diff --git a/llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h b/llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
--- a/llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
+++ b/llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
@@ -510,6 +510,8 @@
                     std::function<void(MachineIRBuilder &)> &MatchInfo);
   bool matchFunnelShiftToRotate(MachineInstr &MI);
   void applyFunnelShiftToRotate(MachineInstr &MI);
+  bool matchRotateOutOfRange(MachineInstr &MI);
+  void applyRotateOutOfRange(MachineInstr &MI);
 
   /// Try to transform \p MI by using all of the above
   /// combine functions. Returns true if changed.
diff --git a/llvm/include/llvm/Target/GlobalISel/Combine.td b/llvm/include/llvm/Target/GlobalISel/Combine.td
--- a/llvm/include/llvm/Target/GlobalISel/Combine.td
+++ b/llvm/include/llvm/Target/GlobalISel/Combine.td
@@ -605,7 +605,14 @@
   (apply [{ Helper.applyFunnelShiftToRotate(*${root}); }])
 >;
 
-def funnel_shift_combines : GICombineGroup<[funnel_shift_to_rotate]>;
+def rotate_out_of_range : GICombineRule<
+  (defs root:$root),
+  (match (wip_match_opcode G_ROTR, G_ROTL):$root,
+    [{ return Helper.matchRotateOutOfRange(*${root}); }]),
+  (apply [{ Helper.applyRotateOutOfRange(*${root}); }])
+>;
+
+def funnel_shift_combines : GICombineGroup<[funnel_shift_to_rotate, rotate_out_of_range]>;
 
 // FIXME: These should use the custom predicate feature once it lands.
 def undef_combines : GICombineGroup<[undef_to_fp_zero, undef_to_int_zero,
diff --git a/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
--- a/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
@@ -9,6 +9,7 @@
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallBitVector.h"
 #include "llvm/CodeGen/GlobalISel/Combiner.h"
+#include "llvm/CodeGen/GlobalISel/ConstantFoldingMIRBuilder.h"
 #include "llvm/CodeGen/GlobalISel/GISelChangeObserver.h"
 #include "llvm/CodeGen/GlobalISel/GISelKnownBits.h"
 #include "llvm/CodeGen/GlobalISel/LegalizerInfo.h"
@@ -25,6 +26,7 @@
 #include "llvm/CodeGen/TargetInstrInfo.h"
 #include "llvm/CodeGen/TargetLowering.h"
 #include "llvm/CodeGen/TargetOpcodes.h"
+#include "llvm/Support/Casting.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Target/TargetMachine.h"
 
