Revert "cmd/compile: walk the progs to generate debug_lines"

This reverts CL 196661.

Reason for revert: broke TestGdb* tests on mips64le, ppc64le, and s390x builders.

Change-Id: I3b5c97c840819a0d407b943f7cf7e2d97f06042d
Reviewed-on: https://go-review.googlesource.com/c/go/+/198697
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/dwarf.go

@@ -8,7 +8,6 @@ package obj
 
 import (
 	"cmd/internal/dwarf"
-	"cmd/internal/src"
 	"fmt"
 )
 
@@ -32,51 +31,95 @@ const (
 func (ctxt *Link) generateDebugLinesSymbol(s, lines *LSym) {
 	dctxt := dwCtxt{ctxt}
 
+	// The Pcfile table is used to generate the debug_lines section, and the file
+	// indices for that data could differ from the files we write out for the
+	// debug_lines section. Here we generate a LUT between those two indices.
+	fileNums := make(map[int32]int64)
+	for i, filename := range s.Func.Pcln.File {
+		if symbolIndex := ctxt.PosTable.FileIndex(filename); symbolIndex >= 0 {
+			fileNums[int32(i)] = int64(symbolIndex) + 1
+		} else {
+			panic(fmt.Sprintf("First time we've seen filename: %q", filename))
+		}
+	}
+
 	// Set up the debug_lines state machine.
 	// NB: This state machine is reset to this state when we've finished
 	// generating the line table. See below.
 	// TODO: Once delve can support multiple DW_LNS_end_statements, we don't have
 	// to do this.
-	stmt := true
-	line := int64(1)
+	is_stmt := uint8(1)
 	pc := s.Func.Text.Pc
-	name := ""
-	prologue, wrotePrologue := false, false
+	line := 1
+	file := 1
 
-	// Walk the progs, generating the DWARF table.
-	for p := s.Func.Text; p != nil; p = p.Link {
-		prologue = prologue || (p.Pos.Xlogue() == src.PosPrologueEnd)
-		// If we're not at a real instruction, keep looping!
-		if p.Pos.Line() == 0 || (p.Link != nil && p.Link.Pc == pc) {
-			continue
-		}
-		newStmt := p.Pos.IsStmt() != src.PosNotStmt
-		newName, newLine := linkgetlineFromPos(ctxt, p.Pos)
+	// The linker will insert the DW_LNE_set_address once determined; therefore,
+	// it's omitted here.
 
-		// Output debug info.
-		wrote := false
-		if name != newName {
-			newFile := ctxt.PosTable.FileIndex(newName) + 1 // 1 indexing for the table.
+	// Generate the actual line information.
+	// We use the pcline and pcfile to generate this section, and it's suboptimal.
+	// Likely better would be to generate this dirrectly from the progs and not
+	// parse those tables.
+	// TODO: Generate from the progs if it's faster.
+	pcfile := NewPCIter(uint32(ctxt.Arch.Arch.MinLC))
+	pcline := NewPCIter(uint32(ctxt.Arch.Arch.MinLC))
+	pcstmt := NewPCIter(uint32(ctxt.Arch.Arch.MinLC))
+	pcfile.Init(s.Func.Pcln.Pcfile.P)
+	pcline.Init(s.Func.Pcln.Pcline.P)
+	var pctostmtData Pcdata
+	funcpctab(ctxt, &pctostmtData, s, "pctostmt", pctostmt, nil)
+	pcstmt.Init(pctostmtData.P)
+	var thispc uint32
+
+	for !pcfile.Done && !pcline.Done {
+		// Only changed if it advanced
+		if int32(file) != pcfile.Value {
 			dctxt.AddUint8(lines, dwarf.DW_LNS_set_file)
-			dwarf.Uleb128put(dctxt, lines, int64(newFile))
-			name = newName
-			wrote = true
-		}
-		if prologue && !wrotePrologue {
-			dctxt.AddUint8(lines, uint8(dwarf.DW_LNS_set_prologue_end))
-			wrotePrologue = true
-			wrote = true
-		}
-		if stmt != newStmt {
-			dctxt.AddUint8(lines, uint8(dwarf.DW_LNS_negate_stmt))
-			stmt = newStmt
-			wrote = true
+			dwarf.Uleb128put(dctxt, lines, fileNums[pcfile.Value])
+			file = int(pcfile.Value)
 		}
 
