[LegalizeTypes][RISCV] Correctly sign-extend comparison for ATOMIC_CMP_XCHG

Currently, the comparison argument used for ATOMIC_CMP_XCHG is legalised
with GetPromotedInteger, which leaves the upper bits of the value
undefind. Since this is used for comparing in an LR/SC loop with a
full-width comparison, we must sign extend it on RISC-V.

This is related to https://reviews.llvm.org/D58829, which solved the
issue for ATOMIC_CMP_SWAP_WITH_SUCCESS, but not the simpler
ATOMIC_CMP_SWAP.

This patch is a modified form of
616289ed29225c0ddfe5699c7fdf42a0fcbe0ab4 by Jessica Clarke. It localises
the changes to LegalizeIntegerTypes and avoids adding a new virtual
method to TargetLowering to avoid changing the ABI of libLLVM.so.

1 files changed, 13 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp
index 6aed5796acc6..015b3d99fb0f 100644
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp
@@ -271,8 +271,20 @@ SDValue DAGTypeLegalizer::PromoteIntRes_AtomicCmpSwap(AtomicSDNode *N,
     return Res.getValue(1);
   }
 
-  SDValue Op2 = GetPromotedInteger(N->getOperand(2));
+  // Op2 is used for the comparison and thus must be extended according to the
+  // target's atomic operations. Op3 is merely stored and so can be left alone.
+  SDValue Op2 = N->getOperand(2);
   SDValue Op3 = GetPromotedInteger(N->getOperand(3));
+  if (TLI.getTargetMachine().getTargetTriple().isRISCV()) {
+    // The comparison argument must be sign-extended for RISC-V. This is
+    // abstracted using a new TargetLowering hook in the main LLVM development
+    // branch, but handled here directly in order to fix the codegen bug for
+    // 10.x without breaking the libLLVM.so ABI.
+    Op2 = SExtPromotedInteger(Op2);
+  } else {
+    Op2 = GetPromotedInteger(Op2);
+  }
+
   SDVTList VTs =
       DAG.getVTList(Op2.getValueType(), N->getValueType(1), MVT::Other);
   SDValue Res = DAG.getAtomicCmpSwap(