Index: llvm/docs/GlobalISel/GenericOpcode.rst
===================================================================
--- llvm/docs/GlobalISel/GenericOpcode.rst
+++ llvm/docs/GlobalISel/GenericOpcode.rst
@@ -735,3 +735,24 @@
 .. code-block:: none
 
   %8:_(p0) = G_DYN_STACKALLOC %7(s64), 32
+
+Optimization Hints
+------------------
+
+These instructions do not correspond to any target instructions. They act as
+hints for various combines.
+
+G_ASSERT_ZEXT
+^^^^^^^^^^^^^
+
+Signifies that the contents of a register were previously zero-extended from a
+smaller type. This behaves identically to a COPY.
+
+The smaller type is denoted using an immediate operand. For scalars, this is the
+width of the entire smaller type. For vectors, this is the width of the smaller
+element type.
+
+.. code-block:: none
+
+  %x_assert:_(s32) = G_ASSERT_ZEXT %x(s32), 16
+  %y_assert:_(<2 x s32>) = G_ASSERT_ZEXT %y(<2 x s32>), 16
Index: llvm/include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h
===================================================================
--- llvm/include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h
+++ llvm/include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h
@@ -810,6 +810,12 @@
   /// \return a MachineInstrBuilder for the newly created instruction.
   MachineInstrBuilder buildCopy(const DstOp &Res, const SrcOp &Op);
 
+  /// Build and insert \p Res = G_ASSERT_ZEXT Op, Size
+  ///
+  /// \return a MachineInstrBuilder for the newly created instruction.
+  MachineInstrBuilder buildAssertZExt(const DstOp &Res, const SrcOp &Op,
+                                      unsigned Size);
+
   /// Build and insert `Res = G_LOAD Addr, MMO`.
   ///
   /// Loads the value stored at \p Addr. Puts the result in \p Res.
Index: llvm/include/llvm/CodeGen/TargetOpcodes.h
===================================================================
--- llvm/include/llvm/CodeGen/TargetOpcodes.h
+++ llvm/include/llvm/CodeGen/TargetOpcodes.h
@@ -36,6 +36,14 @@
 inline bool isTargetSpecificOpcode(unsigned Opcode) {
   return Opcode > TargetOpcode::PRE_ISEL_GENERIC_OPCODE_END;
 }
+
+/// \returns true if \p Opcode is an optimization hint opcode which is not
+/// supposed to appear after ISel.
+inline bool isPreISelGenericOptimizationHint(unsigned Opcode) {
+  return Opcode >= TargetOpcode::PRE_ISEL_GENERIC_OPTIMIZATION_HINT_START &&
+         Opcode <= TargetOpcode::PRE_ISEL_GENERIC_OPTIMIZATION_HINT_END;
+}
+
 } // end namespace llvm
 
 #endif
Index: llvm/include/llvm/Support/TargetOpcodes.def
===================================================================
--- llvm/include/llvm/Support/TargetOpcodes.def
+++ llvm/include/llvm/Support/TargetOpcodes.def
@@ -213,6 +213,14 @@
 /// This is something we might want to relax, but for now, this is convenient
 /// to produce diagnostics.
 
+/// Instructions which should not exist past instruction selection, but do not
+/// generate code. These instructions only act as optimization hints.
+HANDLE_TARGET_OPCODE(G_ASSERT_ZEXT)
+HANDLE_TARGET_OPCODE_MARKER(PRE_ISEL_GENERIC_OPTIMIZATION_HINT_START,
+                            G_ASSERT_ZEXT)
+HANDLE_TARGET_OPCODE_MARKER(PRE_ISEL_GENERIC_OPTIMIZATION_HINT_END,
+                            G_ASSERT_ZEXT)
+
 /// Generic ADD instruction. This is an integer add.
 HANDLE_TARGET_OPCODE(G_ADD)
 HANDLE_TARGET_OPCODE_MARKER(PRE_ISEL_GENERIC_OPCODE_START, G_ADD)
Index: llvm/include/llvm/Target/GenericOpcodes.td
===================================================================
--- llvm/include/llvm/Target/GenericOpcodes.td
+++ llvm/include/llvm/Target/GenericOpcodes.td
@@ -1337,3 +1337,15 @@
   let hasSideEffects = false;
   let mayStore = true;
 }
+
+//------------------------------------------------------------------------------
+// Optimization hints
+//------------------------------------------------------------------------------
+
+// Asserts that an operation has already been zero-extended from a specific
+// type.
+def G_ASSERT_ZEXT : GenericInstruction {
+  let OutOperandList = (outs type0:$dst);
+  let InOperandList = (ins type0:$src, untyped_imm_0:$sz);
+  let hasSideEffects = false;
+}
Index: llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp
+++ llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp
@@ -240,6 +240,12 @@
   return buildInstr(TargetOpcode::COPY, Res, Op);
 }
 
+MachineInstrBuilder MachineIRBuilder::buildAssertZExt(const DstOp &Res,
+                                                      const SrcOp &Op,
+                                                      unsigned Size) {
+  return buildInstr(TargetOpcode::G_ASSERT_ZEXT, Res, Op).addImm(Size);
+}
+
 MachineInstrBuilder MachineIRBuilder::buildConstant(const DstOp &Res,
                                                     const ConstantInt &Val) {
   LLT Ty = Res.getLLTTy(*getMRI());
Index: llvm/lib/CodeGen/MachineVerifier.cpp
===================================================================
--- llvm/lib/CodeGen/MachineVerifier.cpp
+++ llvm/lib/CodeGen/MachineVerifier.cpp
@@ -941,6 +941,22 @@
 
   // Verify properties of various specific instruction types
   switch (MI->getOpcode()) {
+  case TargetOpcode::G_ASSERT_ZEXT: {
+    if (!MI->getOperand(2).isImm()) {
+      report("G_ASSERT_ZEXT expects an immediate operand #2", MI);
+      break;
+    }
+    LLT DstTy = MRI->getType(MI->getOperand(0).getReg());
+    LLT SrcTy = MRI->getType(MI->getOperand(1).getReg());
+    verifyVectorElementMatch(DstTy, SrcTy, MI);
+    int64_t Imm = MI->getOperand(2).getImm();
+    if (Imm <= 0)
+      report("G_ASSERT_ZEXT size must be >= 1", MI);
+    else if (Imm >= SrcTy.getScalarSizeInBits())
+      report("G_ASSERT_ZEXT size must be less than source bit width", MI);
+    break;
+  }
+
   case TargetOpcode::G_CONSTANT:
   case TargetOpcode::G_FCONSTANT: {
     LLT DstTy = MRI->getType(MI->getOperand(0).getReg());
@@ -1594,7 +1610,8 @@
     }
   }
 
