cmd/compile,cmd/link,runtime: add start line numbers to func metadata

This adds the function "start line number" to runtime._func and
runtime.inlinedCall objects. The "start line number" is the line number
of the func keyword or TEXT directive for assembly.

Subtracting the start line number from PC line number provides the
relative line offset of a PC from the the start of the function. This
helps with source stability by allowing code above the function to move
without invalidating samples within the function.

Encoding start line rather than relative lines directly is convenient
because the pprof format already contains a start line field.

This CL uses a straightforward encoding of explictly including a start
line field in every _func and inlinedCall. It is possible that we could
compress this further in the future. e.g., functions with a prologue
usually have <line of PC 0> == <start line>. In runtime.test, 95% of
functions have <line of PC 0> == <start line>.

According to bent, this is geomean +0.83% binary size vs master and
-0.31% binary size vs 1.19.

Note that //line directives can change the file and line numbers
arbitrarily. The encoded start line is as adjusted by //line directives.
Since this can change in the middle of a function, `line - start line`
offset calculations may not be meaningful if //line directives are in
use.

For #55022.

Change-Id: Iaabbc6dd4f85ffdda294266ef982ae838cc692f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/429638
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

2 files changed, 13 insertions, 9 deletions

diff --git a/src/cmd/internal/goobj/funcinfo.go b/src/cmd/internal/goobj/funcinfo.go
index 59cb957fa7..fbcf9d9bb5 100644
--- a/src/cmd/internal/goobj/funcinfo.go
+++ b/src/cmd/internal/goobj/funcinfo.go
@@ -17,12 +17,13 @@ type CUFileIndex uint32
 // FuncInfo is serialized as a symbol (aux symbol). The symbol data is
 // the binary encoding of the struct below.
 type FuncInfo struct {
-	Args     uint32
-	Locals   uint32
-	FuncID   objabi.FuncID
-	FuncFlag objabi.FuncFlag
-	File     []CUFileIndex
-	InlTree  []InlTreeNode
+	Args      uint32
+	Locals    uint32
+	FuncID    objabi.FuncID
+	FuncFlag  objabi.FuncFlag
+	StartLine int32
+	File      []CUFileIndex
+	InlTree   []InlTreeNode
 }
 
 func (a *FuncInfo) Write(w *bytes.Buffer) {
@@ -41,6 +42,7 @@ func (a *FuncInfo) Write(w *bytes.Buffer) {
 	writeUint8(uint8(a.FuncFlag))
 	writeUint8(0) // pad to uint32 boundary
 	writeUint8(0)
+	writeUint32(uint32(a.StartLine))
 
 	writeUint32(uint32(len(a.File)))
 	for _, f := range a.File {
@@ -70,7 +72,7 @@ func (*FuncInfo) ReadFuncInfoLengths(b []byte) FuncInfoLengths {
 
 	// Offset to the number of the file table. This value is determined by counting
 	// the number of bytes until we write funcdataoff to the file.
-	const numfileOff = 12
+	const numfileOff = 16
 	result.NumFile = binary.LittleEndian.Uint32(b[numfileOff:])
 	result.FileOff = numfileOff + 4
 
@@ -91,6 +93,8 @@ func (*FuncInfo) ReadFuncID(b []byte) objabi.FuncID { return objabi.FuncID(b[8])
 
 func (*FuncInfo) ReadFuncFlag(b []byte) objabi.FuncFlag { return objabi.FuncFlag(b[9]) }
 
+func (*FuncInfo) ReadStartLine(b []byte) int32 { return int32(binary.LittleEndian.Uint32(b[12:])) }
+
 func (*FuncInfo) ReadFile(b []byte, filesoff uint32, k uint32) CUFileIndex {
 	return CUFileIndex(binary.LittleEndian.Uint32(b[filesoff+4*k:]))
 }
diff --git a/src/cmd/internal/goobj/objfile.go b/src/cmd/internal/goobj/objfile.go
index 39b86b0f8f..4276df3d19 100644
--- a/src/cmd/internal/goobj/objfile.go
+++ b/src/cmd/internal/goobj/objfile.go
@@ -30,7 +30,7 @@ import (
 // New object file format.
 //
 //    Header struct {
-//       Magic       [...]byte   // "\x00go118ld"
+//       Magic       [...]byte   // "\x00go120ld"
 //       Fingerprint [8]byte
 //       Flags       uint32
 //       Offsets     [...]uint32 // byte offset of each block below
@@ -215,7 +215,7 @@ type Header struct {
 	Offsets     [NBlk]uint32
 }
 
-const Magic = "\x00go118ld"
+const Magic = "\x00go120ld"
 
 func (h *Header) Write(w *Writer) {
 	w.RawString(h.Magic)