diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -545,6 +545,9 @@
     setOperationAction(ISD::CTPOP, MVT::i64, Legal);
     setOperationAction(ISD::CTPOP, MVT::i128, Expand);
     setOperationAction(ISD::PARITY, MVT::i128, Expand);
+    setOperationAction(ISD::CTTZ, MVT::i32, Legal);
+    setOperationAction(ISD::CTTZ, MVT::i64, Legal);
+    setOperationAction(ISD::CTTZ, MVT::i128, Expand);
   } else {
     setOperationAction(ISD::CTPOP, MVT::i32, Custom);
     setOperationAction(ISD::CTPOP, MVT::i64, Custom);
diff --git a/llvm/lib/Target/AArch64/AArch64InstrInfo.td b/llvm/lib/Target/AArch64/AArch64InstrInfo.td
--- a/llvm/lib/Target/AArch64/AArch64InstrInfo.td
+++ b/llvm/lib/Target/AArch64/AArch64InstrInfo.td
@@ -8494,7 +8494,16 @@
 //===----------------------------------------------------------------------===//
 defm ABS : OneOperandData<0b001000, "abs">, Requires<[HasCSSC]>;
 defm CNT : OneOperandData<0b000111, "cnt", ctpop>, Requires<[HasCSSC]>;
-defm CTZ : OneOperandData<0b000110, "ctz">, Requires<[HasCSSC]>;
+defm CTZ : OneOperandData<0b000110, "ctz", cttz>, Requires<[HasCSSC]>;
+
+// SDCOMP-61561: Before upstreaming, let's make sure this is what upstream would
+// want. See Gerrit review comments.
+let Predicates = [HasCSSC] in {
+  def : Pat<(i32 (ctlz (bitreverse GPR32:$Rn))),
+            (CTZWr GPR32:$Rn)>;
+  def : Pat<(i64 (ctlz (bitreverse GPR64:$Rn))),
+            (CTZXr GPR64:$Rn)>;
+}
 
 defm SMAX : ComparisonOp<0, 0, "smax">, Requires<[HasCSSC]>;
 defm SMIN : ComparisonOp<0, 1, "smin">, Requires<[HasCSSC]>;
diff --git a/llvm/test/CodeGen/AArch64/gpr_cttz.ll b/llvm/test/CodeGen/AArch64/gpr_cttz.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/gpr_cttz.ll
@@ -0,0 +1,162 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -verify-machineinstrs -o - %s -mtriple=aarch64-linux-gnu | FileCheck %s
+; RUN: llc -verify-machineinstrs -o - %s -mtriple=aarch64-linux-gnu -mattr=+cssc | FileCheck %s -check-prefix=CHECK-CSSC
+
+define i4 @cttz4(i4 %x) {
+; CHECK-LABEL: cttz4:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    orr w8, w0, #0x10
+; CHECK-NEXT:    rbit w8, w8
+; CHECK-NEXT:    clz w0, w8
+; CHECK-NEXT:    ret
+;
+; CHECK-CSSC-LABEL: cttz4:
+; CHECK-CSSC:       // %bb.0:
+; CHECK-CSSC-NEXT:    orr w8, w0, #0x10
+; CHECK-CSSC-NEXT:    ctz w0, w8
+; CHECK-CSSC-NEXT:    ret
+  %ctz = tail call i4 @llvm.cttz.i4(i4 %x)
+  ret i4 %ctz
+}
+
+define i8 @cttz8(i8 %x) {
+; CHECK-LABEL: cttz8:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    orr w8, w0, #0x100
+; CHECK-NEXT:    rbit w8, w8
+; CHECK-NEXT:    clz w0, w8
+; CHECK-NEXT:    ret
+;
+; CHECK-CSSC-LABEL: cttz8:
+; CHECK-CSSC:       // %bb.0:
+; CHECK-CSSC-NEXT:    orr w8, w0, #0x100
+; CHECK-CSSC-NEXT:    ctz w0, w8
+; CHECK-CSSC-NEXT:    ret
+  %ctz = tail call i8 @llvm.cttz.i8(i8 %x)
+  ret i8 %ctz
+}
+
+define i16 @cttz16(i16 %x) {
+; CHECK-LABEL: cttz16:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    orr w8, w0, #0x10000
+; CHECK-NEXT:    rbit w8, w8
+; CHECK-NEXT:    clz w0, w8
+; CHECK-NEXT:    ret
+;
+; CHECK-CSSC-LABEL: cttz16:
+; CHECK-CSSC:       // %bb.0:
+; CHECK-CSSC-NEXT:    orr w8, w0, #0x10000
+; CHECK-CSSC-NEXT:    ctz w0, w8
+; CHECK-CSSC-NEXT:    ret
+  %ctz = tail call i16 @llvm.cttz.i16(i16 %x)
