[LLVMdev] Inverse of ConstantFP::get and similar functions?

[Eli Friedman]

On Mon, Jul 22, 2013 at 10:19 AM, Stephen Lin <swlin at post.harvard.edu> wrote:
> Hi,
>
> I noticed that ConstantFP::get automatically returns the appropriately
> types Constant depending on the LLVM type passed in (i.e. if called
> with a vector, it returns a splat vector with the given constant).
>
> Is there any simple way to do the inverse of this function? i.e.,
> given a llvm::Value, check whether it is either a scalar of the given
> constant value or a splat vector with the given constant value? I
> can't seem to find any, and it doesn't look like the pattern matching
> interface provides something similar to this either.
>
> If this doesn't exist, then I propose adding static versions of all
> the various ConstantFoo::isBar() functions, which take a value as a
> parameter and check that the value is of a constant of the appropriate
> type and value (checking for vectors matching the predicate in the
> vector case).
>
> For example:
>
>    static bool ConstantFP::isExactlyValue(Value *V, double D);
>
> would return true is V is ConstantFP, a splat ConstantVector, or a
> ConstantDataVector with the appropriate type. Similarly,
>
>    static bool ConstantFP::isZero(Value *V);
>
> would return true if V is a ConstantFP with zero of either sign, a
> ConstantVector or ConstantDataVector with all zeros of either sign, or
> a zero initializer...
>
> Anyone have any thoughts, and/or can point me to somewhere where this
> kind of thing is already implemented?

We do already have Constant::isZeroValue().  There's also m_SpecificFP
and m_AnyZero in PatternMatch.h.

-Eli