Index: llvm/trunk/lib/Transforms/Scalar/SROA.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/SROA.cpp
+++ llvm/trunk/lib/Transforms/Scalar/SROA.cpp
@@ -2422,6 +2422,7 @@
     if (!VecTy && !IntTy &&
         (BeginOffset > NewAllocaBeginOffset ||
          EndOffset < NewAllocaEndOffset ||
+         SliceSize != DL.getTypeStoreSize(AllocaTy) ||
          !AllocaTy->isSingleValueType() ||
          !DL.isLegalInteger(DL.getTypeSizeInBits(ScalarTy)) ||
          DL.getTypeSizeInBits(ScalarTy)%8 != 0)) {
@@ -2544,10 +2545,11 @@
 
     // If this doesn't map cleanly onto the alloca type, and that type isn't
     // a single value type, just emit a memcpy.
-    bool EmitMemCpy
-      = !VecTy && !IntTy && (BeginOffset > NewAllocaBeginOffset ||
-                             EndOffset < NewAllocaEndOffset ||
-                             !NewAI.getAllocatedType()->isSingleValueType());
+    bool EmitMemCpy =
+        !VecTy && !IntTy &&
+        (BeginOffset > NewAllocaBeginOffset || EndOffset < NewAllocaEndOffset ||
+         SliceSize != DL.getTypeStoreSize(NewAI.getAllocatedType()) ||
+         !NewAI.getAllocatedType()->isSingleValueType());
 
     // If we're just going to emit a memcpy, the alloca hasn't changed, and the
     // size hasn't been shrunk based on analysis of the viable range, this is
Index: llvm/trunk/test/Transforms/SROA/slice-width.ll
===================================================================
--- llvm/trunk/test/Transforms/SROA/slice-width.ll
+++ llvm/trunk/test/Transforms/SROA/slice-width.ll
@@ -1,7 +1,8 @@
 ; RUN: opt < %s -sroa -S | FileCheck %s
-target datalayout = "e-p:64:64:64-p1:16:16:16-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-n8:16:32:64"
+target datalayout = "e-p:64:64:64-p1:16:16:16-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:64:64-f80:128-v64:64:64-v128:128:128-a0:0:64-n8:16:32:64"
 
 declare void @llvm.memcpy.p0i8.p0i8.i32(i8* nocapture, i8* nocapture, i32, i32, i1) nounwind
+declare void @llvm.memset.p0i8.i32(i8* nocapture, i8, i32, i32, i1) nounwind
 
 define void @no_split_on_non_byte_width(i32) {
 ; This tests that allocas are not split into slices that are not byte width multiple
@@ -23,3 +24,49 @@
   %t1 = load i1* %p1
   ret void
 }
+
+; PR18726: Check that we use memcpy and memset to fill out padding when we have
+; a slice with a simple single type whose store size is smaller than the slice
+; size.
+
+%union.Foo = type { x86_fp80, i64, i64 }
+
+@foo_copy_source = external constant %union.Foo
+@i64_sink = global i64 0
+
+define void @memcpy_fp80_padding() {
+  %x = alloca %union.Foo
+
+  ; Copy from a global.
+  %x_i8 = bitcast %union.Foo* %x to i8*
+  call void @llvm.memcpy.p0i8.p0i8.i32(i8* %x_i8, i8* bitcast (%union.Foo* @foo_copy_source to i8*), i32 32, i32 16, i1 false)
+
+  ; Access a slice of the alloca to trigger SROA.
+  %mid_p = getelementptr %union.Foo* %x, i32 0, i32 1
+  %elt = load i64* %mid_p
+  store i64 %elt, i64* @i64_sink
+  ret void
+}
+; CHECK-LABEL: define void @memcpy_fp80_padding
+; CHECK: alloca x86_fp80
+; CHECK: call void @llvm.memcpy.p0i8.p0i8.i32
+; CHECK: load i64* getelementptr inbounds (%union.Foo* @foo_copy_source, i64 0, i32 1)
+; CHECK: load i64* getelementptr inbounds (%union.Foo* @foo_copy_source, i64 0, i32 2)
+
+define void @memset_fp80_padding() {
+  %x = alloca %union.Foo
+
+  ; Set to all ones.
+  %x_i8 = bitcast %union.Foo* %x to i8*
+  call void @llvm.memset.p0i8.i32(i8* %x_i8, i8 -1, i32 32, i32 16, i1 false)
+
+  ; Access a slice of the alloca to trigger SROA.
+  %mid_p = getelementptr %union.Foo* %x, i32 0, i32 1
+  %elt = load i64* %mid_p
+  store i64 %elt, i64* @i64_sink
+  ret void
+}
+; CHECK-LABEL: define void @memset_fp80_padding
+; CHECK: alloca x86_fp80
+; CHECK: call void @llvm.memset.p0i8.i32(i8* %{{.*}}, i8 -1, i32 16, i32 16, i1 false)
+; CHECK: store i64 -1, i64* @i64_sink