Index: llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
===================================================================
--- llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
+++ llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
@@ -234,6 +234,9 @@
   /// Delete \p MI and replace all of its uses with its \p OpIdx-th operand.
   bool replaceSingleDefInstWithOperand(MachineInstr &MI, unsigned OpIdx);
 
+  /// Delete \p MI and replace all of its uses with \p Replacement.
+  bool replaceSingleDefInstWithReg(MachineInstr &MI, Register Replacement);
+
   /// Return true if \p MOP1 and \p MOP2 are register operands are defined by
   /// equivalent instructions.
   bool matchEqualDefs(const MachineOperand &MOP1, const MachineOperand &MOP2);
@@ -260,6 +263,14 @@
   bool applySimplifyAddToSub(MachineInstr &MI,
                              std::tuple<Register, Register> &MatchInfo);
 
+  /// \return true if \p MI is a G_AND instruction whose RHS is a mask where
+  /// LHS & mask == LHS. (E.g., an all-ones value.)
+  ///
+  /// \param [in] MI - The G_AND instruction.
+  /// \param [out] Reg - A register the G_AND should be replaced with on
+  /// success.
+  bool matchAndWithTrivialMask(MachineInstr &MI, Register &Replacement);
+
   /// Try to transform \p MI by using all of the above
   /// combine functions. Returns true if changed.
   bool tryCombine(MachineInstr &MI);
Index: llvm/include/llvm/Target/GlobalISel/Combine.td
===================================================================
--- llvm/include/llvm/Target/GlobalISel/Combine.td
+++ llvm/include/llvm/Target/GlobalISel/Combine.td
@@ -275,6 +275,15 @@
   (apply [{ return Helper.applyCombineP2IToI2P(*${root}, ${info}); }])
 >;
 
+// Fold (x & mask) -> x when (x & mask) is known to equal x.
+def and_trivial_mask_matchinfo : GIDefMatchData<"Register">;
+def and_trivial_mask: GICombineRule <
+  (defs root:$root, and_trivial_mask_matchinfo:$matchinfo),
+  (match (wip_match_opcode G_AND):$root,
+         [{ return Helper.matchAndWithTrivialMask(*${root}, ${matchinfo}); }]),
+  (apply [{ return Helper.replaceSingleDefInstWithReg(*${root}, ${matchinfo}); }])
+>;
+
 // FIXME: These should use the custom predicate feature once it lands.
 def undef_combines : GICombineGroup<[undef_to_fp_zero, undef_to_int_zero,
                                      undef_to_negative_one,
Index: llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
+++ llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
@@ -1700,6 +1700,16 @@
   return true;
 }
 
+bool CombinerHelper::replaceSingleDefInstWithReg(MachineInstr &MI,
+                                                 Register Replacement) {
+  assert(MI.getNumExplicitDefs() == 1 && "Expected one explicit def?");
+  Register OldReg = MI.getOperand(0).getReg();
+  assert(canReplaceReg(OldReg, Replacement, MRI) && "Cannot replace register?");
+  MI.eraseFromParent();
+  replaceRegWith(MRI, OldReg, Replacement);
+  return true;
+}
+
 bool CombinerHelper::matchSelectSameVal(MachineInstr &MI) {
   assert(MI.getOpcode() == TargetOpcode::G_SELECT);
   // Match (cond ? x : x)
@@ -1776,6 +1786,50 @@
   return true;
 }
 
+bool CombinerHelper::matchAndWithTrivialMask(MachineInstr &MI,
+                                             Register &Replacement) {
+  // Given
+  //
+  // %mask:_(sN) = G_CONSTANT iN 000...0111...1
+  // %x:_(sN) = G_SOMETHING
+  // %y:_(sN) = G_AND %x, %mask
+  //
+  // Eliminate the G_AND when it is known that x & mask == x.
+  //
+  // Patterns like this can appear as a result of legalization. E.g.
