sdesmalen added inline comments.
================
Comment at: clang/utils/TableGen/SveEmitter.cpp:302
unsigned Shift = llvm::countr_zero(Mask);
+ assert(Shift >= 64 && "Shift is out of encodable range");
return (V << Shift) & Mask;
----------------
erichkeane wrote:
> sdesmalen wrote:
> > erichkeane wrote:
> > > Shouldn't this be: `assert(Shift < 64 &&"...")`?
> > >
> > > `expr.shift` (https://eel.is/c++draft/expr.shift) says:
> > > ```
> > > The operands shall be of integral or unscoped enumeration type and
> > > integral promotions are performed.
> > > The type of the result is that of the promoted left operand.
> > > The behavior is undefined if the right operand is negative, or greater
> > > than or equal to the width of the promoted left operand.```
> > >
> > > uint64 stays as an `unsigned long`, so it is still 64 bits, so the only
> > > invalid value for `Shift` is 64 (though >64 is 'nonsense', but only
> > > impossible because of `llvm::countr_zero`).
> > >
> > > One thing to consider: I wonder if we should instead be changing the
> > > 'shift' to be:
> > >
> > > `(V << (Shift % 64)) && Mask` ? It looks like `arm_sve.td` has the
> > > `NoFlags` value as zero, which I think will end up going through here
> > > possibly (or at least, inserted into `FlagTypes`.
> > >
> > > So I suspect an assert might not be sufficient, since a 64 bit shift is
> > > possible in that case (since a zero 'Mask' is the only case where
> > > `countr_zero` will end up being 64).
> > >
> > >
> > > So I suspect an assert might not be sufficient, since a 64 bit shift is
> > > possible in that case (since a zero 'Mask' is the only case where
> > > countr_zero will end up being 64).
> > It should be fine to assert that `Mask != 0`, since that would be an
> > invalid mask.
> Thanks for the comment @sdesmalen! Is there something that prevents the
> `NoFlags` from being passed as the `MaskName` here?
There's nothing that actively prevents it, but `encodeFlag` is a utility
function that has no uses outside this file and has only 4 uses. Adding an
assert should be sufficient.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D150140/new/
https://reviews.llvm.org/D150140
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