[llvm-commits] [patch] Add TLS hook to MCTargetExpr (AArch64 generic change)

[Tim Northover]

On Wednesday 09 Jan 2013 19:02:07 Tim Northover wrote:
> > No objection per se, but it'd be nice to see the cleanups in at least one
> > target as the motivation for the change?
> 
> Fair enough. I'll work on porting the ARM target tomorrow.

Hmm. This turned out to be more difficult than anticipated. Unlike the AArch64 
backend, the ARM one uses a combination of MCFixup kinds and MCSymbolRef kinds 
to determine the correct relocation in ARMELFObjectWriter (AArch64 uses 
different fixups only).

Unfortunately, the TargetMCExprs seem to have been discarded by this point 
(only an MCSymbolRefExpr is available in MCValue). A messy but small problem 
for instructions, but a larger one for constant pool entries, which always 
seem to get FK_Data_4 (or similar) MCFixup kind.

An alternative viewpoint is that without this patch we're adopting the view 
that TargetMCExpr's cannot contain any TLS symbols via any means. Obviously I 
don't agree with that.

I've put the AArch64 implementation of this new callback at the end of this 
message to show some kind of motivation. I've also attached the incomplete ARM 
conversion: it's completely broken for object file output for the reasons 
mentioned above (ignore ARMELFObjectWriter in particular), but gives some idea 
of the kind of backend changes I'm talking about.

Tim.

---------------------------------

static void fixELFSymbolsInTLSFixupsImpl(const MCEXpr *Expr, MCAssembler 
&Asm);

void AArch64MCExpr::fixELFSymbolsInTLSFixups(MCAssembler &Asm) const {
  switch (getKind()) {
  default:
    return;
  case VK_AARCH64_DTPREL_G2:
  case VK_AARCH64_DTPREL_G1:
  case VK_AARCH64_DTPREL_G1_NC:
  case VK_AARCH64_DTPREL_G0:
  case VK_AARCH64_DTPREL_G0_NC:
  case VK_AARCH64_DTPREL_HI12:
  case VK_AARCH64_DTPREL_LO12:
  case VK_AARCH64_DTPREL_LO12_NC:
  case VK_AARCH64_GOTTPREL_G1:
  case VK_AARCH64_GOTTPREL_G0_NC:
  case VK_AARCH64_GOTTPREL:
  case VK_AARCH64_GOTTPREL_LO12:
  case VK_AARCH64_TPREL_G2:
  case VK_AARCH64_TPREL_G1:
  case VK_AARCH64_TPREL_G1_NC:
  case VK_AARCH64_TPREL_G0:
  case VK_AARCH64_TPREL_G0_NC:
  case VK_AARCH64_TPREL_HI12:
  case VK_AARCH64_TPREL_LO12:
  case VK_AARCH64_TPREL_LO12_NC:
  case VK_AARCH64_TLSDESC:
  case VK_AARCH64_TLSDESC_LO12:
    break;
  }

  fixELFSymbolsInTLSFixupsImpl(getSubExpr(), Asm);
}


static void fixELFSymbolsInTLSFixupsImpl(const MCExpr *Expr, MCAssembler &Asm) 
{
  switch (Expr->getKind()) {
  case MCExpr::Target:
    llvm_unreachable("Can't handle nested target expression");
    break;
  case MCExpr::Constant:
    break;

  case MCExpr::Binary: {
    const MCBinaryExpr *BE = cast<MCBinaryExpr>(Expr);
    fixELFSymbolsInTLSFixupsImpl(BE->getLHS(), Asm);
    fixELFSymbolsInTLSFixupsImpl(BE->getRHS(), Asm);
    break;
  }

  case MCExpr::SymbolRef: {
    // We're known to be under a TLS fixup, so any symbol should be
    // modified. There should be only one.
    const MCSymbolRefExpr &SymRef = *cast<MCSymbolRefExpr>(Expr);
    MCSymbolData &SD = Asm.getOrCreateSymbolData(SymRef.getSymbol());
    MCELF::SetType(SD, ELF::STT_TLS);
    break;
  }

  case MCExpr::Unary:
    fixELFSymbolsInTLSFixupsImpl(cast<MCUnaryExpr>(Expr)->getSubExpr(), Asm);
    break;
  }
}
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