diff --git a/llvm/include/llvm/CodeGen/TargetLowering.h b/llvm/include/llvm/CodeGen/TargetLowering.h
--- a/llvm/include/llvm/CodeGen/TargetLowering.h
+++ b/llvm/include/llvm/CodeGen/TargetLowering.h
@@ -530,6 +530,12 @@
     return false;
   }
 
+  /// \returns true if GlobalISel should over-align allocas to the preferred
+  /// alignment.
+  virtual bool shouldGISelOverAlignAllocas() const {
+    return false;
+  }
+
   /// Return true if the following transform is beneficial:
   /// fold (conv (load x)) -> (load (conv*)x)
   /// On architectures that don't natively support some vector loads
diff --git a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
--- a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
@@ -245,8 +245,36 @@
   // Always allocate at least one byte.
   Size = std::max<uint64_t>(Size, 1u);
 
+  auto &ST = MF->getSubtarget();
+  const TargetFrameLowering *TFI = ST.getFrameLowering();
+  const Align StackAlign = TFI->getStackAlign();
+  Align FIAlignment = AI.getAlign();
+  if (ST.getTargetLowering()->shouldGISelOverAlignAllocas()) {
+    // The following code mimics the alignment optimizations done in
+    // FunctionLoweringInfo for SelectionDAG.
+    Type *Ty = AI.getAllocatedType();
+    Align TyPrefAlign = MF->getDataLayout().getPrefTypeAlign(Ty);
+    // The "specified" alignment is the alignment written on the alloca,
+    // or the preferred alignment of the type if none is specified.
+    Align SpecifiedAlign = AI.getAlign();
+
+    // If the preferred alignment of the type is higher than the specified
+    // alignment of the alloca, promote the alignment, as long as it doesn't
+    // require realigning the stack.
+    //
+    // FIXME: Do we really want to second-guess the IR in isel?
+    Align Alignment =
+        std::max(std::min(TyPrefAlign, StackAlign), SpecifiedAlign);
+
+    // Static allocas can be folded into the initial stack frame
+    // adjustment. For targets that don't realign the stack, don't
+    // do this if there is an extra alignment requirement.
+    if (TFI->isStackRealignable() || (Alignment <= StackAlign))
+      FIAlignment = Alignment;
+  }
+
   int &FI = FrameIndices[&AI];
-  FI = MF->getFrameInfo().CreateStackObject(Size, AI.getAlign(), false, &AI);
+  FI = MF->getFrameInfo().CreateStackObject(Size, FIAlignment, false, &AI);
   return FI;
 }
 
diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.h b/llvm/lib/Target/AArch64/AArch64ISelLowering.h
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.h
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.h
@@ -825,6 +825,10 @@
 
   bool isAllActivePredicate(SDValue N) const;
 
+  bool shouldGISelOverAlignAllocas() const override {
+    return true;
+  }
+
 private:
   /// Keep a pointer to the AArch64Subtarget around so that we can
   /// make the right decision when generating code for different targets.
diff --git a/llvm/test/CodeGen/AArch64/GlobalISel/arm64-irtranslator.ll b/llvm/test/CodeGen/AArch64/GlobalISel/arm64-irtranslator.ll
--- a/llvm/test/CodeGen/AArch64/GlobalISel/arm64-irtranslator.ll
+++ b/llvm/test/CodeGen/AArch64/GlobalISel/arm64-irtranslator.ll
@@ -35,7 +35,7 @@
 ; CHECK-NEXT:   - { id: 0, name: ptr1, type: default, offset: 0, size: 8, alignment: 8,
 ; CHECK-NEXT:       stack-id: default, callee-saved-register: '', callee-saved-restored: true,
 ; CHECK-NEXT: debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
-; CHECK-NEXT:   - { id: 1, name: ptr2, type: default, offset: 0, size: 8, alignment: 1,
+; CHECK-NEXT:   - { id: 1, name: ptr2, type: default, offset: 0, size: 8, alignment: 8,
 ; CHECK-NEXT:       stack-id: default, callee-saved-register: '', callee-saved-restored: true,
 ; CHECK-NEXT:       debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
 ; CHECK-NEXT:   - { id: 2, name: ptr3, type: default, offset: 0, size: 128, alignment: 8,
@@ -54,6 +54,17 @@
   ret void
 }
 
+define void @alloca_increase_alignment()  {
+; The alignment of the frame index should be 8 here since it's the preferred alignment.
+; CHECK: stack:
+; CHECK:   - { id: 0, name: ai, type: default, offset: 0, size: 8, alignment: 8,
+; CHECK:       stack-id: default, callee-saved-register: '', callee-saved-restored: true,
+; CHECK:       debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+  %ai = alloca i8*, align 4
+  ret void
+}
+
+
 ; Tests for br.
 ; CHECK-LABEL: name: uncondbr
 ; CHECK: body: