Index: llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp
+++ llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp
@@ -207,6 +207,7 @@
   if (OptForSize) {
     UP.Threshold = UP.OptSizeThreshold;
     UP.PartialThreshold = UP.PartialOptSizeThreshold;
+    UP.MaxPercentThresholdBoost = 100;
   }
 
   // Apply any user values specified by cl::opt
@@ -993,6 +994,7 @@
   if (OnlyWhenForced && !(TM & TM_Enable))
     return LoopUnrollResult::Unmodified;
 
+  bool OptForSize = L->getHeader()->getParent()->hasOptSize();
   unsigned NumInlineCandidates;
   bool NotDuplicatable;
   bool Convergent;
@@ -1000,8 +1002,11 @@
       L, SE, TTI, BFI, PSI, OptLevel, ProvidedThreshold, ProvidedCount,
       ProvidedAllowPartial, ProvidedRuntime, ProvidedUpperBound,
       ProvidedAllowPeeling);
-  // Exit early if unrolling is disabled.
-  if (UP.Threshold == 0 && (!UP.Partial || UP.PartialThreshold == 0))
+
+  // Exit early if unrolling is disabled. For OptForSize, we pick the loop size
+  // as threshold later on.
+  if (UP.Threshold == 0 && (!UP.Partial || UP.PartialThreshold == 0) &&
+      !OptForSize)
     return LoopUnrollResult::Unmodified;
 