-  if (isPreISelGenericOpcode(MCID.getOpcode())) {
+  unsigned Opc = MCID.getOpcode();
+  if (isPreISelGenericOpcode(Opc) || isPreISelGenericOptimizationHint(Opc)) {
     verifyPreISelGenericInstruction(MI);
     return;
   }
Index: llvm/test/CodeGen/AArch64/GlobalISel/legalize-ignore-hint.mir
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AArch64/GlobalISel/legalize-ignore-hint.mir
@@ -0,0 +1,21 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=aarch64 -run-pass=legalizer -verify-machineinstrs %s -o - | FileCheck %s
+#
+# Verify that hint instructions are ignored by the legalizer.
+
+---
+name:            assert_zext
+tracksRegLiveness: true
+body: |
+  bb.0:
+    liveins: $w0, $w1
+    ; CHECK-LABEL: name: assert_zext
+    ; CHECK: %copy:_(s32) = COPY $w1
+    ; CHECK: %hint:_(s32) = G_ASSERT_ZEXT %copy, 16
+    ; CHECK: $w0 = COPY %hint(s32)
+    ; CHECK: RET_ReallyLR implicit $w0
+    %copy:_(s32) = COPY $w1
+    %hint:_(s32) = G_ASSERT_ZEXT %copy, 16
+    $w0 = COPY %hint
+    RET_ReallyLR implicit $w0
+...
Index: llvm/test/MachineVerifier/test_g_assert_ext.mir
===================================================================
--- /dev/null
+++ llvm/test/MachineVerifier/test_g_assert_ext.mir
@@ -0,0 +1,50 @@
+# RUN: not --crash llc -verify-machineinstrs -run-pass none -o /dev/null %s 2>&1 | FileCheck %s
+# REQUIRES: aarch64-registered-target
+
+--- |
+  target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
+  target triple = "aarch64-unknown-unknown"
+  define void @test() { ret void }
+...
+---
+name:            test
+body: |
+  bb.0:
+   liveins: $x0, $w0
+   %0:_(s64) = COPY $x0
+   %1:_(<4 x s16>) = COPY $x0
+   %2:_(s32) = COPY $w0
+
+   ; CHECK: *** Bad machine code: G_ASSERT_ZEXT expects an immediate operand #2 ***
+   ; CHECK: instruction: %assert_zext_1:_(s64) = G_ASSERT_ZEXT
+   %assert_zext_1:_(s64) = G_ASSERT_ZEXT %0, %0
+
+   ; CHECK: *** Bad machine code: G_ASSERT_ZEXT expects an immediate operand #2 ***
+   ; CHECK: instruction: %assert_zext_2:_(s64) = G_ASSERT_ZEXT
+   %assert_zext_2:_(s64) = G_ASSERT_ZEXT %0, i8 8
+
+   ; CHECK: *** Bad machine code: Type mismatch in generic instruction ***
+   ; CHECK: instruction: %assert_zext_3:_(<2 x s32>) = G_ASSERT_ZEXT
+   ; CHECK: *** Bad machine code: operand types must be all-vector or all-scalar ***
+   ; CHECK: instruction: %assert_zext_3:_(<2 x s32>) = G_ASSERT_ZEXT
+   %assert_zext_3:_(<2 x s32>) = G_ASSERT_ZEXT %0, 8
+
+   ; CHECK: *** Bad machine code: operand types must preserve number of vector elements ***
+   ; CHECK: instruction: %assert_zext_4:_(<2 x s32>) = G_ASSERT_ZEXT
+   %assert_zext_4:_(<2 x s32>) = G_ASSERT_ZEXT %1, 8
+
+   ; CHECK: *** Bad machine code: G_ASSERT_ZEXT size must be >= 1 ***
+   ; CHECK: instruction: %assert_zext_5:_(s64) = G_ASSERT_ZEXT
+   %assert_zext_5:_(s64) = G_ASSERT_ZEXT %0, 0
+
+   ; CHECK: *** Bad machine code: G_ASSERT_ZEXT size must be less than source bit width ***
+   ; CHECK: instruction: %assert_zext_6:_(s64) = G_ASSERT_ZEXT
+   %assert_zext_6:_(s64) = G_ASSERT_ZEXT %0, 128
+
+   ; CHECK: *** Bad machine code: Type mismatch in generic instruction ***
+   ; CHECK: instruction: %assert_zext_7:_(s64) = G_ASSERT_ZEXT %2:_, 8
+   %assert_zext_7:_(s64) = G_ASSERT_ZEXT %2, 8
+
+   ; CHECK: *** Bad machine code: Generic instruction cannot have physical register ***
+   ; CHECK: instruction: %assert_zext_8:_(s64) = G_ASSERT_ZEXT $x0, 8
+   %assert_zext_8:_(s64) = G_ASSERT_ZEXT $x0, 8