+  //
+  // %cmp:_(s32) = G_ICMP intpred(pred), %x(s32), %y
+  // %one:_(s32) = G_CONSTANT i32 1
+  // %and:_(s32) = G_AND %cmp, %one
+  //
+  // In this case, G_ICMP only produces a single bit, so x & 1 == x.
+  assert(MI.getOpcode() == TargetOpcode::G_AND);
+  if (!KB)
+    return false;
+
+  // Replacement = %x, AndDst = %y. Check that we can replace AndDst with the
+  // LHS of the G_AND.
+  Replacement = MI.getOperand(1).getReg();
+  Register AndDst = MI.getOperand(0).getReg();
+  LLT DstTy = MRI.getType(AndDst);
+  if (DstTy.isVector())
+    return false;
+  if (!canReplaceReg(AndDst, Replacement, MRI))
+    return false;
+
+  // Check that we have a constant on the RHS of the G_AND, which is of the form
+  // 000...0111...1.
+  int64_t Cst;
+  if (!mi_match(MI.getOperand(2).getReg(), MRI, m_ICst(Cst)))
+    return false;
+  APInt Mask(DstTy.getSizeInBits(), Cst);
+  if (!Mask.isMask())
+    return false;
+
+  // Now, let's check that x & Mask == x. If this is true, then x & ~Mask == 0.
+  return KB->maskedValueIsZero(Replacement, ~Mask);
+}
+
 bool CombinerHelper::tryCombine(MachineInstr &MI) {
   if (tryCombineCopy(MI))
     return true;
Index: llvm/lib/Target/AArch64/AArch64Combine.td
===================================================================
--- llvm/lib/Target/AArch64/AArch64Combine.td
+++ llvm/lib/Target/AArch64/AArch64Combine.td
@@ -79,6 +79,7 @@
 def AArch64PostLegalizerCombinerHelper
     : GICombinerHelper<"AArch64GenPostLegalizerCombinerHelper",
                        [erase_undef_store, combines_for_extload,
-                        sext_trunc_sextload, shuffle_vector_pseudos]> {
+                        sext_trunc_sextload, shuffle_vector_pseudos,
+                        and_trivial_mask]> {
   let DisableRuleOption = "aarch64postlegalizercombiner-disable-rule";
 }
Index: llvm/test/CodeGen/AArch64/GlobalISel/postlegalizer-combiner-and-trivial-mask.mir
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AArch64/GlobalISel/postlegalizer-combiner-and-trivial-mask.mir
@@ -0,0 +1,222 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+#
+# Check that we can fold (x & mask) -> x when (x & mask) is known to equal x.
+#
+# RUN: llc -mtriple aarch64 -run-pass=aarch64-postlegalizer-combiner -verify-machineinstrs %s -o - | FileCheck %s
+
+name:            remove_and_with_one_bit
+legalized:       true
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $w0, $w1
+    ; G_ICMP produces a single bit. The mask is 1.
+    ;
+    ; cmp = 000...0?
+    ; mask = 000...01
+    ; cmp & mask = 000...0?
+    ;
+    ; Remove the G_AND.
+    ;
+    ; CHECK-LABEL: name: remove_and_with_one_bit
+    ; CHECK: liveins: $w0, $w1
+    ; CHECK: %x:_(s32) = COPY $w0
+    ; CHECK: %y:_(s32) = COPY $w1
+    ; CHECK: %cmp:_(s32) = G_ICMP intpred(eq), %x(s32), %y
+    ; CHECK: $w0 = COPY %cmp(s32)
+    ; CHECK: RET_ReallyLR implicit $w0
+    %x:_(s32) = COPY $w0
+    %y:_(s32) = COPY $w1
+    %cmp:_(s32) = G_ICMP intpred(eq), %x(s32), %y
+    %mask:_(s32) = G_CONSTANT i32 1
+    %and:_(s32) = G_AND %cmp(s32), %mask
+    $w0 = COPY %and(s32)
+    RET_ReallyLR implicit $w0
+
+...
