[llvm] r233119 - AArch64: use a different means to determine whether to byte swap relocations.

[Peter Collingbourne]

I still haven't figured out how to test this. For example:

.section .data
.word x + .Lbar - (.Lfoo + x)
.word .Lbar - .Lfoo
.word (.Lbar - .Lfoo) + 1
.word 1 + (.Lbar - .Lfoo)
.word (.Lbar - .Lfoo) - 1
.word .Lbar2 - .Lfoo2

.section .eh_frame
.Lfoo:
.byte 1
.Lbar:
.byte 1

.section .not_eh_frame
.Lfoo2:
.byte 1
.Lbar2:
.byte 1

(test with "llvm-mc -triple=aarch64_be-linux-gnu -filetype=obj -o - eh-frame.s")

In all of these cases, the new code produces the same result. It seems
that the effects are only visible if Value is non-zero, which it will
only be if both symbols are in the same section. But if that is the case,
EvaluateAsRelocatable will return the same result.

Peter

On Tue, Mar 24, 2015 at 06:10:27PM -0400, Rafael Espíndola wrote:
> Oh, and the code should be checking SymB too.
> 
> A logic similar to getBaseSymbol is probably what is needed.
> 
> On 24 March 2015 at 18:03, Rafael Espíndola <rafael.espindola at gmail.com> wrote:
> > This is testable. The new code should be able to handle more situations.
> >
> > On 24 March 2015 at 17:47, Peter Collingbourne <peter at pcc.me.uk> wrote:
> >> Author: pcc
> >> Date: Tue Mar 24 16:47:03 2015
> >> New Revision: 233119
> >>
> >> URL: http://llvm.org/viewvc/llvm-project?rev=233119&view=rev
> >> Log:
> >> AArch64: use a different means to determine whether to byte swap relocations.
> >>
> >> This code depended on a bug in the FindAssociatedSection function that would
> >> cause it to return the wrong result for certain absolute expressions. Instead,
> >> use EvaluateAsRelocatable.
> >>
> >> Modified:
> >>     llvm/trunk/lib/Target/AArch64/MCTargetDesc/AArch64AsmBackend.cpp
> >>
> >> Modified: llvm/trunk/lib/Target/AArch64/MCTargetDesc/AArch64AsmBackend.cpp
> >> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/AArch64/MCTargetDesc/AArch64AsmBackend.cpp?rev=233119&r1=233118&r2=233119&view=diff
> >> ==============================================================================
> >> --- llvm/trunk/lib/Target/AArch64/MCTargetDesc/AArch64AsmBackend.cpp (original)
> >> +++ llvm/trunk/lib/Target/AArch64/MCTargetDesc/AArch64AsmBackend.cpp Tue Mar 24 16:47:03 2015
> >> @@ -18,6 +18,7 @@
> >>  #include "llvm/MC/MCObjectWriter.h"
> >>  #include "llvm/MC/MCSectionELF.h"
> >>  #include "llvm/MC/MCSectionMachO.h"
> >> +#include "llvm/MC/MCValue.h"
> >>  #include "llvm/Support/ErrorHandling.h"
> >>  #include "llvm/Support/MachO.h"
> >>  using namespace llvm;
> >> @@ -493,14 +494,28 @@ void ELFAArch64AsmBackend::processFixupV
> >>      IsResolved = false;
> >>  }
> >>
> >> +// Returns whether this fixup is based on an address in the .eh_frame section,
> >> +// and therefore should be byte swapped.
> >> +// FIXME: Should be replaced with something more principled.
> >> +static bool isByteSwappedFixup(const MCExpr *E) {
> >> +  MCValue Val;
> >> +  if (!E->EvaluateAsRelocatable(Val, nullptr, nullptr))
> >> +    return false;
> >> +
> >> +  if (!Val.getSymA() || Val.getSymA()->getSymbol().isUndefined())
> >> +    return false;
> >> +
> >> +  const MCSectionELF *SecELF =
> >> +      dyn_cast<MCSectionELF>(&Val.getSymA()->getSymbol().getSection());
> >> +  return SecELF->getSectionName() == ".eh_frame";
> >> +}
> >> +
> >>  void ELFAArch64AsmBackend::applyFixup(const MCFixup &Fixup, char *Data,
> >>                                        unsigned DataSize, uint64_t Value,
> >>                                        bool IsPCRel) const {
> >>    // store fixups in .eh_frame section in big endian order
> >>    if (!IsLittleEndian && Fixup.getKind() == FK_Data_4) {
> >> -    const MCSection *Sec = Fixup.getValue()->FindAssociatedSection();
> >> -    const MCSectionELF *SecELF = dyn_cast_or_null<const MCSectionELF>(Sec);
> >> -    if (SecELF && SecELF->getSectionName() == ".eh_frame")
> >> +    if (isByteSwappedFixup(Fixup.getValue()))
> >>        Value = ByteSwap_32(unsigned(Value));
> >>    }
> >>    AArch64AsmBackend::applyFixup (Fixup, Data, DataSize, Value, IsPCRel);
> >>
> >>
> >> _______________________________________________
> >> llvm-commits mailing list
> >> llvm-commits at cs.uiuc.edu
> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> 

-- 
Peter