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cmd/link: process is_stmt data into dwarf line tables

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To improve debugging, instructions should be annotated with
DWARF is_stmt.  The DWARF default before was is_stmt=1, and
to remove "jumpy" stepping the optimizer was tagging
instructions with a no-position position, which interferes
with the accuracy of profiling information.  This allows
that to be corrected, and also allows more "jumpy" positions
to be annotated with is_stmt=0 (these changes were not made
for 1.10 because of worries about further messing up
profiling).

The is_stmt values are placed in a pc-encoded table and
passed through a symbol derived from the name of the
function and processed in the linker alongside its
processing of each function's pc/line tables.

The only change in binary size is in the .debug_line tables
measured with "objdump -h --section=.debug_line go1.test"
For go1.test, these are 2614 bytes larger,
or 0.72% of the size of .debug_line,
or 0.025% of the file size.

This will increase in proportion to how much the is_stmt
flag is used (toggled).

Change-Id: Ic1f1aeccff44591ad0494d29e1a0202a3c506a7a
Reviewed-on: https://go-review.googlesource.com/93664
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
This commit is contained in:
David Chase 2018-02-13 17:39:38 -05:00
parent 619679a397
commit dead03b794
11 changed files with 100 additions and 29 deletions
Download patch file Download diff file Expand all files Collapse all files

8
src/cmd/internal/obj/objfile.go
View file

@ -497,7 +497,7 @@ func (c dwCtxt) Logf(format string, args ...interface{}) {
c.Link.Logf(format, args...)
}
func (ctxt *Link) dwarfSym(s *LSym) (dwarfInfoSym, dwarfLocSym, dwarfRangesSym, dwarfAbsFnSym *LSym) {
func (ctxt *Link) dwarfSym(s *LSym) (dwarfInfoSym, dwarfLocSym, dwarfRangesSym, dwarfAbsFnSym, dwarfIsStmtSym *LSym) {
if s.Type != objabi.STEXT {
ctxt.Diag("dwarfSym of non-TEXT %v", s)
}
@ -510,9 +510,10 @@ func (ctxt *Link) dwarfSym(s *LSym) (dwarfInfoSym, dwarfLocSym, dwarfRangesSym,
if s.WasInlined() {
s.Func.dwarfAbsFnSym = ctxt.DwFixups.AbsFuncDwarfSym(s)
}
s.Func.dwarfIsStmtSym = ctxt.LookupDerived(s, dwarf.IsStmtPrefix+s.Name)
}
return s.Func.dwarfInfoSym, s.Func.dwarfLocSym, s.Func.dwarfRangesSym, s.Func.dwarfAbsFnSym
return s.Func.dwarfInfoSym, s.Func.dwarfLocSym, s.Func.dwarfRangesSym, s.Func.dwarfAbsFnSym, s.Func.dwarfIsStmtSym
}
func (s *LSym) Len() int64 {
@ -536,13 +537,14 @@ func (ctxt *Link) fileSymbol(fn *LSym) *LSym {
// TEXT symbol 's'. The various DWARF symbols must already have been
// initialized in InitTextSym.
func (ctxt *Link) populateDWARF(curfn interface{}, s *LSym, myimportpath string) {
info, loc, ranges, absfunc := ctxt.dwarfSym(s)
info, loc, ranges, absfunc, _ := ctxt.dwarfSym(s)
if info.Size != 0 {
ctxt.Diag("makeFuncDebugEntry double process %v", s)
}
var scopes []dwarf.Scope
var inlcalls dwarf.InlCalls
if ctxt.DebugInfo != nil {
stmtData(ctxt, s)
scopes, inlcalls = ctxt.DebugInfo(s, curfn)
}
var err error