+---
+name:            remove_and_all_ones_mask
+legalized:       true
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $w0, $w1, $w2
+    ; -1 is all ones. Therefore z & -1 = z. Remove the G_AND.
+    ;
+    ; CHECK-LABEL: name: remove_and_all_ones_mask
+    ; CHECK: liveins: $w0, $w1, $w2
+    ; CHECK: %z:_(s32) = COPY $w2
+    ; CHECK: $w0 = COPY %z(s32)
+    ; CHECK: RET_ReallyLR implicit $w0
+    %x:_(s32) = COPY $w0
+    %y:_(s32) = COPY $w1
+    %z:_(s32) = COPY $w2
+    %mask:_(s32) = G_CONSTANT i32 -1
+    %and:_(s32) = G_AND %z(s32), %mask
+    $w0 = COPY %and(s32)
+    RET_ReallyLR implicit $w0
+
+...
+---
+name:            remove_and_all_ones_zext
+legalized:       true
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $w0, $w1, $w2
+    ; %z is a s32, so it can be at most the all-ones value on 32 bits.
+    ; In decimal this is 4294967295. Any zero-extension of %z is at most this
+    ; value.
+    ;
+    ; Therefore, zext(z) & 4294967295 == z. Remove the G_AND.
+    ;
+    ; CHECK-LABEL: name: remove_and_all_ones_zext
+    ; CHECK: liveins: $w0, $w1, $w2
+    ; CHECK: %z:_(s32) = COPY $w2
+    ; CHECK: %ext:_(s64) = G_ZEXT %z(s32)
+    ; CHECK: $x0 = COPY %ext(s64)
+    ; CHECK: RET_ReallyLR implicit $x0
+    %x:_(s32) = COPY $w0
+    %y:_(s32) = COPY $w1
+    %z:_(s32) = COPY $w2
+    %ext:_(s64) = G_ZEXT %z
+    %mask:_(s64) = G_CONSTANT i64 4294967295
+    %and:_(s64) = G_AND %ext(s64), %mask
+    $x0 = COPY %and(s64)
+    RET_ReallyLR implicit $x0
+
+...
+---
+name:            remove_and_all_ones_anyext
+legalized:       true
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $w0, $w1, $w2
+    ; This is the same as the zext case.
+    ;
+    ; CHECK-LABEL: name: remove_and_all_ones_anyext
+    ; CHECK: liveins: $w0, $w1, $w2
+    ; CHECK: %z:_(s32) = COPY $w2
+    ; CHECK: %ext:_(s64) = G_ZEXT %z(s32)
+    ; CHECK: $x0 = COPY %ext(s64)
+    ; CHECK: RET_ReallyLR implicit $x0
+    %x:_(s32) = COPY $w0
+    %y:_(s32) = COPY $w1
+    %z:_(s32) = COPY $w2
+    %ext:_(s64) = G_ZEXT %z
+    %mask:_(s64) = G_CONSTANT i64 4294967295
+    %and:_(s64) = G_AND %ext(s64), %mask
+    $x0 = COPY %and(s64)
+    RET_ReallyLR implicit $x0
+
+...
+---
+name:            dont_remove_all_ones_sext
+legalized:       true
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $w0, $w1, $w2
+    ; We don't know if the sign bit is set on %z. So, the value in %ext may have
+    ; higher bits set than 4294967295.
