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Implement external weak (ELF) symbols on AArch64

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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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174518 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Tim Northover 2013-02-06 16:43:33 +00:00
parent 2e402d5b5f
commit 8a06229c89
3 changed files with 30 additions and 4 deletions
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15
lib/Target/AArch64/AArch64ISelLowering.cpp
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@ -1863,7 +1863,7 @@ AArch64TargetLowering::LowerGlobalAddressELF(SDValue Op,
// 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::LowerGlobalAddressELF(SDValue Op,
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::LowerGlobalAddressELF(SDValue Op,
}
unsigned char HiFixup, LoFixup;
Reloc::Model RelocM = getTargetMachine().getRelocationModel();
bool UseGOT = Subtarget->GVIsIndirectSymbol(GV, RelocM);
if (UseGOT) {

6
lib/Target/AArch64/AArch64InstrInfo.td
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@ -2409,8 +2409,10 @@ class A64I_LDRlitSimple<bits<2> opc, bit v, RegisterClass OutReg,
"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,

13
test/CodeGen/AArch64/extern-weak.ll Normal file
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@ -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
}