diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -11843,6 +11843,10 @@
   EVT VT = N->getValueType(0);
   SDLoc DL(N);
 
+  // sext(undef) = 0 because the top bit will all be the same.
+  if (N0.isUndef())
+    return DAG.getConstant(0, DL, VT);
+
   if (SDValue Res = tryToFoldExtendOfConstant(N, TLI, DAG, LegalTypes))
     return Res;
 
@@ -12088,6 +12092,10 @@
   SDValue N0 = N->getOperand(0);
   EVT VT = N->getValueType(0);
 
+  // zext(undef) = 0
+  if (N0.isUndef())
+    return DAG.getConstant(0, SDLoc(N), VT);
+
   if (SDValue Res = tryToFoldExtendOfConstant(N, TLI, DAG, LegalTypes))
     return Res;
 
@@ -12347,6 +12355,10 @@
   SDValue N0 = N->getOperand(0);
   EVT VT = N->getValueType(0);
 
+  // aext(undef) = undef
+  if (N0.isUndef())
+    return DAG.getUNDEF(VT);
+
   if (SDValue Res = tryToFoldExtendOfConstant(N, TLI, DAG, LegalTypes))
     return Res;
 
diff --git a/llvm/test/CodeGen/AMDGPU/llvm.amdgcn.class.ll b/llvm/test/CodeGen/AMDGPU/llvm.amdgcn.class.ll
--- a/llvm/test/CodeGen/AMDGPU/llvm.amdgcn.class.ll
+++ b/llvm/test/CodeGen/AMDGPU/llvm.amdgcn.class.ll
@@ -495,11 +495,11 @@
   ret void
 }
 
-; FIXME: Why is the extension still here?
 ; SI-LABEL: {{^}}test_class_undef_f32:
 ; SI-NOT: v_cmp_class
-; SI: v_cndmask_b32_e64 v{{[0-9]+}}, 0, -1,
-; SI: buffer_store_dword
+; SI: v_mov_b32_e32 [[RESULT:v[0-9]+]], 0
+; SI: buffer_store_dword [[RESULT]]
+; SI: s_endpgm
 define amdgpu_kernel void @test_class_undef_f32(i32 addrspace(1)* %out, float %a, i32 %b) #0 {
   %result = call i1 @llvm.amdgcn.class.f32(float undef, i32 %b) #1
   %sext = sext i1 %result to i32
diff --git a/llvm/test/CodeGen/RISCV/min-max.ll b/llvm/test/CodeGen/RISCV/min-max.ll
--- a/llvm/test/CodeGen/RISCV/min-max.ll
+++ b/llvm/test/CodeGen/RISCV/min-max.ll
@@ -531,52 +531,91 @@
 }
 
 ; Tests with undef operands. These should fold to undef for RV32 or 0 for RV64.
-; FIXME: The RV64 cases are hitting pr55178.
 
 define signext i32 @smin_undef_i32() {
-; NOZBB-LABEL: smin_undef_i32:
-; NOZBB:       # %bb.0:
-; NOZBB-NEXT:    ret
+; RV32I-LABEL: smin_undef_i32:
+; RV32I:       # %bb.0:
+; RV32I-NEXT:    ret
 ;
-; ZBB-LABEL: smin_undef_i32:
-; ZBB:       # %bb.0:
-; ZBB-NEXT:    ret
+; RV64I-LABEL: smin_undef_i32:
+; RV64I:       # %bb.0:
+; RV64I-NEXT:    li a0, 0
+; RV64I-NEXT:    ret
+;
+; RV32ZBB-LABEL: smin_undef_i32:
+; RV32ZBB:       # %bb.0:
+; RV32ZBB-NEXT:    ret
+;
+; RV64ZBB-LABEL: smin_undef_i32:
+; RV64ZBB:       # %bb.0:
+; RV64ZBB-NEXT:    li a0, 0
+; RV64ZBB-NEXT:    ret
   %c = call i32 @llvm.smin.i32(i32 undef, i32 undef)
   ret i32 %c
 }
 
 define signext i32 @smax_undef_i32() {
-; NOZBB-LABEL: smax_undef_i32:
-; NOZBB:       # %bb.0:
-; NOZBB-NEXT:    ret
+; RV32I-LABEL: smax_undef_i32:
+; RV32I:       # %bb.0:
+; RV32I-NEXT:    ret
 ;
-; ZBB-LABEL: smax_undef_i32:
-; ZBB:       # %bb.0:
-; ZBB-NEXT:    ret
+; RV64I-LABEL: smax_undef_i32:
+; RV64I:       # %bb.0:
+; RV64I-NEXT:    li a0, 0
+; RV64I-NEXT:    ret
+;
+; RV32ZBB-LABEL: smax_undef_i32:
+; RV32ZBB:       # %bb.0:
+; RV32ZBB-NEXT:    ret
+;
+; RV64ZBB-LABEL: smax_undef_i32:
+; RV64ZBB:       # %bb.0:
+; RV64ZBB-NEXT:    li a0, 0
+; RV64ZBB-NEXT:    ret
   %c = call i32 @llvm.smax.i32(i32 undef, i32 undef)
   ret i32 %c
 }
 
 define signext i32 @umin_undef_i32() {
-; NOZBB-LABEL: umin_undef_i32:
-; NOZBB:       # %bb.0:
-; NOZBB-NEXT:    ret
+; RV32I-LABEL: umin_undef_i32:
+; RV32I:       # %bb.0:
+; RV32I-NEXT:    ret
 ;
-; ZBB-LABEL: umin_undef_i32:
-; ZBB:       # %bb.0:
-; ZBB-NEXT:    ret
+; RV64I-LABEL: umin_undef_i32:
+; RV64I:       # %bb.0:
+; RV64I-NEXT:    li a0, 0
+; RV64I-NEXT:    ret
+;
+; RV32ZBB-LABEL: umin_undef_i32:
+; RV32ZBB:       # %bb.0:
+; RV32ZBB-NEXT:    ret
+;
+; RV64ZBB-LABEL: umin_undef_i32:
+; RV64ZBB:       # %bb.0:
+; RV64ZBB-NEXT:    li a0, 0
+; RV64ZBB-NEXT:    ret
   %c = call i32 @llvm.umin.i32(i32 undef, i32 undef)
   ret i32 %c
 }
 
 define signext i32 @umax_undef_i32() {
-; NOZBB-LABEL: umax_undef_i32:
-; NOZBB:       # %bb.0:
-; NOZBB-NEXT:    ret
+; RV32I-LABEL: umax_undef_i32:
+; RV32I:       # %bb.0:
+; RV32I-NEXT:    ret
 ;
-; ZBB-LABEL: umax_undef_i32:
-; ZBB:       # %bb.0:
-; ZBB-NEXT:    ret
+; RV64I-LABEL: umax_undef_i32:
+; RV64I:       # %bb.0:
+; RV64I-NEXT:    li a0, 0
+; RV64I-NEXT:    ret
+;
+; RV32ZBB-LABEL: umax_undef_i32:
+; RV32ZBB:       # %bb.0:
+; RV32ZBB-NEXT:    ret
+;
+; RV64ZBB-LABEL: umax_undef_i32:
+; RV64ZBB:       # %bb.0:
+; RV64ZBB-NEXT:    li a0, 0
+; RV64ZBB-NEXT:    ret
   %c = call i32 @llvm.umax.i32(i32 undef, i32 undef)
   ret i32 %c
 }