Index: lib/Transforms/Scalar/SROA.cpp
===================================================================
--- lib/Transforms/Scalar/SROA.cpp
+++ lib/Transforms/Scalar/SROA.cpp
@@ -30,6 +30,7 @@
 #include "llvm/ADT/PointerIntPair.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SetVector.h"
+#include "llvm/ADT/SmallBitVector.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
@@ -4047,21 +4048,31 @@
   // First try to pre-split loads and stores.
   Changed |= presplitLoadsAndStores(AI, AS);
 
-  // Now that we have identified any pre-splitting opportunities, mark any
-  // splittable (non-whole-alloca) loads and stores as unsplittable. If we fail
-  // to split these during pre-splitting, we want to force them to be
-  // rewritten into a partition.
+  // Now that we have identified any pre-splitting opportunities,
+  // mark loads and stores unsplittable except for the following case.
+  // We leave a slice splittable if all other slices are disjoint or fully
+  // included in the slice, such as whole-alloca loads and stores.
+  // If we fail to split these during pre-splitting, we want to force them
+  // to be rewritten into a partition.
   bool IsSorted = true;
+
+  // If a byte boundary is included in any load or store, a slice starting or
+  // ending at the boundary is not splittable.
+  unsigned AllocaSize = DL.getTypeAllocSize(AI.getAllocatedType());
+  SmallBitVector SplittableOffset(AllocaSize+1, true);
+  for (Slice &S : AS)
+    for (unsigned O = S.beginOffset() + 1; O < S.endOffset() && O < AllocaSize;
+         O++)
+      SplittableOffset.reset(O);
+
   for (Slice &S : AS) {
     if (!S.isSplittable())
       continue;
-    // FIXME: We currently leave whole-alloca splittable loads and stores. This
-    // used to be the only splittable loads and stores and we need to be
-    // confident that the above handling of splittable loads and stores is
-    // completely sufficient before we forcibly disable the remaining handling.
-    if (S.beginOffset() == 0 &&
-        S.endOffset() >= DL.getTypeAllocSize(AI.getAllocatedType()))
+
+    if ((S.beginOffset() > AllocaSize || SplittableOffset[S.beginOffset()]) &&
+        (S.endOffset() > AllocaSize || SplittableOffset[S.endOffset()]))
       continue;
+
     if (isa<LoadInst>(S.getUse()->getUser()) ||
         isa<StoreInst>(S.getUse()->getUser())) {
       S.makeUnsplittable();
Index: test/DebugInfo/X86/sroasplit-2.ll
===================================================================
--- test/DebugInfo/X86/sroasplit-2.ll
+++ test/DebugInfo/X86/sroasplit-2.ll
@@ -21,7 +21,8 @@
 
 ; Verify that SROA creates a variable piece when splitting i1.
 ; CHECK:  call void @llvm.dbg.value(metadata i64 %outer.coerce0, metadata ![[O:[0-9]+]], metadata !DIExpression(DW_OP_LLVM_fragment, 0, 64)),
-; CHECK:  call void @llvm.dbg.value(metadata i64 %outer.coerce1, metadata ![[O]], metadata !DIExpression(DW_OP_LLVM_fragment, 64, 64)),
+; CHECK:  call void @llvm.dbg.value(metadata i32 {{.*}}, metadata ![[O]], metadata !DIExpression(DW_OP_LLVM_fragment, 64, 32)),
+; CHECK:  call void @llvm.dbg.value(metadata i32 {{.*}}, metadata ![[O]], metadata !DIExpression(DW_OP_LLVM_fragment, 96, 32)),
 ; CHECK:  call void @llvm.dbg.value({{.*}}, metadata ![[I1:[0-9]+]], metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32)),
 ; CHECK-DAG: ![[O]] = !DILocalVariable(name: "outer",{{.*}} line: 10
 ; CHECK-DAG: ![[I1]] = !DILocalVariable(name: "i1",{{.*}} line: 11
Index: test/Transforms/SROA/basictest.ll
===================================================================
--- test/Transforms/SROA/basictest.ll
+++ test/Transforms/SROA/basictest.ll
@@ -1615,13 +1615,13 @@
 ; Ensure we can handle a very interesting case where there is an integer-based
 ; rewrite of the uses of the alloca, but where one of the integers in that is
 ; a sub-integer that requires extraction *and* extends past the end of the
-; alloca. In this case, we should extract the i8 and then zext it to i16.
+; alloca. SROA can split the alloca to avoid shift or trunc.
 ;
 ; CHECK-LABEL: @PR24463(
 ; CHECK-NOT: alloca
-; CHECK: %[[SHIFT:.*]] = lshr i16 0, 8
-; CHECK: %[[TRUNC:.*]] = trunc i16 %[[SHIFT]] to i8
-; CHECK: %[[ZEXT:.*]] = zext i8 %[[TRUNC]] to i16
+; CHECK-NOT: trunc
+; CHECK-NOT: lshr
+; CHECK: %[[ZEXT:.*]] = zext i8 {{.*}} to i16
 ; CHECK: ret i16 %[[ZEXT]]
 entry:
   %alloca = alloca [3 x i8]
@@ -1695,3 +1695,28 @@
   call void @llvm.lifetime.end.p0i8(i64 2, i8* %0)
   ret void
 }
+
+define void @test28(i64 %v) #0 {
+; SROA should split the first i64 store to avoid additional and/or instructions
+; when storing into i32 fields
+
+; CHECK-LABEL: @test28(
+; CHECK-NOT: alloca
+; CHECK-NOT: and
+; CHECK-NOT: or
+; CHECK:      %[[shift:.*]] = lshr i64 %v, 32
+; CHECK-NEXT: %{{.*}} = trunc i64 %[[shift]] to i32
+; CHECK-NEXT: ret void
+
+entry:
+  %t = alloca { i64, i32, i32 }
+
+  %b = getelementptr { i64, i32, i32 }, { i64, i32, i32 }* %t, i32 0, i32 1
+  %0 = bitcast i32* %b to i64*
+  store i64 %v, i64* %0
+
+  %1 = load i32, i32* %b
+  %c = getelementptr { i64, i32, i32 }, { i64, i32, i32 }* %t, i32 0, i32 2
+  store i32 %1, i32* %c
+  ret void
+}
Index: test/Transforms/SROA/big-endian.ll
===================================================================
--- test/Transforms/SROA/big-endian.ll
+++ test/Transforms/SROA/big-endian.ll
@@ -83,19 +83,34 @@
   store i16 1, i16* %a0i16ptr
 
