[llvm] r174518 - Implement external weak (ELF) symbols on AArch64

[Nadav Rotem]

Hi Tim, 

Can you please document the AArch64 changes in the release notes ? 

Thanks,
Nadav

On Feb 6, 2013, at 8:43 AM, Tim Northover <Tim.Northover at arm.com> wrote:

> Author: tnorthover
> Date: Wed Feb  6 10:43:33 2013
> New Revision: 174518
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=174518&view=rev
> Log:
> Implement external weak (ELF) symbols on AArch64
> 
> Weakly defined symbols should evaluate to 0 if they're undefined at
> link-time. This is impossible to do with the usual address generation
> patterns, so we should use a literal pool entry to materlialise the
> address.
> 
> Added:
>    llvm/trunk/test/CodeGen/AArch64/extern-weak.ll
> Modified:
>    llvm/trunk/lib/Target/AArch64/AArch64ISelLowering.cpp
>    llvm/trunk/lib/Target/AArch64/AArch64InstrInfo.td
> 
> Modified: llvm/trunk/lib/Target/AArch64/AArch64ISelLowering.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/AArch64/AArch64ISelLowering.cpp?rev=174518&r1=174517&r2=174518&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Target/AArch64/AArch64ISelLowering.cpp (original)
> +++ llvm/trunk/lib/Target/AArch64/AArch64ISelLowering.cpp Wed Feb  6 10:43:33 2013
> @@ -1863,7 +1863,7 @@ AArch64TargetLowering::LowerGlobalAddres
>   // TableGen doesn't have easy access to the CodeModel or RelocationModel, so
>   // we make that distinction here.
> 
> -  // We support the static, small memory model for now.
> +  // We support the small memory model for now.
>   assert(getTargetMachine().getCodeModel() == CodeModel::Small);
> 
>   EVT PtrVT = getPointerTy();
> @@ -1871,6 +1871,18 @@ AArch64TargetLowering::LowerGlobalAddres
>   const GlobalAddressSDNode *GN = cast<GlobalAddressSDNode>(Op);
>   const GlobalValue *GV = GN->getGlobal();
>   unsigned Alignment = GV->getAlignment();
> +  Reloc::Model RelocM = getTargetMachine().getRelocationModel();
> +
> +  if (GV->isWeakForLinker() && RelocM == Reloc::Static) {
> +    // Weak symbols can't use ADRP/ADD pair since they should evaluate to
> +    // zero when undefined. In PIC mode the GOT can take care of this, but in
> +    // absolute mode we use a constant pool load.
> +    return DAG.getLoad(PtrVT, dl, DAG.getEntryNode(),
> +                       DAG.getConstantPool(GV, GN->getValueType(0)),
> +                       MachinePointerInfo::getConstantPool(),
> +                       /*isVolatile=*/ false,  /*isNonTemporal=*/ true,
> +                       /*isInvariant=*/ true, 8);
> +  }
> 
>   if (Alignment == 0) {
>     const PointerType *GVPtrTy = cast<PointerType>(GV->getType());
> @@ -1886,7 +1898,6 @@ AArch64TargetLowering::LowerGlobalAddres
>   }
> 
>   unsigned char HiFixup, LoFixup;
> -  Reloc::Model RelocM = getTargetMachine().getRelocationModel();
>   bool UseGOT = Subtarget->GVIsIndirectSymbol(GV, RelocM);
> 
>   if (UseGOT) {
> 
> Modified: llvm/trunk/lib/Target/AArch64/AArch64InstrInfo.td
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/AArch64/AArch64InstrInfo.td?rev=174518&r1=174517&r2=174518&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Target/AArch64/AArch64InstrInfo.td (original)
> +++ llvm/trunk/lib/Target/AArch64/AArch64InstrInfo.td Wed Feb  6 10:43:33 2013
> @@ -2409,8 +2409,10 @@ class A64I_LDRlitSimple<bits<2> opc, bit
>                  "ldr\t$Rt, $Imm19", patterns, NoItinerary>;
> 
> let mayLoad = 1 in {
> -  def LDRw_lit : A64I_LDRlitSimple<0b00, 0b0, GPR32>;
> -  def LDRx_lit : A64I_LDRlitSimple<0b01, 0b0, GPR64>;
> +  def LDRw_lit : A64I_LDRlitSimple<0b00, 0b0, GPR32,
> +                              [(set (i32 GPR32:$Rt), (load constpool:$Imm19))]>;
> +  def LDRx_lit : A64I_LDRlitSimple<0b01, 0b0, GPR64,
> +                              [(set (i64 GPR64:$Rt), (load constpool:$Imm19))]>;
> }
> 
> def LDRs_lit  : A64I_LDRlitSimple<0b00, 0b1, FPR32,
> 
> Added: llvm/trunk/test/CodeGen/AArch64/extern-weak.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/AArch64/extern-weak.ll?rev=174518&view=auto
> ==============================================================================
> --- llvm/trunk/test/CodeGen/AArch64/extern-weak.ll (added)
> +++ llvm/trunk/test/CodeGen/AArch64/extern-weak.ll Wed Feb  6 10:43:33 2013
> @@ -0,0 +1,13 @@
> +; RUN: llc -mtriple=aarch64-none-linux-gnu -o - < %s | FileCheck %s
> +
> +declare extern_weak i32 @var()
> +
> +define i32()* @foo() {
> +; The usual ADRP/ADD pair can't be used for a weak reference because it must
> +; evaluate to 0 if the symbol is undefined. We use a litpool entry.
> +  ret i32()* @var
> +; CHECK: ldr x0, .LCPI0_0
> +
> +; CHECK: .LCPI0_0:
> +; CHECK-NEXT: .xword var
> +}
> 
> 
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