llvm dev - Feb 2019 - [RFC] Enforcing immediate operands for intrinsics

> On Feb 5, 2019, at 6:55 PM, Krzysztof Parzyszek via llvm-dev <llvm-dev
at lists.llvm.org> wrote:
> 
> There is something similar going on in clang in SemaChecking.cpp. The
difference is that each target can write its own verification code, which may
check things like value ranges, for example.
> The scheme you're proposing would invent a new attribute (a widespread
change) that only implements partial checks (i.e. "is argument immediate or
not”).
This is for the benefit of IR transform passes and codegen. I’m not attempting
to diagnose frontend builtins or do anything for the end user. A backend can
always truncate bits or whatever to fit the actual instruction constraints if
necessary. It has to do something to not crash on the IR otherwise, which is
part of the problem I’m trying to solve.
> 
> Why not simply add a function to TTI that tells you whether a particular
Value is a valid n-th argument to the given intrinsic?
> Something like
>  if (TTI.isValidArgument(IntOpc, Val, OpIdx))
>    Int->setOperand(Val, OpIdx);
> 
This is required for correctness, so TTI is not appropriate. Reasonably
implementing this would still require adding something in TableGen (which then
just brings you back to adding an attribute). I would also like to be able to
rely on this for emission of G_INTRINISIC_* instructions in GlobalISel, so that
intermediate illegal G_CONSTANT instructions can be avoided

-Matt

On 2/5/2019 6:17 PM, Matt Arsenault wrote:
> This is for the benefit of IR transform passes and codegen. I’m not
attempting to diagnose frontend builtins or do anything for the end user.
Ah, I thought you wanted to have the ability to verify an intrinsic in 
IR. Yeah, the attribute should be enough.

-Krzysztof

There's a stale proposal with similar motivation here:
https://reviews.llvm.org/D23798

Setting the const-ness in the tablegen definition sounds better than that,
but that patch may provide a starting point for intrinsics that have immarg
potential.

On Tue, Feb 5, 2019 at 6:37 PM Krzysztof Parzyszek via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> On 2/5/2019 6:17 PM, Matt Arsenault wrote:
> > This is for the benefit of IR transform passes and codegen. I’m not
> attempting to diagnose frontend builtins or do anything for the end user.
>
> Ah, I thought you wanted to have the ability to verify an intrinsic in
> IR. Yeah, the attribute should be enough.
>
> -Krzysztof
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