   store i8 1, i8* %a2ptr
-; CHECK:      %[[mask1:.*]] = and i40 undef, 4294967295
-; CHECK-NEXT: %[[insert1:.*]] = or i40 %[[mask1]], 4294967296
 
   %a3i24ptr = bitcast i8* %a3ptr to i24*
   store i24 1, i24* %a3i24ptr
-; CHECK-NEXT: %[[mask2:.*]] = and i40 %[[insert1]], -4294967041
-; CHECK-NEXT: %[[insert2:.*]] = or i40 %[[mask2]], 256
 
   %a2i40ptr = bitcast i8* %a2ptr to i40*
   store i40 1, i40* %a2i40ptr
-; CHECK-NEXT: %[[ext3:.*]] = zext i40 1 to i56
-; CHECK-NEXT: %[[mask3:.*]] = and i56 undef, -1099511627776
-; CHECK-NEXT: %[[insert3:.*]] = or i56 %[[mask3]], %[[ext3]]
+
+; the alloca is splitted into multiple slices
+; Here, i8 1 is for %a[6]
+; CHECK: %[[ext1:.*]] = zext i8 1 to i40
+; CHECK-NEXT: %[[mask1:.*]] = and i40 undef, -256
+; CHECK-NEXT: %[[insert1:.*]] = or i40 %[[mask1]], %[[ext1]]
+
+; Here, i24 0 is for %a[3] to %a[5]
+; CHECK-NEXT: %[[ext2:.*]] = zext i24 0 to i40
+; CHECK-NEXT: %[[shift2:.*]] = shl i40 %[[ext2]], 8
+; CHECK-NEXT: %[[mask2:.*]] = and i40 %[[insert1]], -4294967041
+; CHECK-NEXT: %[[insert2:.*]] = or i40 %[[mask2]], %[[shift2]]
+
+; Here, i8 0 is for %a[2]
+; CHECK-NEXT: %[[ext3:.*]] = zext i8 0 to i40
+; CHECK-NEXT: %[[shift3:.*]] = shl i40 %[[ext3]], 32
+; CHECK-NEXT: %[[mask3:.*]] = and i40 %[[insert2]], 4294967295
+; CHECK-NEXT: %[[insert3:.*]] = or i40 %[[mask3]], %[[shift3]]
+
+; CHECK-NEXT: %[[ext4:.*]] = zext i40 %[[insert3]] to i56
+; CHECK-NEXT: %[[mask4:.*]] = and i56 undef, -1099511627776
+; CHECK-NEXT: %[[insert4:.*]] = or i56 %[[mask4]], %[[ext4]]
 
 ; CHECK-NOT: store
 ; CHECK-NOT: load
@@ -104,11 +119,12 @@
   %ai = load i56, i56* %aiptr
   %ret = zext i56 %ai to i64
   ret i64 %ret
-; CHECK-NEXT: %[[ext4:.*]] = zext i16 1 to i56
-; CHECK-NEXT: %[[shift4:.*]] = shl i56 %[[ext4]], 40
-; CHECK-NEXT: %[[mask4:.*]] = and i56 %[[insert3]], 1099511627775
-; CHECK-NEXT: %[[insert4:.*]] = or i56 %[[mask4]], %[[shift4]]
-; CHECK-NEXT: %[[ret:.*]] = zext i56 %[[insert4]] to i64
+; Here, i16 1 is for %a[0] to %a[1]
+; CHECK-NEXT: %[[ext5:.*]] = zext i16 1 to i56
+; CHECK-NEXT: %[[shift5:.*]] = shl i56 %[[ext5]], 40
+; CHECK-NEXT: %[[mask5:.*]] = and i56 %[[insert4]], 1099511627775
+; CHECK-NEXT: %[[insert5:.*]] = or i56 %[[mask5]], %[[shift5]]
+; CHECK-NEXT: %[[ret:.*]] = zext i56 %[[insert5]] to i64
 ; CHECK-NEXT: ret i64 %[[ret]]
 }