+  ret i16 %ctz
+}
+
+define i17 @cttz17(i17 %x) {
+; CHECK-LABEL: cttz17:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    orr w8, w0, #0x20000
+; CHECK-NEXT:    rbit w8, w8
+; CHECK-NEXT:    clz w0, w8
+; CHECK-NEXT:    ret
+;
+; CHECK-CSSC-LABEL: cttz17:
+; CHECK-CSSC:       // %bb.0:
+; CHECK-CSSC-NEXT:    orr w8, w0, #0x20000
+; CHECK-CSSC-NEXT:    ctz w0, w8
+; CHECK-CSSC-NEXT:    ret
+  %ctz = tail call i17 @llvm.cttz.i17(i17 %x)
+  ret i17 %ctz
+}
+
+define i32 @cttz32(i32 %x) nounwind readnone {
+; CHECK-LABEL: cttz32:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    rbit w8, w0
+; CHECK-NEXT:    clz w0, w8
+; CHECK-NEXT:    ret
+;
+; CHECK-CSSC-LABEL: cttz32:
+; CHECK-CSSC:       // %bb.0:
+; CHECK-CSSC-NEXT:    ctz w0, w0
+; CHECK-CSSC-NEXT:    ret
+  %ctz = tail call i32 @llvm.cttz.i32(i32 %x)
+  ret i32 %ctz
+}
+
+define i64 @cttz64(i64 %x) nounwind readnone {
+; CHECK-LABEL: cttz64:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    rbit x8, x0
+; CHECK-NEXT:    clz x0, x8
+; CHECK-NEXT:    ret
+;
+; CHECK-CSSC-LABEL: cttz64:
+; CHECK-CSSC:       // %bb.0:
+; CHECK-CSSC-NEXT:    ctz x0, x0
+; CHECK-CSSC-NEXT:    ret
+  %ctz = tail call i64 @llvm.cttz.i64(i64 %x)
+  ret i64 %ctz
+}
+
+define i128 @cttz128(i128 %x) nounwind readnone {
+; CHECK-LABEL: cttz128:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    rbit x9, x1
+; CHECK-NEXT:    rbit x8, x0
+; CHECK-NEXT:    clz x9, x9
+; CHECK-NEXT:    clz x8, x8
+; CHECK-NEXT:    add x9, x9, #64
+; CHECK-NEXT:    cmp x0, #0
+; CHECK-NEXT:    csel x0, x8, x9, ne
+; CHECK-NEXT:    mov x1, xzr
+; CHECK-NEXT:    ret
+;
+; CHECK-CSSC-LABEL: cttz128:
+; CHECK-CSSC:       // %bb.0:
+; CHECK-CSSC-NEXT:    ctz x9, x1
+; CHECK-CSSC-NEXT:    ctz x8, x0
+; CHECK-CSSC-NEXT:    add x9, x9, #64
+; CHECK-CSSC-NEXT:    cmp x0, #0
+; CHECK-CSSC-NEXT:    csel x0, x8, x9, ne
+; CHECK-CSSC-NEXT:    mov x1, xzr
+; CHECK-CSSC-NEXT:    ret
+  %ctz = tail call i128 @llvm.cttz.i128(i128 %x)
+  ret i128 %ctz
+}
+
+define i32 @cttz32combine(i32 %x) nounwind readnone {
+; CHECK-LABEL: cttz32combine:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    rbit w8, w0
+; CHECK-NEXT:    clz w0, w8
+; CHECK-NEXT:    ret
+;
+; CHECK-CSSC-LABEL: cttz32combine:
+; CHECK-CSSC:       // %bb.0:
+; CHECK-CSSC-NEXT:    ctz w0, w0
+; CHECK-CSSC-NEXT:    ret
+  %rev = tail call i32 @llvm.bitreverse.i32(i32 %x)
+  %ctz = tail call i32 @llvm.ctlz.i32(i32 %rev)
+  ret i32 %ctz
+}
+
+define i64 @cttz64combine(i64 %x) nounwind readnone {
+  %rev = tail call i64 @llvm.bitreverse.i64(i64 %x)
+  %ctz = tail call i64 @llvm.ctlz.i64(i64 %rev)
+  ret i64 %ctz
+}
+
+declare i4 @llvm.cttz.i4(i4 %x) nounwind readnone
+declare i8 @llvm.cttz.i8(i8 %x) nounwind readnone
+declare i16 @llvm.cttz.i16(i16 %x) nounwind readnone
+declare i17 @llvm.cttz.i17(i17 %x) nounwind readnone
+declare i32 @llvm.cttz.i32(i32) nounwind readnone
+declare i64 @llvm.cttz.i64(i64) nounwind readnone
+declare i128 @llvm.cttz.i128(i128) nounwind readnone
+
+declare i32 @llvm.ctlz.i32(i32 %x) nounwind readnone
+declare i32 @llvm.bitreverse.i32(i32 %x) nounwind readnone
+declare i64 @llvm.ctlz.i64(i64 %x) nounwind readnone
+declare i64 @llvm.bitreverse.i64(i64 %x) nounwind readnone