@@ -3894,6 +3896,38 @@
   Observer.changedInstr(MI);
 }
 
+// Fold (rot x, c) -> (rot x, c % BitSize)
+bool CombinerHelper::matchRotateOutOfRange(MachineInstr &MI) {
+  assert(MI.getOpcode() == TargetOpcode::G_ROTL ||
+         MI.getOpcode() == TargetOpcode::G_ROTR);
+  unsigned Bitsize =
+      MRI.getType(MI.getOperand(0).getReg()).getScalarSizeInBits();
+  bool OutOfRange = false;
+  auto MatchOutOfRange = [Bitsize, &OutOfRange](const Constant *C) {
+    if (auto *CI = dyn_cast<ConstantInt>(C))
+      OutOfRange |= CI->getValue().uge(Bitsize);
+    return true;
+  };
+  return matchUnaryPredicate(MRI, MI.getOperand(2).getReg(), MatchOutOfRange) &&
+         OutOfRange;
+}
+
+void CombinerHelper::applyRotateOutOfRange(MachineInstr &MI) {
+  assert(MI.getOpcode() == TargetOpcode::G_ROTL ||
+         MI.getOpcode() == TargetOpcode::G_ROTR);
+  unsigned Bitsize =
+      MRI.getType(MI.getOperand(0).getReg()).getScalarSizeInBits();
+  Builder.setInstrAndDebugLoc(MI);
+  Register Amt = MI.getOperand(2).getReg();
+  LLT AmtTy = MRI.getType(Amt);
+  auto Bits = Builder.buildConstant(AmtTy, Bitsize);
+  ConstantFoldingMIRBuilder CFMIB(MI);
+  Amt = CFMIB.buildURem(AmtTy, MI.getOperand(2).getReg(), Bits).getReg(0);
+  Observer.changingInstr(MI);
+  MI.getOperand(2).setReg(Amt);
+  Observer.changedInstr(MI);
+}
+
 bool CombinerHelper::tryCombine(MachineInstr &MI) {
   if (tryCombineCopy(MI))
     return true;
diff --git a/llvm/lib/Target/AArch64/AArch64Combine.td b/llvm/lib/Target/AArch64/AArch64Combine.td
--- a/llvm/lib/Target/AArch64/AArch64Combine.td
+++ b/llvm/lib/Target/AArch64/AArch64Combine.td
@@ -188,6 +188,6 @@
                         redundant_and, xor_of_and_with_same_reg,
                         extractvecelt_pairwise_add, redundant_or,
                         mul_const, redundant_sext_inreg,
-                        form_bitfield_extract]> {
+                        form_bitfield_extract, rotate_out_of_range]> {
   let DisableRuleOption = "aarch64postlegalizercombiner-disable-rule";
 }
diff --git a/llvm/test/CodeGen/AArch64/GlobalISel/postlegalizercombiner-rotate.mir b/llvm/test/CodeGen/AArch64/GlobalISel/postlegalizercombiner-rotate.mir
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/GlobalISel/postlegalizercombiner-rotate.mir
@@ -0,0 +1,54 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple aarch64 -run-pass=aarch64-postlegalizer-combiner -verify-machineinstrs %s -o - | FileCheck %s
+
+# Check that we simplify the constant rotate amount to be in range.
+---
+name:            rotl
+alignment:       4
+legalized:       true
+tracksRegLiveness: true
+liveins:
+  - { reg: '$w0' }
+body:             |
+  bb.1.entry:
+    liveins: $w0
+
+    ; CHECK-LABEL: name: rotl
+    ; CHECK: liveins: $w0
+    ; CHECK: [[COPY:%[0-9]+]]:_(s32) = COPY $w0
+    ; CHECK: [[C:%[0-9]+]]:_(s64) = G_CONSTANT i64 16
+    ; CHECK: [[ROTL:%[0-9]+]]:_(s32) = G_ROTL [[COPY]], [[C]](s64)
+    ; CHECK: $w0 = COPY [[ROTL]](s32)
+    ; CHECK: RET_ReallyLR implicit $w0
+    %0:_(s32) = COPY $w0
+    %5:_(s64) = G_CONSTANT i64 -16
+    %2:_(s32) = G_ROTL %0, %5(s64)
+    $w0 = COPY %2(s32)
+    RET_ReallyLR implicit $w0
+
+...
+---
+name:            rotr
+alignment:       4
+legalized:       true
+tracksRegLiveness: true
+liveins:
+  - { reg: '$w0' }
+body:             |
+  bb.1.entry:
+    liveins: $w0
+
+    ; CHECK-LABEL: name: rotr
+    ; CHECK: liveins: $w0
+    ; CHECK: [[COPY:%[0-9]+]]:_(s32) = COPY $w0
+    ; CHECK: [[C:%[0-9]+]]:_(s64) = G_CONSTANT i64 16
+    ; CHECK: [[ROTR:%[0-9]+]]:_(s32) = G_ROTR [[COPY]], [[C]](s64)
+    ; CHECK: $w0 = COPY [[ROTR]](s32)
+    ; CHECK: RET_ReallyLR implicit $w0
+    %0:_(s32) = COPY $w0
+    %5:_(s64) = G_CONSTANT i64 -16
+    %2:_(s32) = G_ROTR %0, %5(s64)
+    $w0 = COPY %2(s32)
+    RET_ReallyLR implicit $w0
+
+...