   SmallPtrSet<const Value *, 32> EphValues;
@@ -1016,6 +1021,12 @@
                       << " instructions.\n");
     return LoopUnrollResult::Unmodified;
   }
+
+  // When optimizing for size, use LoopSize as threshold, to (fully) unroll
+  // loops, if it does not increase code size.
+  if (OptForSize)
+    UP.Threshold = std::max(UP.Threshold, LoopSize);
+
   if (NumInlineCandidates != 0) {
     LLVM_DEBUG(dbgs() << "  Not unrolling loop with inlinable calls.\n");
     return LoopUnrollResult::Unmodified;
Index: llvm/trunk/test/Transforms/LoopUnroll/AArch64/unroll-optsize.ll
===================================================================
--- llvm/trunk/test/Transforms/LoopUnroll/AArch64/unroll-optsize.ll
+++ llvm/trunk/test/Transforms/LoopUnroll/AArch64/unroll-optsize.ll
@@ -0,0 +1,171 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -loop-unroll -mtriple=arm64-apple-iphoneos -S %s | FileCheck %s
+
+; Check we unroll even with optsize, if the result is smaller, either because
+; we have single iteration loops or bodies with constant folding opportunities
+; after fully unrolling.
+
+declare i32 @get()
+
+define void @fully_unrolled_single_iteration(i32* %src) #0 {
+; CHECK-LABEL: @fully_unrolled_single_iteration(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[ARR:%.*]] = alloca [4 x i32], align 4
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[V:%.*]] = load i32, i32* [[SRC:%.*]]
+; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [4 x i32], [4 x i32]* [[ARR]], i64 0, i64 0
+; CHECK-NEXT:    store i32 [[V]], i32* [[ARRAYIDX]], align 4
+; CHECK-NEXT:    [[PTR:%.*]] = bitcast [4 x i32]* [[ARR]] to i32*
+; CHECK-NEXT:    call void @use(i32* nonnull [[PTR]])
+; CHECK-NEXT:    ret void
+;
+entry:
+  %arr = alloca [4 x i32], align 4
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ %indvars.iv.next, %for.body ], [ 0, %entry ]
+  %src.idx = getelementptr inbounds i32, i32* %src, i64 %indvars.iv
+  %v = load i32, i32* %src.idx
+  %arrayidx = getelementptr inbounds [4 x i32], [4 x i32]* %arr, i64 0, i64 %indvars.iv
+  store i32 %v, i32* %arrayidx, align 4
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp eq i64 %indvars.iv.next, 1
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body
+
+for.cond.cleanup:                                 ; preds = %for.cond
+  %ptr = bitcast [4 x i32]* %arr to i32*
+  call void @use(i32* nonnull %ptr) #4
+  ret void
+}
+
+
+define void @fully_unrolled_smaller() #0 {
+; CHECK-LABEL: @fully_unrolled_smaller(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[ARR:%.*]] = alloca [4 x i32], align 4
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [4 x i32], [4 x i32]* [[ARR]], i64 0, i64 0
+; CHECK-NEXT:    store i32 16, i32* [[ARRAYIDX]], align 4
+; CHECK-NEXT:    [[ARRAYIDX_1:%.*]] = getelementptr inbounds [4 x i32], [4 x i32]* [[ARR]], i64 0, i64 1
+; CHECK-NEXT:    store i32 4104, i32* [[ARRAYIDX_1]], align 4
+; CHECK-NEXT:    [[ARRAYIDX_2:%.*]] = getelementptr inbounds [4 x i32], [4 x i32]* [[ARR]], i64 0, i64 2
+; CHECK-NEXT:    store i32 1048592, i32* [[ARRAYIDX_2]], align 4
+; CHECK-NEXT:    [[ARRAYIDX_3:%.*]] = getelementptr inbounds [4 x i32], [4 x i32]* [[ARR]], i64 0, i64 3
+; CHECK-NEXT:    store i32 268435480, i32* [[ARRAYIDX_3]], align 4
+; CHECK-NEXT:    [[PTR:%.*]] = bitcast [4 x i32]* [[ARR]] to i32*
+; CHECK-NEXT:    call void @use(i32* nonnull [[PTR]])
+; CHECK-NEXT:    ret void
+;
+entry:
+  %arr = alloca [4 x i32], align 4
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ %indvars.iv.next, %for.body ], [ 0, %entry ]
+  %indvars.iv.tr = trunc i64 %indvars.iv to i32
+  %shl.0 = shl i32 %indvars.iv.tr, 3
+  %shl.1 = shl i32 16, %shl.0
+  %or = or i32 %shl.1, %shl.0
+  %arrayidx = getelementptr inbounds [4 x i32], [4 x i32]* %arr, i64 0, i64 %indvars.iv
+  store i32 %or, i32* %arrayidx, align 4
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp eq i64 %indvars.iv, 3
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body
+
+for.cond.cleanup:                                 ; preds = %for.cond
+  %ptr = bitcast [4 x i32]* %arr to i32*
+  call void @use(i32* nonnull %ptr) #4
+  ret void
+}
+
+define void @fully_unrolled_smaller_Oz() #1 {
+; CHECK-LABEL: @fully_unrolled_smaller_Oz(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[ARR:%.*]] = alloca [4 x i32], align 4
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [4 x i32], [4 x i32]* [[ARR]], i64 0, i64 0
+; CHECK-NEXT:    store i32 16, i32* [[ARRAYIDX]], align 4
+; CHECK-NEXT:    [[ARRAYIDX_1:%.*]] = getelementptr inbounds [4 x i32], [4 x i32]* [[ARR]], i64 0, i64 1
+; CHECK-NEXT:    store i32 4104, i32* [[ARRAYIDX_1]], align 4
+; CHECK-NEXT:    [[ARRAYIDX_2:%.*]] = getelementptr inbounds [4 x i32], [4 x i32]* [[ARR]], i64 0, i64 2
+; CHECK-NEXT:    store i32 1048592, i32* [[ARRAYIDX_2]], align 4
+; CHECK-NEXT:    [[ARRAYIDX_3:%.*]] = getelementptr inbounds [4 x i32], [4 x i32]* [[ARR]], i64 0, i64 3
+; CHECK-NEXT:    store i32 268435480, i32* [[ARRAYIDX_3]], align 4
+; CHECK-NEXT:    [[PTR:%.*]] = bitcast [4 x i32]* [[ARR]] to i32*
+; CHECK-NEXT:    call void @use(i32* nonnull [[PTR]])
+; CHECK-NEXT:    ret void
+;
+entry:
+  %arr = alloca [4 x i32], align 4
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ %indvars.iv.next, %for.body ], [ 0, %entry ]
+  %indvars.iv.tr = trunc i64 %indvars.iv to i32
+  %shl.0 = shl i32 %indvars.iv.tr, 3
+  %shl.1 = shl i32 16, %shl.0
+  %or = or i32 %shl.1, %shl.0
+  %arrayidx = getelementptr inbounds [4 x i32], [4 x i32]* %arr, i64 0, i64 %indvars.iv
+  store i32 %or, i32* %arrayidx, align 4
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp eq i64 %indvars.iv, 3
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body
+
+for.cond.cleanup:                                 ; preds = %for.cond
+  %ptr = bitcast [4 x i32]* %arr to i32*
+  call void @use(i32* nonnull %ptr) #4
+  ret void
+}
+
+
+define void @fully_unrolled_bigger() #0 {
+; CHECK-LABEL: @fully_unrolled_bigger(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[ARR:%.*]] = alloca [4 x i32], align 4
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[INDVARS_IV:%.*]] = phi i64 [ [[INDVARS_IV_NEXT:%.*]], [[FOR_BODY]] ], [ 0, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[INDVARS_IV_TR:%.*]] = trunc i64 [[INDVARS_IV]] to i32
+; CHECK-NEXT:    [[SHL_0:%.*]] = shl i32 [[INDVARS_IV_TR]], 3
+; CHECK-NEXT:    [[SHL_1:%.*]] = shl i32 16, [[SHL_0]]
+; CHECK-NEXT:    [[OR:%.*]] = or i32 [[SHL_1]], [[SHL_0]]
+; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [4 x i32], [4 x i32]* [[ARR]], i64 0, i64 [[INDVARS_IV]]
+; CHECK-NEXT:    store i32 [[OR]], i32* [[ARRAYIDX]], align 4
+; CHECK-NEXT:    [[INDVARS_IV_NEXT]] = add nuw nsw i64 [[INDVARS_IV]], 1
+; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp eq i64 [[INDVARS_IV]], 6
+; CHECK-NEXT:    br i1 [[EXITCOND]], label [[FOR_COND_CLEANUP:%.*]], label [[FOR_BODY]]
+; CHECK:       for.cond.cleanup:
+; CHECK-NEXT:    [[PTR:%.*]] = bitcast [4 x i32]* [[ARR]] to i32*
+; CHECK-NEXT:    call void @use(i32* nonnull [[PTR]])
+; CHECK-NEXT:    ret void
+;
+entry:
+  %arr = alloca [4 x i32], align 4
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ %indvars.iv.next, %for.body ], [ 0, %entry ]
+  %indvars.iv.tr = trunc i64 %indvars.iv to i32
+  %shl.0 = shl i32 %indvars.iv.tr, 3
+  %shl.1 = shl i32 16, %shl.0
+  %or = or i32 %shl.1, %shl.0
+  %arrayidx = getelementptr inbounds [4 x i32], [4 x i32]* %arr, i64 0, i64 %indvars.iv
+  store i32 %or, i32* %arrayidx, align 4
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp eq i64 %indvars.iv, 6
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body
+
+for.cond.cleanup:                                 ; preds = %for.cond
+  %ptr = bitcast [4 x i32]* %arr to i32*
+  call void @use(i32* nonnull %ptr) #4
+  ret void
+}
+
+declare void @use(i32*)
+
+attributes #0 = { optsize }
+attributes #1 = { minsize optsize }