Index: lib/CodeGen/SelectionDAG/FastISel.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/FastISel.cpp
+++ lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -497,7 +497,7 @@
        OI != E; ++OI) {
     const Value *Idx = *OI;
     if (auto *StTy = dyn_cast<StructType>(Ty)) {
-      unsigned Field = cast<ConstantInt>(Idx)->getZExtValue();
+      uint64_t Field = cast<ConstantInt>(Idx)->getZExtValue();
       if (Field) {
         // N = N + Offset
         TotalOffs += DL.getStructLayout(StTy)->getElementOffset(Field);
@@ -518,8 +518,8 @@
         if (CI->isZero())
           continue;
         // N = N + Offset
-        TotalOffs +=
-            DL.getTypeAllocSize(Ty) * cast<ConstantInt>(CI)->getSExtValue();
+        uint64_t IdxN = CI->getValue().sextOrTrunc(64).getSExtValue();
+        TotalOffs += DL.getTypeAllocSize(Ty) * IdxN;
         if (TotalOffs >= MaxOffs) {
           N = fastEmit_ri_(VT, ISD::ADD, N, NIsKill, TotalOffs, VT);
           if (!N) // Unhandled operand. Halt "fast" selection and bail.
Index: lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -3448,30 +3448,21 @@
       Ty = StTy->getElementType(Field);
     } else {
       Ty = cast<SequentialType>(Ty)->getElementType();
+      MVT PtrTy = DAG.getTargetLoweringInfo().getPointerTy(AS);
+      unsigned PtrSize = PtrTy.getSizeInBits();
+      auto ElementSize = APInt{PtrSize, DL->getTypeAllocSize(Ty)};
 
       // If this is a constant subscript, handle it quickly.
-      const TargetLowering &TLI = DAG.getTargetLoweringInfo();
-      if (const ConstantInt *CI = dyn_cast<ConstantInt>(Idx)) {
-        if (CI->isZero()) continue;
-        uint64_t Offs =
-            DL->getTypeAllocSize(Ty)*cast<ConstantInt>(CI)->getSExtValue();
-        SDValue OffsVal;
-        EVT PTy = TLI.getPointerTy(AS);
-        unsigned PtrBits = PTy.getSizeInBits();
-        if (PtrBits < 64)
-          OffsVal = DAG.getNode(ISD::TRUNCATE, getCurSDLoc(), PTy,
-                                DAG.getConstant(Offs, MVT::i64));
-        else
-          OffsVal = DAG.getConstant(Offs, PTy);
-
-        N = DAG.getNode(ISD::ADD, getCurSDLoc(), N.getValueType(), N,
-                        OffsVal);
+      if (auto CI = dyn_cast<ConstantInt>(Idx)) {
+        if (CI->isZero())
+          continue;
+        APInt Offs = ElementSize * CI->getValue().sextOrTrunc(PtrSize);
+        SDValue OffsVal = DAG.getConstant(Offs, PtrTy);
+        N = DAG.getNode(ISD::ADD, getCurSDLoc(), N.getValueType(), N, OffsVal);
         continue;
       }
 
       // N = N + Idx * ElementSize;
-      APInt ElementSize =
-          APInt(TLI.getPointerSizeInBits(AS), DL->getTypeAllocSize(Ty));
       SDValue IdxN = getValue(Idx);
 
       // If the index is smaller or larger than intptr_t, truncate or extend
Index: test/CodeGen/X86/getelementptr.ll
===================================================================
--- /dev/null
+++ test/CodeGen/X86/getelementptr.ll
@@ -0,0 +1,81 @@
+; RUN: llc < %s -O0 -march=x86
+; RUN: llc < %s -O0 -march=x86-64
+; RUN: llc < %s -O2 -march=x86
+; RUN: llc < %s -O2 -march=x86-64
+
+
+; Test big index trunc to pointer size:
+
+define i8* @test_trunc65(i8* %ptr) nounwind {
+; CHECK-LABEL: test_trunc65
+; CHECK: 3
+  %d = getelementptr i8, i8* %ptr, i65 18446744073709551619 ; 2^64 + 3
+  ret i8* %d
+}
+
+define i8* @test_trunc128(i8* %ptr) nounwind {
+; CHECK-LABEL: test_trunc128
+; CHECK: 5
+  %d = getelementptr i8, i8* %ptr, i128 18446744073709551621 ; 2^64 + 5
+  ret i8* %d
+}
+
+define i8* @test_trunc160(i8* %ptr) nounwind {
+; CHECK-LABEL: test_trunc160
+; CHECK: 8
+  %d = getelementptr i8, i8* %ptr, i160 18446744073709551624 ; 2^64 + 8
+  ret i8* %d
+}
+
+define i8* @test_trunc256(i8* %ptr) nounwind {
+; CHECK-LABEL: test_trunc256
+; CHECK: 13
+  %d = getelementptr i8, i8* %ptr, i256 18446744073709551629 ; 2^64 + 13
+  ret i8* %d
+}
+
+define i8* @test_trunc2048(i8* %ptr) nounwind {
+; CHECK-LABEL: test_trunc2048
+; CHECK: 21
+  %d = getelementptr i8, i8* %ptr, i2048 18446744073709551637 ; 2^64 + 21
+  ret i8* %d
+}
+
+
+; Test small index sext to pointer size
+
+define i8* @test_sext3(i8* %ptr) nounwind {
+; CHECK-LABEL: test_sext3
+; CHECK: -3
+  %d = getelementptr i8, i8* %ptr, i3 -3
+  ret i8* %d
+}
+
+define i8* @test_sext5(i8* %ptr) nounwind {
+; CHECK-LABEL: test_sext5
+; CHECK: -5
+  %d = getelementptr i8, i8* %ptr, i5 -5
+  ret i8* %d
+}
+
+define i8* @test_sext8(i8* %ptr) nounwind {
+; CHECK-LABEL: test_sext8
+; CHECK: -8
+  %d = getelementptr i8, i8* %ptr, i8 -8
+  ret i8* %d
+}
+
+define i8* @test_sext13(i8* %ptr) nounwind {
+; CHECK-LABEL: test_sext13
+; CHECK: -13
+  %d = getelementptr i8, i8* %ptr, i8 -13
+  ret i8* %d
+}
+
+define i8* @test_sext16(i8* %ptr) nounwind {
+; CHECK-LABEL: test_sext16
+; CHECK: -21
+  %d = getelementptr i8, i8* %ptr, i8 -21
+  ret i8* %d
+}
+