[PATCH] D107280: [RISCV] Lower extern_weak symbols using the GOT for the medany model

[Jessica Clarke via Phabricator via llvm-commits]

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Such symbols may be undefined at link time and thus resolve to 0, which
may be further than 2GiB away from PC, causing the immediate to be out
of range for PC-relative addressing. Using the GOT avoids this, and is
the approach taken by AArch64.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D107280

Files:
  llvm/lib/Target/RISCV/RISCVISelLowering.cpp
  llvm/lib/Target/RISCV/RISCVISelLowering.h
  llvm/test/CodeGen/RISCV/codemodel-lowering.ll


Index: llvm/test/CodeGen/RISCV/codemodel-lowering.ll
===================================================================
--- llvm/test/CodeGen/RISCV/codemodel-lowering.ll
+++ llvm/test/CodeGen/RISCV/codemodel-lowering.ll
@@ -161,8 +161,8 @@
 ; RV32I-MEDIUM-LABEL: lower_extern_weak:
 ; RV32I-MEDIUM:       # %bb.0:
 ; RV32I-MEDIUM-NEXT:  .LBB4_1: # Label of block must be emitted
-; RV32I-MEDIUM-NEXT:    auipc a0, %pcrel_hi(W)
-; RV32I-MEDIUM-NEXT:    addi a0, a0, %pcrel_lo(.LBB4_1)
+; RV32I-MEDIUM-NEXT:    auipc a0, %got_pcrel_hi(W)
+; RV32I-MEDIUM-NEXT:    lw a0, %pcrel_lo(.LBB4_1)(a0)
 ; RV32I-MEDIUM-NEXT:    lw a0, 0(a0)
 ; RV32I-MEDIUM-NEXT:    ret
   %1 = load volatile i32, i32* @W
Index: llvm/lib/Target/RISCV/RISCVISelLowering.h
===================================================================
--- llvm/lib/Target/RISCV/RISCVISelLowering.h
+++ llvm/lib/Target/RISCV/RISCVISelLowering.h
@@ -520,7 +520,8 @@
                          RISCVCCAssignFn Fn) const;
 
   template <class NodeTy>
-  SDValue getAddr(NodeTy *N, SelectionDAG &DAG, bool IsLocal = true) const;
+  SDValue getAddr(NodeTy *N, SelectionDAG &DAG, bool IsLocal = true,
+                  bool IsExternWeak = false) const;
 
   SDValue getStaticTLSAddr(GlobalAddressSDNode *N, SelectionDAG &DAG,
                            bool UseGOT) const;
Index: llvm/lib/Target/RISCV/RISCVISelLowering.cpp
===================================================================
--- llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -2700,7 +2700,7 @@
 
 template <class NodeTy>
 SDValue RISCVTargetLowering::getAddr(NodeTy *N, SelectionDAG &DAG,
-                                     bool IsLocal) const {
+                                     bool IsLocal, bool IsExternWeak) const {
   SDLoc DL(N);
   EVT Ty = getPointerTy(DAG.getDataLayout());
 
@@ -2730,10 +2730,16 @@
     return SDValue(DAG.getMachineNode(RISCV::ADDI, DL, Ty, MNHi, AddrLo), 0);
   }
   case CodeModel::Medium: {
+    SDValue Addr = getTargetNode(N, DL, Ty, DAG, 0);
+    if (IsExternWeak)
+      // Extern weak symbols must be addressed indirectly as 0 may not be
+      // within 2GiB of PC. This generates the pattern (PseudoLA sym), which
+      // expands to (ld (addi (auipc %got_pcrel_hi(sym)) %pcrel_lo(auipc))).
+      return SDValue(DAG.getMachineNode(RISCV::PseudoLGA, DL, Ty, Addr), 0);
+
     // Generate a sequence for accessing addresses within any 2GiB range within
     // the address space. This generates the pattern (PseudoLLA sym), which
     // expands to (addi (auipc %pcrel_hi(sym)) %pcrel_lo(auipc)).
-    SDValue Addr = getTargetNode(N, DL, Ty, DAG, 0);
     return SDValue(DAG.getMachineNode(RISCV::PseudoLLA, DL, Ty, Addr), 0);
   }
   }
@@ -2749,7 +2755,8 @@
 
   const GlobalValue *GV = N->getGlobal();
   bool IsLocal = getTargetMachine().shouldAssumeDSOLocal(*GV->getParent(), GV);
-  SDValue Addr = getAddr(N, DAG, IsLocal);
+  bool IsExternWeak = GV->hasExternalWeakLinkage();
+  SDValue Addr = getAddr(N, DAG, IsLocal, IsExternWeak);
 
   // In order to maximise the opportunity for common subexpression elimination,
   // emit a separate ADD node for the global address offset instead of folding


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