tra added a subscriber: yaxunl.
tra added inline comments.
================
Comment at: clang/lib/CodeGen/CGCUDANV.cpp:491
new llvm::LoadInst(Var->getType(), ManagedVar, "ld.managed", false,
- llvm::Align(Var->getAlignment()), I);
+ Var->getAlign().valueOrOne(), I);
WorkItem.pop_back();
----------------
gchatelet wrote:
> tra wrote:
> > This appears to be a change in behavior. AFAICT, previously used
> > Var->getAlignment() could return alignment value or 0. Now it's value or 1.
> >
> > Is it intentional?
> The previous statement was constructing an `llvm::Align` from a value, and
> `llvm::Align` [asserts when the provided value is
> 0](https://github.com/llvm/llvm-project/blob/4ab2246d486ba30c4b2d654323a0c0b97565c0f1/llvm/include/llvm/Support/Alignment.h#L76-L81).
> This means that it is undefined to pass the value `0`.
>
> As far as `LoadInst` is concerned it can only accept valid alignments (i.e.,
> powers of two => non zero).
>
> So you're right that it is not strictly NFC and that `*Var->getAlign()`would
> be a more rigorous transformation but I //think// that converting the `0`
> value to `1` moves us from UB to semantically valid code.
>
> I don't feel strongly about it though and I'm fine changing this to
> `*Var->getAlign()` to keep this patch NFC. WDYT?
Enforcing alignment of 1 would potentially force us to generate overly
conservative one byte at a time loads/stores.
I agree that passing 0 is a wrong choice here, but 1 does not seem to be
correct, either.
Unfortunately LoadInst does not have overloads accepting MaybeAlign so we need
to use different `LoadInst` overload depending on whether alignment is
specified.
```
NewV = Var->getAlign().isAligned()
? llvm::LoadInst(Var->getType(), ManagedVar, "ld.managed", false,
Var->getAlign().value(), I)
: llvm::LoadInst(Var->getType(), ManagedVar, "ld.managed", false, I);
```
@yaxunl -- Sam, does it make sense? This seems to be largely HIP-specific.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D142459/new/
https://reviews.llvm.org/D142459
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