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
@@ -107,6 +107,9 @@
   bool matchCombineIndexedLoadStore(MachineInstr &MI, IndexedLoadStoreMatchInfo &MatchInfo);
   void applyCombineIndexedLoadStore(MachineInstr &MI, IndexedLoadStoreMatchInfo &MatchInfo);
 
+  bool matchSextTruncSextLoad(MachineInstr &MI, Register &LoadReg);
+  bool applySextTruncSextLoad(MachineInstr &MI, Register &LoadReg);
+
   bool matchElideBrByInvertingCond(MachineInstr &MI);
   void applyElideBrByInvertingCond(MachineInstr &MI);
   bool tryElideBrByInvertingCond(MachineInstr &MI);
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
@@ -114,6 +114,13 @@
   (apply [{ Helper.applyCombineExtendingLoads(*${root}, ${matchinfo}); }])>;
 def combines_for_extload: GICombineGroup<[extending_loads]>;
 
+def sext_trunc_sextload_matchinfo: GIDefMatchData<"Register">;
+def sext_trunc_sextload : GICombineRule<
+  (defs root:$d, sext_trunc_sextload_matchinfo:$matchinfo),
+  (match (wip_match_opcode G_SEXT_INREG):$d,
+         [{ return Helper.matchSextTruncSextLoad(*${d}, ${matchinfo}); }]),
+  (apply [{ Helper.applySextTruncSextLoad(*${d}, ${matchinfo}); }])>;
+
 def combine_indexed_load_store : GICombineRule<
   (defs root:$root, indexed_load_store_matchdata:$matchinfo),
   (match (wip_match_opcode G_LOAD, G_SEXTLOAD, G_ZEXTLOAD, G_STORE):$root,
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
@@ -576,6 +576,41 @@
   return isPredecessor(DefMI, UseMI);
 }
 
