diff --git a/llvm/include/llvm/CodeGen/TargetLowering.h b/llvm/include/llvm/CodeGen/TargetLowering.h
--- a/llvm/include/llvm/CodeGen/TargetLowering.h
+++ b/llvm/include/llvm/CodeGen/TargetLowering.h
@@ -1875,6 +1875,12 @@
     return ISD::ZERO_EXTEND;
   }
 
+  /// Returns how the platform's atomic compare and swap expects its comparison
+  /// value to be extended (ZERO_EXTEND, SIGN_EXTEND, or ANY_EXTEND).
+  virtual ISD::NodeType getExtendForAtomicCmpSwapArg() const {
+    return ISD::ANY_EXTEND;
+  }
+
   /// @}
 
   /// Returns true if we should normalize
diff --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp
@@ -272,8 +272,24 @@
     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));
+  switch (TLI.getExtendForAtomicCmpSwapArg()) {
+  case ISD::SIGN_EXTEND:
+    Op2 = SExtPromotedInteger(Op2);
+    break;
+  case ISD::ZERO_EXTEND:
+    Op2 = ZExtPromotedInteger(Op2);
+    break;
+  case ISD::ANY_EXTEND:
+    Op2 = GetPromotedInteger(Op2);
+    break;
+  default:
+    llvm_unreachable("Invalid atomic op extension");
+  }
+
   SDVTList VTs =
       DAG.getVTList(Op2.getValueType(), N->getValueType(1), MVT::Other);
   SDValue Res = DAG.getAtomicCmpSwap(
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.h b/llvm/lib/Target/RISCV/RISCVISelLowering.h
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.h
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.h
@@ -127,6 +127,10 @@
     return ISD::SIGN_EXTEND;
   }
 
+  ISD::NodeType getExtendForAtomicCmpSwapArg() const override {
+    return ISD::SIGN_EXTEND;
+  }
+
   bool shouldExpandShift(SelectionDAG &DAG, SDNode *N) const override {
     if (DAG.getMachineFunction().getFunction().hasMinSize())
       return false;
diff --git a/llvm/test/CodeGen/RISCV/atomic-cmpxchg.ll b/llvm/test/CodeGen/RISCV/atomic-cmpxchg.ll
--- a/llvm/test/CodeGen/RISCV/atomic-cmpxchg.ll
+++ b/llvm/test/CodeGen/RISCV/atomic-cmpxchg.ll
@@ -1628,6 +1628,7 @@
 ;
 ; RV64IA-LABEL: cmpxchg_i32_monotonic_monotonic:
 ; RV64IA:       # %bb.0:
+; RV64IA-NEXT:    sext.w a1, a1
 ; RV64IA-NEXT:  .LBB20_1: # =>This Inner Loop Header: Depth=1
 ; RV64IA-NEXT:    lr.w a3, (a0)
 ; RV64IA-NEXT:    bne a3, a1, .LBB20_3
@@ -1680,6 +1681,7 @@
 ;
 ; RV64IA-LABEL: cmpxchg_i32_acquire_monotonic:
 ; RV64IA:       # %bb.0:
+; RV64IA-NEXT:    sext.w a1, a1
 ; RV64IA-NEXT:  .LBB21_1: # =>This Inner Loop Header: Depth=1
 ; RV64IA-NEXT:    lr.w.aq a3, (a0)
 ; RV64IA-NEXT:    bne a3, a1, .LBB21_3
@@ -1732,6 +1734,7 @@
 ;
 ; RV64IA-LABEL: cmpxchg_i32_acquire_acquire:
 ; RV64IA:       # %bb.0:
+; RV64IA-NEXT:    sext.w a1, a1
 ; RV64IA-NEXT:  .LBB22_1: # =>This Inner Loop Header: Depth=1
 ; RV64IA-NEXT:    lr.w.aq a3, (a0)
 ; RV64IA-NEXT:    bne a3, a1, .LBB22_3
@@ -1784,6 +1787,7 @@
 ;
 ; RV64IA-LABEL: cmpxchg_i32_release_monotonic:
 ; RV64IA:       # %bb.0:
+; RV64IA-NEXT:    sext.w a1, a1
 ; RV64IA-NEXT:  .LBB23_1: # =>This Inner Loop Header: Depth=1
 ; RV64IA-NEXT:    lr.w a3, (a0)
 ; RV64IA-NEXT:    bne a3, a1, .LBB23_3
@@ -1836,6 +1840,7 @@
 ;
 ; RV64IA-LABEL: cmpxchg_i32_release_acquire:
 ; RV64IA:       # %bb.0:
+; RV64IA-NEXT:    sext.w a1, a1
 ; RV64IA-NEXT:  .LBB24_1: # =>This Inner Loop Header: Depth=1
 ; RV64IA-NEXT:    lr.w a3, (a0)
 ; RV64IA-NEXT:    bne a3, a1, .LBB24_3
@@ -1888,6 +1893,7 @@
 ;
 ; RV64IA-LABEL: cmpxchg_i32_acq_rel_monotonic:
 ; RV64IA:       # %bb.0:
+; RV64IA-NEXT:    sext.w a1, a1
 ; RV64IA-NEXT:  .LBB25_1: # =>This Inner Loop Header: Depth=1
 ; RV64IA-NEXT:    lr.w.aq a3, (a0)
 ; RV64IA-NEXT:    bne a3, a1, .LBB25_3
@@ -1940,6 +1946,7 @@
 ;
 ; RV64IA-LABEL: cmpxchg_i32_acq_rel_acquire:
 ; RV64IA:       # %bb.0:
+; RV64IA-NEXT:    sext.w a1, a1
 ; RV64IA-NEXT:  .LBB26_1: # =>This Inner Loop Header: Depth=1
 ; RV64IA-NEXT:    lr.w.aq a3, (a0)
 ; RV64IA-NEXT:    bne a3, a1, .LBB26_3
@@ -1992,6 +1999,7 @@
 ;
 ; RV64IA-LABEL: cmpxchg_i32_seq_cst_monotonic:
 ; RV64IA:       # %bb.0:
+; RV64IA-NEXT:    sext.w a1, a1
 ; RV64IA-NEXT:  .LBB27_1: # =>This Inner Loop Header: Depth=1
 ; RV64IA-NEXT:    lr.w.aqrl a3, (a0)
 ; RV64IA-NEXT:    bne a3, a1, .LBB27_3
@@ -2044,6 +2052,7 @@
 ;
 ; RV64IA-LABEL: cmpxchg_i32_seq_cst_acquire:
 ; RV64IA:       # %bb.0:
+; RV64IA-NEXT:    sext.w a1, a1
 ; RV64IA-NEXT:  .LBB28_1: # =>This Inner Loop Header: Depth=1
 ; RV64IA-NEXT:    lr.w.aqrl a3, (a0)
 ; RV64IA-NEXT:    bne a3, a1, .LBB28_3
@@ -2096,6 +2105,7 @@
 ;
 ; RV64IA-LABEL: cmpxchg_i32_seq_cst_seq_cst:
 ; RV64IA:       # %bb.0:
+; RV64IA-NEXT:    sext.w a1, a1
 ; RV64IA-NEXT:  .LBB29_1: # =>This Inner Loop Header: Depth=1
 ; RV64IA-NEXT:    lr.w.aqrl a3, (a0)
 ; RV64IA-NEXT:    bne a3, a1, .LBB29_3