[PATCH] D88409: [SVE] Make ElementCount and TypeSize use a new PolySize class

[David Sherwood via Phabricator via llvm-commits]

david-arm added inline comments.


================
Comment at: llvm/include/llvm/Support/TypeSize.h:38
+public:
+  PolySize() = default;
 
----------------
ctetreau wrote:
> david-arm wrote:
> > ctetreau wrote:
> > > david-arm wrote:
> > > > ctetreau wrote:
> > > > > Why does there need to be a default constructor?
> > > > I think because there are places in the code base that rely upon it - this isn't something new as it was already in ElementCount. I think there is one example in llvm/include/llvm/IR/Intrinsics.h where the ElementCount is a member of a union. I'm pretty sure that removing the default constructor causes compilation errors, but I can try again to make sure.
> > > Assuming it's necessary to have this constructor, what should it do? Should it explicitly be defined to be {0, false}? Is it a garbage value?
> > > 
> > > I suppose {0, false} sort of is already a garbage value, so maybe everything can just assert that it's not this?
> > I think it's just like someone declaring a variable and then initialising it later, no different from a normal integer type that has a default constructor, i.e.
> > 
> > uint64_t SomeVal;
> > ...
> > SomeVal = name == "Bob" ? 3 : 7;
> > 
> > If we want to remove the default constructor we could do it, but it will require some work to fix up cases where it's relied upon. I can certainly see what breaks when I remove it?
> I guess it might be nice to be able to do this. When ElementCount and TypeSize were just PODs, I was more OK with expecting the user to not mess it up, but since we're making them proper classes now...
> 
> I think if you make isScalable() and getKnownMinValue() assert that it's not {0, false}, then we can leave the default constructor for ergonomic purposes. If everything else were implemented in terms of these functions, then everything would be covered by the assert.
I tried replacing "PolySize() = default" with something explicit like "PolySize() : MinVal(0), IsScalable(false) {}", but sadly the compiler says this is not good enough for IITDescriptor. Since we can't be sure exactly what the default value is (I assume it's {0,false} but perhaps someone more knowledgeable can confirm?) I'd have to add asserts in isScalable, getKnownMinValue, getFixedSize and so on along the lines of:

assert(*this != PolySize())


Repository:
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https://reviews.llvm.org/D88409