0" + not + pro, 0, 0},
+		{makeLico(0, 0).withXlogue(PosEpilogueBegin), ":0" + not + epi, 0, 0},
+
+		{makeLico(0, 1).withXlogue(PosDefaultLogue), ":0:1" + defs + defp, 0, 1},
+		{makeLico(0, 1).withXlogue(PosPrologueEnd), ":0:1" + defs + pro, 0, 1},
+		{makeLico(0, 1).withXlogue(PosEpilogueBegin), ":0:1" + defs + epi, 0, 1},
+
+		{makeLico(1, 0).withXlogue(PosDefaultLogue), ":1" + defs + defp, 1, 0},
+		{makeLico(1, 0).withXlogue(PosPrologueEnd), ":1" + defs + pro, 1, 0},
+		{makeLico(1, 0).withXlogue(PosEpilogueBegin), ":1" + defs + epi, 1, 0},
+
+		{makeLico(1, 1).withXlogue(PosDefaultLogue), ":1:1" + defs + defp, 1, 1},
+		{makeLico(1, 1).withXlogue(PosPrologueEnd), ":1:1" + defs + pro, 1, 1},
+		{makeLico(1, 1).withXlogue(PosEpilogueBegin), ":1:1" + defs + epi, 1, 1},
+
+		{makeLico(lineMax, 1).withXlogue(PosDefaultLogue), fmt.Sprintf(":%d:1", lineMax) + defs + defp, lineMax, 1},
+		{makeLico(lineMax, 1).withXlogue(PosPrologueEnd), fmt.Sprintf(":%d:1", lineMax) + defs + pro, lineMax, 1},
+		{makeLico(lineMax, 1).withXlogue(PosEpilogueBegin), fmt.Sprintf(":%d:1", lineMax) + defs + epi, lineMax, 1},
+	} {
+		x := test.x
+		if got := format("", x.Line(), x.Col(), true) + fmt.Sprintf(":%d:%d", x.IsStmt(), x.Xlogue()); got != test.string {
+			t.Errorf("%d: %s: got %q", i, test.string, got)
+		}
+	}
+}

src/cmd/internal/src/xpos.go

@@ -60,6 +60,12 @@ func (p XPos) WithIsStmt() XPos {
 	return p
 }
 
+// WithXlogue returns the same location but marked with DWARF function prologue/epilogue
+func (p XPos) WithXlogue(x PosXlogue) XPos {
+	p.lico = p.lico.withXlogue(x)
+	return p
+}
+
 func (p XPos) LineNumber() string {
 	if !p.IsKnown() {
 		return "?"

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

@@ -15,6 +15,7 @@ package ld
 
 import (
 	"cmd/internal/dwarf"
+	"cmd/internal/obj"
 	"cmd/internal/objabi"
 	"cmd/internal/sys"
 	"cmd/link/internal/sym"
@@ -953,7 +954,7 @@ const (
 	LINE_BASE   = -4
 	LINE_RANGE  = 10
 	PC_RANGE    = (255 - OPCODE_BASE) / LINE_RANGE
-	OPCODE_BASE = 10
+	OPCODE_BASE = 11
 )
 
 func putpclcdelta(linkctxt *Link, ctxt dwarf.Context, s *sym.Symbol, deltaPC uint64, deltaLC int64) {
@@ -1157,7 +1158,7 @@ func writelines(ctxt *Link, lib *sym.Library, textp []*sym.Symbol, ls *sym.Symbo
 	unitLengthOffset := ls.Size
 	ls.AddUint32(ctxt.Arch, 0) // unit_length (*), filled in at end.
 	unitstart = ls.Size
-	ls.AddUint16(ctxt.Arch, 2) // dwarf version (appendix F)
+	ls.AddUint16(ctxt.Arch, 3) // dwarf version (appendix F)
 	headerLengthOffset := ls.Size
 	ls.AddUint32(ctxt.Arch, 0) // header_length (*), filled in at end.
 	headerstart = ls.Size
@@ -1177,6 +1178,7 @@ func writelines(ctxt *Link, lib *sym.Library, textp []*sym.Symbol, ls *sym.Symbo
 	ls.AddUint8(0)                // standard_opcode_lengths[7]
 	ls.AddUint8(0)                // standard_opcode_lengths[8]
 	ls.AddUint8(1)                // standard_opcode_lengths[9]
+	ls.AddUint8(0)                // standard_opcode_lengths[10]
 	ls.AddUint8(0)                // include_directories  (empty)
 
 	// Create the file table. fileNums maps from global file
@@ -1271,8 +1273,19 @@ func writelines(ctxt *Link, lib *sym.Library, textp []*sym.Symbol, ls *sym.Symbo
 
 			// Only changed if it advanced
 			if is_stmt != uint8(pcstmt.value) {
-				is_stmt = uint8(pcstmt.value)
-				ls.AddUint8(uint8(dwarf.DW_LNS_negate_stmt))
+				new_stmt := uint8(pcstmt.value)
+				switch new_stmt &^ 1 {
+				case obj.PrologueEnd:
+					ls.AddUint8(uint8(dwarf.DW_LNS_set_prologue_end))
+				case obj.EpilogueBegin:
+					// TODO if there is a use for this, add it.
+					// Don't forget to increase OPCODE_BASE by 1 and add entry for standard_opcode_lengths[11]
+				}
+				new_stmt &= 1
+				if is_stmt != new_stmt {
+					is_stmt = new_stmt
+					ls.AddUint8(uint8(dwarf.DW_LNS_negate_stmt))
+				}
 			}
 
 			// putpcldelta makes a row in the DWARF matrix, always, even if line is unchanged.