Stowage/go
2
0
Fork 0
mirror of https://github.com/golang/go.git synced 2025-12-08 06:10:04 +00:00
Code Issues Projects Releases Packages Wiki Activity

cmd/link: process is_stmt data into dwarf line tables

Browse source 
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

69
src/cmd/link/internal/ld/dwarf.go
View file

@ -1113,6 +1113,7 @@ func collectAbstractFunctions(ctxt *Link, fn *sym.Symbol, dsym *sym.Symbol, absf
func writelines(ctxt *Link, lib *sym.Library, textp []*sym.Symbol, ls *sym.Symbol) (dwinfo *dwarf.DWDie, funcs []*sym.Symbol, absfuncs []*sym.Symbol) {
var dwarfctxt dwarf.Context = dwctxt{ctxt}
is_stmt := uint8(1) // initially = recommended default_is_stmt = 1, tracks is_stmt toggles.
unitstart := int64(-1)
headerstart := int64(-1)
@ -1154,7 +1155,7 @@ func writelines(ctxt *Link, lib *sym.Library, textp []*sym.Symbol, ls *sym.Symbo
// cpos == unitstart + 4 + 2 + 4
ls.AddUint8(1) // minimum_instruction_length
ls.AddUint8(1) // default_is_stmt
ls.AddUint8(is_stmt) // default_is_stmt
ls.AddUint8(LINE_BASE & 0xFF) // line_base
ls.AddUint8(LINE_RANGE) // line_range
ls.AddUint8(OPCODE_BASE) // opcode_base
@ -1172,7 +1173,7 @@ func writelines(ctxt *Link, lib *sym.Library, textp []*sym.Symbol, ls *sym.Symbo
// Create the file table. fileNums maps from global file
// indexes (created by numberfile) to CU-local indexes.
fileNums := make(map[int]int)
for _, s := range textp {
for _, s := range textp { // textp has been dead-code-eliminated already.
for _, f := range s.FuncInfo.File {
if _, ok := fileNums[int(f.Value)]; ok {
continue
@ -1224,27 +1225,25 @@ func writelines(ctxt *Link, lib *sym.Library, textp []*sym.Symbol, ls *sym.Symbo
var pcfile Pciter
var pcline Pciter
for _, s := range textp {
var pcstmt Pciter
for i, s := range textp {
dsym := ctxt.Syms.Lookup(dwarf.InfoPrefix+s.Name, int(s.Version))
funcs = append(funcs, dsym)
absfuncs = collectAbstractFunctions(ctxt, s, dsym, absfuncs)
finddebugruntimepath(s)
isStmtsSym := ctxt.Syms.ROLookup(dwarf.IsStmtPrefix+s.Name, int(s.Version))
pctostmtData := sym.Pcdata{P: isStmtsSym.P}
pciterinit(ctxt, &pcfile, &s.FuncInfo.Pcfile)
pciterinit(ctxt, &pcline, &s.FuncInfo.Pcline)
epc := pc
for pcfile.done == 0 && pcline.done == 0 {
if epc-s.Value >= int64(pcfile.nextpc) {
pciternext(&pcfile)
continue
}
if epc-s.Value >= int64(pcline.nextpc) {
pciternext(&pcline)
continue
}
pciterinit(ctxt, &pcstmt, &pctostmtData)
var thispc uint32
// TODO this loop looks like it could exit with work remaining.
for pcfile.done == 0 && pcline.done == 0 && pcstmt.done == 0 {
// Only changed if it advanced
if int32(file) != pcfile.value {
ls.AddUint8(dwarf.DW_LNS_set_file)
idx, ok := fileNums[int(pcfile.value)]
@ -1255,16 +1254,40 @@ func writelines(ctxt *Link, lib *sym.Library, textp []*sym.Symbol, ls *sym.Symbo
file = int(pcfile.value)
}
putpclcdelta(ctxt, dwarfctxt, ls, uint64(s.Value+int64(pcline.pc)-pc), int64(pcline.value)-int64(line))
pc = s.Value + int64(pcline.pc)
line = int(pcline.value)
if pcfile.nextpc < pcline.nextpc {
epc = int64(pcfile.nextpc)
} else {
epc = int64(pcline.nextpc)
// Only changed if it advanced
if is_stmt != uint8(pcstmt.value) {
is_stmt = uint8(pcstmt.value)
ls.AddUint8(uint8(dwarf.DW_LNS_negate_stmt))
}
epc += s.Value
// putpcldelta makes a row in the DWARF matrix, always, even if line is unchanged.
putpclcdelta(ctxt, dwarfctxt, ls, uint64(s.Value+int64(thispc)-pc), int64(pcline.value)-int64(line))
pc = s.Value + 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.nextpc < thispc {
thispc = pcstmt.nextpc
}
if pcfile.nextpc == thispc {
pciternext(&pcfile)
}
if pcstmt.nextpc == thispc {
pciternext(&pcstmt)
}
if pcline.nextpc == thispc {
pciternext(&pcline)
}
}
if is_stmt == 0 && i < len(textp)-1 {
// If there is more than one function, ensure default value is established.
ls.AddUint8(uint8(dwarf.DW_LNS_negate_stmt))
}
}