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[dev.link] cmd/link: set direct fn address in dwarf gen where possible

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If we are internal linking a static executable, and address assignment
has happened, then when emitting some parts of DWARF we can just emit
a function address directly instead of generating a relocation. For
external linking or other build modes, we are generating a relocatable
binary so we still need to emit relocations.

This CL inspired by Cherry's similar CL for pclntab at
https://go-review.googlesource.com/c/go/+/228478.

Change-Id: Ib03fbe2dd72d0ba746bf46015e0f2d6c3f3d53ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/228537
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
This commit is contained in:
Than McIntosh 2020-04-16 08:29:43 -04:00
parent 4a0bca37d2
commit da9f383ca1
2 changed files with 29 additions and 2 deletions
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10
src/cmd/link/internal/ld/dwarf.go
View file

@ -1140,6 +1140,9 @@ func (d *dwctxt2) writelines(unit *sym.CompilationUnit, ls loader.Sym) {
lsu := d.ldr.MakeSymbolUpdater(ls)
newattr(unit.DWInfo, dwarf.DW_AT_stmt_list, dwarf.DW_CLS_PTR, lsu.Size(), dwSym(ls))
internalExec := d.linkctxt.BuildMode == BuildModeExe && d.linkctxt.IsInternal()
addAddrPlus := loader.GenAddAddrPlusFunc(internalExec)
// Write .debug_line Line Number Program Header (sec 6.2.4)
// Fields marked with (*) must be changed for 64-bit dwarf
unitLengthOffset := lsu.Size()
@ -1209,7 +1212,7 @@ func (d *dwctxt2) writelines(unit *sym.CompilationUnit, ls loader.Sym) {
lsu.AddUint8(0)
dwarf.Uleb128put(d, lsDwsym, 1+int64(d.arch.PtrSize))
lsu.AddUint8(dwarf.DW_LNE_set_address)
addr := lsu.AddAddrPlus(d.arch, fnSym, 0)
addr := addAddrPlus(lsu, d.arch, fnSym, 0)
// Make sure the units are sorted.
if addr < lastAddr {
d.linkctxt.Errorf(fnSym, "address wasn't increasing %x < %x",
@ -1347,6 +1350,9 @@ func (d *dwctxt2) writeframes(syms []loader.Sym) []loader.Sym {
Exitf("dwarf: cieReserve too small by %d bytes.", -pad)
}
internalExec := d.linkctxt.BuildMode == BuildModeExe && d.linkctxt.IsInternal()
addAddrPlus := loader.GenAddAddrPlusFunc(internalExec)
fsu.AddBytes(zeros[:pad])
var deltaBuf []byte
@ -1428,7 +1434,7 @@ func (d *dwctxt2) writeframes(syms []loader.Sym) []loader.Sym {
} else {
d.addDwarfAddrField(fsu, 0) // CIE offset
}
fsu.AddAddrPlus(d.arch, s, 0)
addAddrPlus(fsu, d.arch, s, 0)
fsu.AddUintXX(d.arch, uint64(len(d.ldr.Data(fn))), d.arch.PtrSize) // address range
fsu.AddBytes(deltaBuf)

21
src/cmd/link/internal/loader/symbolbuilder.go
View file

@ -405,3 +405,24 @@ func (sb *SymbolBuilder) AddSize(arch *sys.Arch, tgt Sym) int64 {
sb.setReachable()
return sb.addSymRef(tgt, 0, objabi.R_SIZE, arch.PtrSize)
}
// GenAddAddrPlusFunc returns a function to be called when capturing
// a function symbol's address. In later stages of the link (when
// address assignment is done) when doing internal linking and
// targeting an executable, we can just emit the address of a function
// directly instead of generating a relocation. Clients can call
// this function (setting 'internalExec' based on build mode and target)
// and then invoke the returned function in roughly the same way that
// loader.*SymbolBuilder.AddAddrPlus would be used.
func GenAddAddrPlusFunc(internalExec bool) func(s *SymbolBuilder, arch *sys.Arch, tgt Sym, add int64) int64 {
if internalExec {
return func(s *SymbolBuilder, arch *sys.Arch, tgt Sym, add int64) int64 {
if v := s.l.SymValue(tgt); v != 0 {
return s.AddUint(arch, uint64(v+add))
}
return s.AddAddrPlus(arch, tgt, add)
}
} else {
return (*SymbolBuilder).AddAddrPlus
}
}