Set load alignment on aggregate loads.

When optimizing a extractvalue(load), we generate a load from the
aggregate type.  This load didn't have alignment set and so would
get the alignment of the type.  This breaks when the type is packed
and so the alignment should be lower.

For example, loading { int, int } would give us alignment of 4, but
the original load from this type may have an alignment of 1 if packed.

Reviewed by David Majnemer

Differential revision: http://reviews.llvm.org/D17158

llvm-svn: 260587

llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp

@@ -552,7 +552,8 @@ static Instruction *unpackLoadToAggregate(InstCombiner &IC, LoadInst &LI) {
         ConstantInt::get(IdxType, i),
       };
       auto *Ptr = IC.Builder->CreateInBoundsGEP(ST, Addr, makeArrayRef(Indices), EltName);
-      auto *L = IC.Builder->CreateLoad(ST->getTypeAtIndex(i), Ptr, LoadName);
+      auto *L = IC.Builder->CreateAlignedLoad(Ptr, LI.getAlignment(),
+                                              LoadName);
       V = IC.Builder->CreateInsertValue(V, L, i);
     }
 

llvm/test/Transforms/InstCombine/unpack-fca.ll

@@ -136,3 +136,18 @@ define %B @structB(%B* %b.ptr) {
   %1 = load %B, %B* %b.ptr, align 8
   ret %B %1
 }
+
+%struct.S = type <{ i8, %struct.T }>
+%struct.T = type { i32, i32 }
+
+; Make sure that we do not increase alignment of packed struct element
+define i32 @packed_alignment(%struct.S* dereferenceable(9) %s) {
+; CHECK-LABEL: packed_alignment
+; CHECK-NEXT: %tv.elt1 = getelementptr inbounds %struct.S, %struct.S* %s, i64 0, i32 1, i32 1
+; CHECK-NEXT: %tv.unpack2 = load i32, i32* %tv.elt1, align 1
+; CHECK-NEXT: ret i32 %tv.unpack2
+  %t = getelementptr inbounds %struct.S, %struct.S* %s, i32 0, i32 1
+  %tv = load %struct.T, %struct.T* %t, align 1
+  %v = extractvalue %struct.T %tv, 1
+  ret i32 %v
+}