-		if line != int64(newLine) || wrote {
-			pcdelta := (p.Pc - pc) / int64(ctxt.Arch.MinLC)
-			putpclcdelta(ctxt, dctxt, lines, uint64(pcdelta), int64(newLine)-line)
-			line, pc = int64(newLine), p.Pc
+		// Only changed if it advanced
+		if is_stmt != uint8(pcstmt.Value) {
+			new_stmt := uint8(pcstmt.Value)
+			switch new_stmt &^ 1 {
+			case PrologueEnd:
+				dctxt.AddUint8(lines, uint8(dwarf.DW_LNS_set_prologue_end))
+			case 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]
+				panic("unsupported EpilogueBegin")
+			}
+			new_stmt &= 1
+			if is_stmt != new_stmt {
+				is_stmt = new_stmt
+				dctxt.AddUint8(lines, uint8(dwarf.DW_LNS_negate_stmt))
+			}
+		}
+
+		// putpcldelta makes a row in the DWARF matrix, always, even if line is unchanged.
+		putpclcdelta(ctxt, dctxt, lines, uint64(s.Func.Text.Pc+int64(thispc)-pc), int64(pcline.Value)-int64(line))
+
+		pc = s.Func.Text.Pc + int64(thispc)
+		line = int(pcline.Value)
+
+		// Take the minimum step forward for the three iterators
+		thispc = pcfile.NextPC
+		if pcline.NextPC < thispc {
+			thispc = pcline.NextPC
+		}
+		if !pcstmt.Done && pcstmt.NextPC < thispc {
+			thispc = pcstmt.NextPC
+		}
+
+		if pcfile.NextPC == thispc {
+			pcfile.Next()
+		}
+		if !pcstmt.Done && pcstmt.NextPC == thispc {
+			pcstmt.Next()
+		}
+		if pcline.NextPC == thispc {
+			pcline.Next()
 		}
 	}
 
@@ -86,16 +129,16 @@ func (ctxt *Link) generateDebugLinesSymbol(s, lines *LSym) {
 	//   file = 1
 	//   line = 1
 	//   column = 0
-	//   stmt = set in header, we assume true
+	//   is_stmt = set in header, we assume true
 	//   basic_block = false
 	// Careful readers of the DWARF specification will note that we don't reset
 	// the address of the state machine -- but this will happen at the beginning
-	// of the NEXT block of opcodes.
+	// of the NEXT block of opcodes. (See the SetAddress call above.)
 	dctxt.AddUint8(lines, dwarf.DW_LNS_set_file)
 	dwarf.Uleb128put(dctxt, lines, 1)
 	dctxt.AddUint8(lines, dwarf.DW_LNS_advance_line)
 	dwarf.Sleb128put(dctxt, lines, int64(1-line))
-	if !stmt {
+	if is_stmt != 1 {
 		dctxt.AddUint8(lines, dwarf.DW_LNS_negate_stmt)
 	}
 	dctxt.AddUint8(lines, dwarf.DW_LNS_copy)

src/cmd/internal/obj/pcln.go

@@ -5,6 +5,7 @@
 package obj
 
 import (
+	"cmd/internal/src"
 	"encoding/binary"
 	"log"
 )
@@ -248,6 +249,34 @@ func pctospadj(ctxt *Link, sym *LSym, oldval int32, p *Prog, phase int32, arg in
 	return oldval + p.Spadj
 }
 
+// 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 {
+	if phase == 1 {
+		return 0 // Ignored; also different from initial value of -1, if that ever matters.
+	}
+	s := p.Pos.IsStmt()
+	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 {
+		is_stmt |= 1 // either PosDefaultStmt from asm, or PosIsStmt from go
+	}
+	return is_stmt
+}
+
 // pctopcdata computes the pcdata value in effect at p.
 // A PCDATA instruction sets the value in effect at future
 // non-PCDATA instructions.