runtime/pprof: set Function.start_line field

Now that we plumb the start line to the runtime, we can include in pprof
files. Since runtime.Frame.startLine is not (currently) exported, we
need a runtime helper to get the value.

For #55022.
Updates #56135.

Change-Id: Ifc5b68a7b7170fd7895e4099deb24df7977b22ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/438255
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/pprof/proto.go

@@ -590,6 +590,7 @@ func (b *profileBuilder) emitLocation() uint64 {
 	type newFunc struct {
 		id         uint64
 		name, file string
+		startLine  int64
 	}
 	newFuncs := make([]newFunc, 0, 8)
 
@@ -610,7 +611,12 @@ func (b *profileBuilder) emitLocation() uint64 {
 		if funcID == 0 {
 			funcID = uint64(len(b.funcs)) + 1
 			b.funcs[frame.Function] = int(funcID)
-			newFuncs = append(newFuncs, newFunc{funcID, frame.Function, frame.File})
+			newFuncs = append(newFuncs, newFunc{
+				id:        funcID,
+				name:      frame.Function,
+				file:      frame.File,
+				startLine: int64(runtime_FrameStartLine(&frame)),
+			})
 		}
 		b.pbLine(tagLocation_Line, funcID, int64(frame.Line))
 	}
@@ -633,6 +639,7 @@ func (b *profileBuilder) emitLocation() uint64 {
 		b.pb.int64Opt(tagFunction_Name, b.stringIndex(fn.name))
 		b.pb.int64Opt(tagFunction_SystemName, b.stringIndex(fn.name))
 		b.pb.int64Opt(tagFunction_Filename, b.stringIndex(fn.file))
+		b.pb.int64Opt(tagFunction_StartLine, fn.startLine)
 		b.pb.endMessage(tagProfile_Function, start)
 	}
 

src/runtime/pprof/runtime.go

@@ -6,9 +6,13 @@ package pprof
 
 import (
 	"context"
+	"runtime"
 	"unsafe"
 )
 
+// runtime_FrameStartLine is defined in runtime/symtab.go.
+func runtime_FrameStartLine(f *runtime.Frame) int
+
 // runtime_expandFinalInlineFrame is defined in runtime/symtab.go.
 func runtime_expandFinalInlineFrame(stk []uintptr) []uintptr
 

src/runtime/symtab.go

@@ -170,6 +170,13 @@ func (ci *Frames) Next() (frame Frame, more bool) {
 	return
 }
 
+// runtime_FrameStartLine returns the start line of the function in a Frame.
+//
+//go:linkname runtime_FrameStartLine runtime/pprof.runtime_FrameStartLine
+func runtime_FrameStartLine(f *Frame) int {
+	return f.startLine
+}
+
 // runtime_expandFinalInlineFrame expands the final pc in stk to include all
 // "callers" if pc is inline.
 //