Add support for one element array store in InstCombine

[deadalnix]

Summary

As per title. This is a continuation of the work toward getting FCA store and load transformed in InstCombine to something the optimiser understands.

[majnemer]

LGTM

[deadalnix]

Rebase, can somebody with merge right can get that in ?

llvm/trunk/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp

if (auto *ST = dyn_cast<StructType>(T)) {
     // If the struct only have one element, we unpack.
     if (ST->getNumElements() == 1) {
       V = IC.Builder->CreateExtractValue(V, 0);
       combineStoreToNewValue(IC, SI, V);
       return true;
     }
   }
 
+  if (auto *AT = dyn_cast<ArrayType>(T)) {
+    // If the array only have one element, we unpack.
+    if (AT->getNumElements() == 1) {
+      V = IC.Builder->CreateExtractValue(V, 0);
+      combineStoreToNewValue(IC, SI, V);
+      return true;
+    }
+  }
+
   return false;
 }
 
 /// equivalentAddressValues - Test if A and B will obviously have the same
 /// value. This includes recognizing that %t0 and %t1 will have the same
 /// value in code like this:
 ///   %t0 = getelementptr \@a, 0, 3
 ///   store i32 0, i32* %t0

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

body:
   %0 = tail call i8* @allocmemory(i64 32)
   %1 = bitcast i8* %0 to { %A }*
 ; CHECK-LABEL: storeStructOfA
 ; CHECK: store %A__vtbl* @A__vtblZ
   store { %A } { %A { %A__vtbl* @A__vtblZ } }, { %A }* %1, align 8
   ret void
 }
 
+define void @storeArrayOfA() {
+body:
+  %0 = tail call i8* @allocmemory(i64 32)
+  %1 = bitcast i8* %0 to [1 x %A]*
+; CHECK-LABEL: storeStructOfA
+; CHECK: store %A__vtbl* @A__vtblZ
+  store [1 x %A] [%A { %A__vtbl* @A__vtblZ }], [1 x %A]* %1, align 8
+  ret void
+}
+
+define void @storeStructOfArrayOfA() {
+body:
+  %0 = tail call i8* @allocmemory(i64 32)
+  %1 = bitcast i8* %0 to { [1 x %A] }*
+; CHECK-LABEL: storeStructOfArrayOfA
+; CHECK: store %A__vtbl* @A__vtblZ
+  store { [1 x %A] } { [1 x %A] [%A { %A__vtbl* @A__vtblZ }] }, { [1 x %A] }* %1, align 8
+  ret void
+}
+
 define %A @loadA() {
 body:
   %0 = tail call i8* @allocmemory(i64 32)
   %1 = bitcast i8* %0 to %A*
 ; CHECK-LABEL: loadA
 ; CHECK: load %A__vtbl*,
 ; CHECK: insertvalue %A undef, %A__vtbl* {{.*}}, 0
   %2 = load %A, %A* %1, align 8