Index: llvm/trunk/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ llvm/trunk/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -567,6 +567,26 @@
     MI.eraseFromParent();
     return Legalized;
   }
+  case TargetOpcode::G_SEXT: {
+    if (TypeIdx != 0)
+      return UnableToLegalize;
+
+    if (NarrowTy.getSizeInBits() != SizeOp0 / 2) {
+      LLVM_DEBUG(dbgs() << "Can't narrow sext to type " << NarrowTy << "\n");
+      return UnableToLegalize;
+    }
+
+    Register SrcReg = MI.getOperand(1).getReg();
+
+    // Shift the sign bit of the low register through the high register.
+    auto ShiftAmt =
+        MIRBuilder.buildConstant(LLT::scalar(64), NarrowTy.getSizeInBits() - 1);
+    auto Shift = MIRBuilder.buildAShr(NarrowTy, SrcReg, ShiftAmt);
+    MIRBuilder.buildMerge(MI.getOperand(0).getReg(), {SrcReg, Shift.getReg(0)});
+    MI.eraseFromParent();
+    return Legalized;
+  }
+
   case TargetOpcode::G_ADD: {
     // FIXME: add support for when SizeOp0 isn't an exact multiple of
     // NarrowSize.
Index: llvm/trunk/lib/Target/AArch64/AArch64LegalizerInfo.cpp
===================================================================
--- llvm/trunk/lib/Target/AArch64/AArch64LegalizerInfo.cpp
+++ llvm/trunk/lib/Target/AArch64/AArch64LegalizerInfo.cpp
@@ -340,30 +340,36 @@
       .widenScalarToNextPow2(1);
 
   // Extensions
-  getActionDefinitionsBuilder({G_ZEXT, G_SEXT, G_ANYEXT})
-      .legalIf([=](const LegalityQuery &Query) {
-        unsigned DstSize = Query.Types[0].getSizeInBits();
-
-        // Make sure that we have something that will fit in a register, and
-        // make sure it's a power of 2.
-        if (DstSize < 8 || DstSize > 128 || !isPowerOf2_32(DstSize))
-          return false;
+  auto ExtLegalFunc = [=](const LegalityQuery &Query) {
+    unsigned DstSize = Query.Types[0].getSizeInBits();
 
-        const LLT &SrcTy = Query.Types[1];
+    if (DstSize == 128 && !Query.Types[0].isVector())
+      return false; // Extending to a scalar s128 is not legal.
+    
+    // Make sure that we have something that will fit in a register, and
+    // make sure it's a power of 2.
+    if (DstSize < 8 || DstSize > 128 || !isPowerOf2_32(DstSize))
+      return false;
 
-        // Special case for s1.
-        if (SrcTy == s1)
-          return true;
+    const LLT &SrcTy = Query.Types[1];
 
-        // Make sure we fit in a register otherwise. Don't bother checking that
-        // the source type is below 128 bits. We shouldn't be allowing anything
-        // through which is wider than the destination in the first place.
-        unsigned SrcSize = SrcTy.getSizeInBits();
-        if (SrcSize < 8 || !isPowerOf2_32(SrcSize))
-          return false;
+    // Special case for s1.
+    if (SrcTy == s1)
+      return true;
+
+    // Make sure we fit in a register otherwise. Don't bother checking that
+    // the source type is below 128 bits. We shouldn't be allowing anything
+    // through which is wider than the destination in the first place.
+    unsigned SrcSize = SrcTy.getSizeInBits();
+    if (SrcSize < 8 || !isPowerOf2_32(SrcSize))
+      return false;
 
-        return true;
-      });
+    return true;
+  };
+  getActionDefinitionsBuilder({G_ZEXT, G_ANYEXT}).legalIf(ExtLegalFunc);
+  getActionDefinitionsBuilder(G_SEXT)
+      .legalIf(ExtLegalFunc)
+      .clampScalar(0, s64, s64); // Just for s128, others are handled above.
 
   getActionDefinitionsBuilder(G_TRUNC).alwaysLegal();
 
Index: llvm/trunk/test/CodeGen/AArch64/GlobalISel/legalize-sext-128.mir
===================================================================
--- llvm/trunk/test/CodeGen/AArch64/GlobalISel/legalize-sext-128.mir
+++ llvm/trunk/test/CodeGen/AArch64/GlobalISel/legalize-sext-128.mir
@@ -0,0 +1,25 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -march=aarch64 -run-pass=legalizer -verify-machineinstrs %s -o - | FileCheck %s
+---
+name:            narrow_s128
+tracksRegLiveness: true
+body:             |
+  bb.1:
+    liveins: $x0, $x1
+
+    ; CHECK-LABEL: name: narrow_s128
+    ; CHECK: liveins: $x0, $x1
+    ; CHECK: [[COPY:%[0-9]+]]:_(s64) = COPY $x0
+    ; CHECK: [[COPY1:%[0-9]+]]:_(p0) = COPY $x1
+    ; CHECK: [[C:%[0-9]+]]:_(s64) = G_CONSTANT i64 63
+    ; CHECK: [[ASHR:%[0-9]+]]:_(s64) = G_ASHR [[COPY]], [[C]](s64)
+    ; CHECK: [[MV:%[0-9]+]]:_(s128) = G_MERGE_VALUES [[COPY]](s64), [[ASHR]](s64)
+    ; CHECK: G_STORE [[MV]](s128), [[COPY1]](p0) :: (store 16)
+    ; CHECK: RET_ReallyLR
+    %0:_(s64) = COPY $x0
+    %1:_(p0) = COPY $x1
+    %2:_(s128) = G_SEXT %0(s64)
+    G_STORE %2(s128), %1(p0) :: (store 16)
+    RET_ReallyLR
+
+...