+bool CombinerHelper::matchSextTruncSextLoad(MachineInstr &MI,
+                                            Register &LoadReg) {
+  assert(MI.getOpcode() == TargetOpcode::G_SEXT_INREG);
+  Register SrcReg = MI.getOperand(1).getReg();
+  Register LoadUser = SrcReg;
+
+  Register TruncSrc;
+  if (mi_match(SrcReg, MRI, m_GTrunc(m_Reg(TruncSrc))))
+    LoadUser = TruncSrc;
+
+  uint64_t SizeInBits = MI.getOperand(2).getImm();
+  // If the source is a G_SEXTLOAD from the same bit width, then we don't
+  // need any extend at all, just a truncate.
+  if (auto *LoadMI = getOpcodeDef(TargetOpcode::G_SEXTLOAD, LoadUser, MRI)) {
+    const auto &MMO = **LoadMI->memoperands_begin();
+    // If truncating more than the original extended value, abort.
+    if (TruncSrc && MRI.getType(TruncSrc).getSizeInBits() < MMO.getSizeInBits())
+      return false;
+    if (MMO.getSizeInBits() == SizeInBits) {
+      LoadReg = LoadMI->getOperand(0).getReg();
+      return true;
+    }
+  }
+  return false;
+}
+
+bool CombinerHelper::applySextTruncSextLoad(MachineInstr &MI,
+                                           Register &LoadReg) {
+  assert(MI.getOpcode() == TargetOpcode::G_SEXT_INREG);
+  MachineIRBuilder MIB(MI);
+  MIB.buildAnyExtOrTrunc(MI.getOperand(0).getReg(), LoadReg);
+  MI.eraseFromParent();
+  return true;
+}
+
 bool CombinerHelper::findPostIndexCandidate(MachineInstr &MI, Register &Addr,
                                             Register &Base, Register &Offset) {
   auto &MF = *MI.getParent()->getParent();
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
@@ -77,6 +77,6 @@
 def AArch64PostLegalizerCombinerHelper
     : GICombinerHelper<"AArch64GenPostLegalizerCombinerHelper",
                        [erase_undef_store, combines_for_extload,
-                        shuffle_vector_pseudos]> {
+                        sext_trunc_sextload, shuffle_vector_pseudos]> {
   let DisableRuleOption = "aarch64postlegalizercombiner-disable-rule";
 }
diff --git a/llvm/test/CodeGen/AArch64/GlobalISel/combine-sext-trunc-sextload.mir b/llvm/test/CodeGen/AArch64/GlobalISel/combine-sext-trunc-sextload.mir
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/GlobalISel/combine-sext-trunc-sextload.mir
@@ -0,0 +1,61 @@
+# 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
+---
+name:            test_combine_sext_trunc_of_sextload
+legalized:       true
+tracksRegLiveness: true
+body: |
+  bb.0.entry:
+    liveins: $x0
+    ; CHECK-LABEL: name: test_combine_sext_trunc_of_sextload
+    ; CHECK: liveins: $x0
+    ; CHECK: [[COPY:%[0-9]+]]:_(p0) = COPY $x0
+    ; CHECK: [[SEXTLOAD:%[0-9]+]]:_(s64) = G_SEXTLOAD [[COPY]](p0) :: (load 2)
+    ; CHECK: [[TRUNC:%[0-9]+]]:_(s32) = G_TRUNC [[SEXTLOAD]](s64)
+    ; CHECK: $w0 = COPY [[TRUNC]](s32)
+    %0:_(p0) = COPY $x0
+    %1:_(s64) = G_SEXTLOAD %0:_(p0) :: (load 2)
+    %2:_(s32) = G_TRUNC %1:_(s64)
+    %3:_(s32) = G_SEXT_INREG %2:_(s32), 16
+    $w0 = COPY %3(s32)
+...
+---
+name:            test_combine_sext_of_sextload
+legalized:       true
+tracksRegLiveness: true
+body: |
+  bb.0.entry:
+    liveins: $x0
+    ; CHECK-LABEL: name: test_combine_sext_of_sextload
+    ; CHECK: liveins: $x0
+    ; CHECK: [[COPY:%[0-9]+]]:_(p0) = COPY $x0
+    ; CHECK: [[SEXTLOAD:%[0-9]+]]:_(s32) = G_SEXTLOAD [[COPY]](p0) :: (load 2)
+    ; CHECK: [[COPY1:%[0-9]+]]:_(s32) = COPY [[SEXTLOAD]](s32)
+    ; CHECK: $w0 = COPY [[COPY1]](s32)
+    %0:_(p0) = COPY $x0
+    %1:_(s32) = G_SEXTLOAD %0:_(p0) :: (load 2)
+    %2:_(s32) = COPY %1:_(s32)
+    %3:_(s32) = G_SEXT_INREG %2:_(s32), 16
+    $w0 = COPY %3(s32)
+...
+---
+name:            test_combine_sext_of_sextload_not_matching
+legalized:       true
+tracksRegLiveness: true
+body: |
+  bb.0.entry:
+    liveins: $x0
+    ; Here we're trying to extend from a smaller value than was extended in the load.
+    ; CHECK-LABEL: name: test_combine_sext_of_sextload_not_matching
+    ; CHECK: liveins: $x0
+    ; CHECK: [[COPY:%[0-9]+]]:_(p0) = COPY $x0
+    ; CHECK: [[SEXTLOAD:%[0-9]+]]:_(s32) = G_SEXTLOAD [[COPY]](p0) :: (load 2)
+    ; CHECK: [[COPY1:%[0-9]+]]:_(s32) = COPY [[SEXTLOAD]](s32)
+    ; CHECK: [[SEXT_INREG:%[0-9]+]]:_(s32) = G_SEXT_INREG [[COPY1]], 24
+    ; CHECK: $w0 = COPY [[SEXT_INREG]](s32)
+    %0:_(p0) = COPY $x0
+    %1:_(s32) = G_SEXTLOAD %0:_(p0) :: (load 2)
+    %2:_(s32) = COPY %1:_(s32)
+    %3:_(s32) = G_SEXT_INREG %2:_(s32), 24
+    $w0 = COPY %3(s32)
+...