+    ;
+    ; CHECK-LABEL: name: dont_remove_all_ones_sext
+    ; CHECK: liveins: $w0, $w1, $w2
+    ; CHECK: %z:_(s32) = COPY $w2
+    ; CHECK: %ext:_(s64) = G_SEXT %z(s32)
+    ; CHECK: %mask:_(s64) = G_CONSTANT i64 4294967295
+    ; CHECK: %and:_(s64) = G_AND %ext, %mask
+    ; CHECK: $x0 = COPY %and(s64)
+    ; CHECK: RET_ReallyLR implicit $x0
+    %x:_(s32) = COPY $w0
+    %y:_(s32) = COPY $w1
+    %z:_(s32) = COPY $w2
+    %ext:_(s64) = G_SEXT %z
+    %mask:_(s64) = G_CONSTANT i64 4294967295
+    %and:_(s64) = G_AND %ext(s64), %mask
+    $x0 = COPY %and(s64)
+    RET_ReallyLR implicit $x0
+
+...
+---
+name:            remove_and_positive_constant_sext
+legalized:       true
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $w0, $w1, $w2
+    ; We know the sign bit is not set on %z. Therefore,
+    ;
+    ; z = ext = 42 = 000...0101010
+    ; mask = 0000...0111111
+    ;
+    ; So z & mask == z
+    ; CHECK-LABEL: name: remove_and_positive_constant_sext
+    ; CHECK: liveins: $w0, $w1, $w2
+    ; CHECK: %z:_(s32) = G_CONSTANT i32 42
+    ; CHECK: %ext:_(s64) = G_SEXT %z(s32)
+    ; CHECK: $x0 = COPY %ext(s64)
+    ; CHECK: RET_ReallyLR implicit $x0
+    %x:_(s32) = COPY $w0
+    %y:_(s32) = COPY $w1
+    %z:_(s32) = G_CONSTANT i32 42
+    %ext:_(s64) = G_SEXT %z
+    %mask:_(s64) = G_CONSTANT i64 63
+    %and:_(s64) = G_AND %ext(s64), %mask
+    $x0 = COPY %and(s64)
+    RET_ReallyLR implicit $x0
+
+...
+---
+name:            not_a_mask
+legalized:       true
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $w0, $w1
+    ; 6 is not a mask, so we should still have the G_AND.
+    ;
+    ; CHECK-LABEL: name: not_a_mask
+    ; CHECK: liveins: $w0, $w1
+    ; CHECK: %x:_(s32) = COPY $w0
+    ; CHECK: %y:_(s32) = COPY $w1
+    ; CHECK: %cmp:_(s32) = G_ICMP intpred(eq), %x(s32), %y
+    ; CHECK: %mask:_(s32) = G_CONSTANT i32 6
+    ; CHECK: %and:_(s32) = G_AND %cmp, %mask
+    ; CHECK: $w0 = COPY %and(s32)
+    ; CHECK: RET_ReallyLR implicit $w0
+    %x:_(s32) = COPY $w0
+    %y:_(s32) = COPY $w1
+    %cmp:_(s32) = G_ICMP intpred(eq), %x(s32), %y
+    %mask:_(s32) = G_CONSTANT i32 6
+    %and:_(s32) = G_AND %cmp(s32), %mask
+    $w0 = COPY %and(s32)
+    RET_ReallyLR implicit $w0
+
+...
+---
+name:            unknown_val
+legalized:       true
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $w0, $w1, $w2
+    ; We don't know what's in $w2, so we can't remove the G_AND without a mask
+    ; that fills every bit in the type.
+    ;
+    ; CHECK-LABEL: name: unknown_val
+    ; CHECK: liveins: $w0, $w1, $w2
+    ; CHECK: %z:_(s32) = COPY $w2
+    ; CHECK: %one:_(s32) = G_CONSTANT i32 32
+    ; CHECK: %and:_(s32) = G_AND %z, %one
+    ; CHECK: $w0 = COPY %and(s32)
+    ; CHECK: RET_ReallyLR implicit $w0
+    %x:_(s32) = COPY $w0
+    %y:_(s32) = COPY $w1
+    %z:_(s32) = COPY $w2
+    %one:_(s32) = G_CONSTANT i32 32
+    %and:_(s32) = G_AND %z(s32), %one
+    $w0 = COPY %and(s32)
+    RET_ReallyLR implicit $w0
+...