[llvm-dev] BUILD_VECTOR disambiguation

[Krzysztof Parzyszek via llvm-dev]

The problem can happen when you want to build a vector of i8, but i8 itself is not legal on your target.  All operands to BUILD_VECTOR will be legalized to the next legal integer type, so you can end up with a BUILD_VECTOR of vNi8 where all operands are i32, for example.

Actual mixed types are less likely to happen, but if you are doing your own target-specific lowering, you can end up with mixed types, simply because your code doesn't have to make them all the same.

--
Krzysztof Parzyszek  kparzysz at quicinc.com   AI tools development

> -----Original Message-----
> From: Cameron McInally <cameron.mcinally at nyu.edu>
> Sent: Tuesday, July 7, 2020 10:10 AM
> To: Krzysztof Parzyszek <kparzysz at quicinc.com>
> Cc: llvm-dev at lists.llvm.org
> Subject: [EXT] Re: [llvm-dev] BUILD_VECTOR disambiguation
>
> Thanks for the clarification. The `except` seemed dangling to me, but it's
> early here.
>
> Just curious... how do we end up with a mixed type BUILD_VECTOR?
> That's counterintuitive.
>
> On Tue, Jul 7, 2020 at 10:58 AM Krzysztof Parzyszek via llvm-dev <llvm-
> dev at lists.llvm.org> wrote:
> >
> > Mixed integer types are ok, but the values will get truncated to the
> element type of the output value.  It's pretty much what the comment says.
> >
> > --
> > Krzysztof Parzyszek  kparzysz at quicinc.com   AI tools development
> >
> > > -----Original Message-----
> > > From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of
> > > Cameron McInally via llvm-dev
> > > Sent: Tuesday, July 7, 2020 9:53 AM
> > > To: LLVM Developers Mailing List <llvm-dev at lists.llvm.org>
> > > Subject: [EXT] [llvm-dev] BUILD_VECTOR disambiguation
> > >
> > > Hey devs,
> > >
> > > From ISDOpcodes.h...
> > >
> > > /// BUILD_VECTOR(ELT0, ELT1, ELT2, ELT3,...) - Return a fixed-width
> > > vector /// with the specified, possibly variable, elements. The
> > > number of elements /// is required to be a power of two. The types
> > > of the operands must all be /// the same and must match the vector
> > > element type, except that integer types /// are allowed to be larger
> > > than the element type, in which case the operands /// are implicitly
> truncated.
> > > BUILD_VECTOR,
> > >
> > > Must the operand types always be the same? Or could we see a
> > > BUILD_VECTOR with mixed integer types? E.g.:
> > >
> > > BUILD_VECTOR(i32, i32, i64, i32)
> > >
> > > Thanks,
> > > Cam