Return Undef from isBytewiseValue for empty arrays or structs

[efriedma]

llvm/lib/Analysis/ValueTracking.cpp

3187

Does this work recursively? For example, [5 x [0 x i32]]. It might be easier to just check the size of the type.

Actually, we should probably constant-fold [0 x i8] zeroinitializer etc., to [0 x i8] undef, since there isn't any actual data anyway. That would make this check unnecessary.

[vitalybuka]

Use size instead of getNumElements

[vitalybuka]

llvm/lib/Analysis/ValueTracking.cpp

3187

Yep, recursive did not work and size looks simpler.

[eugenis]

This is not a lot of code, so LGTM, but I agree that constant folding would be even better.

[efriedma]

LGTM

llvm/lib/Analysis/ValueTracking.cpp

Value *llvm::isBytewiseValue(Value *V, const DataLayout &DL) {
 
   LLVMContext &Ctx = V->getContext();
 
   // Undef don't care.
   auto *UndefInt8 = UndefValue::get(Type::getInt8Ty(Ctx));
   if (isa<UndefValue>(V))
     return UndefInt8;
 
+  const uint64_t Size = DL.getTypeStoreSize(V->getType());
+  if (!Size)
+    return UndefInt8;
+
   Constant *C = dyn_cast<Constant>(V);
   if (!C) {
     // Conceptually, we could handle things like:
     //   %a = zext i8 %X to i16
     //   %b = shl i16 %a, 8
     //   %c = or i16 %a, %b
     // but until there is an example that actually needs this, it doesn't seem
     // worth worrying about.
     return nullptr;
   }

llvm/unittests/Analysis/ValueTrackingTest.cpp

const std::pair<const char *, const char *> IsBytewiseValueTests[] = {
         "",
         "i16* inttoptr (i48 -1 to i16*)",
     },
     {
         "i8 -1",
         "i16* inttoptr (i96 -1 to i16*)",
     },
     {
-        "i8 0",
+        "i8 undef",
         "[0 x i8] zeroinitializer",
     },
     {
         "i8 undef",
         "[0 x i8] undef",
     },
     {
-        "i8 0",
+        "i8 undef",
         "[5 x [0 x i8]] zeroinitializer",
     },
     {
         "i8 undef",
         "[5 x [0 x i8]] undef",
     },
     {
         "i8 0",
const std::pair<const char *, const char *> IsBytewiseValueTests[] = {
         "[2 x i16] [i16 -21846, i16 -21846]]",
     },
     {
         "",
         "[2 x [2 x i16]] [[2 x i16] [i16 -21846, i16 -21846], "
         "[2 x i16] [i16 -21836, i16 -21846]]",
     },
     {
-        "i8 0",
+        "i8 undef",
         "{ } zeroinitializer",
     },
     {
         "i8 undef",
         "{ } undef",
     },
     {
-        "i8 0",
+        "i8 undef",
         "{ {}, {} } zeroinitializer",
     },
     {
         "i8 undef",
         "{ {}, {} } undef",
     },
     {
         "i8 0",