[llvm-dev] sizeof implementation: how to get size as a constantInt?

[Yafei Liu via llvm-dev]

Hi Eli, do you know what's the difference between
"DataLayout::getTypeAllocSize()" and "llvm::ConstantExpr::getSizeOf()"

On Wed, Jan 15, 2020 at 3:10 AM Eli Friedman <efriedma at quicinc.com> wrote:

> I’d recommend just using DataLayout::getTypeAllocSize() directly to get
> the number you want.  (If you really want to fold your Constant to a
> ConstantInt, llvm::ConstantFoldConstant should work, but that’s a really
> convoluted way to do it.)
>
>
>
> -Eli
>
>
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *Yafei
> Liu via llvm-dev
> *Sent:* Tuesday, January 14, 2020 1:52 AM
> *To:* llvm-dev <llvm-dev at lists.llvm.org>
> *Subject:* [EXT] [llvm-dev] sizeof implementation: how to get size as a
> constantInt?
>
>
>
> I'm implementing c style "sizeof()", and I did as
> http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt illuarstrated,
> and it works find, here's an example of my implementation:
>
> auto *p = builder.CreateGEP(structTy,
>                               llvm::ConstantPointerNull::get(pointerTy),
>                               constint1);
>
>   auto *size = builder.CreatePtrToInt(p, llvm::IntegerType::get(context,
> 64));
>
>
>
> and type definitions:
>
> auto *constint1 = llvm::ConstantInt::get(context, llvm::APInt(64, 1));
> auto *int64Ty = llvm::IntegerType::get(context, 64);
> auto *doubleTy = llvm::Type::getDoubleTy(context);
> auto *structTy = llvm::StructType::create(context, "Foo");
> structTy->setBody({int64Ty, doubleTy});
>
> auto *pointerTy = llvm::PointerType::get(structTy, 0);
>
>
>
> take care that the "size" is a "llvm::Value" type, not a
> "llvm::constantInt" type, this leads to a problem:  definitions like "int
> i[sizeof(Foo)]" will fail (please ignore that this definition makes no
> sense), because in my implementation, sizeof() should get the result on
> compile time, but the current implementation seems calculate at runtime( or
> at least an llvm::Value, not an llvm::ConstantInt)
>
>
>
> and I noticed this in the tutorial:
>
> Note that in both of these cases, the expression will be evaluated to a
>
> constant at code generation time, so there is no runtime overhead to using this
>
> technique.
>
> But how can he cast an llvm::Value to llvm::ConstantInt?
>
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