[llvm-dev] [FPEnv] FNEG instruction

Cameron McInally via llvm-dev
Tue, 11 Sep 2018 19:34:41 -0000

On Tue, Sep 11, 2018 at 2:45 PM, Kevin Neal via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Which exactly was the plan?
>
>
>
> Add a new, regular instruction?
>
>
>
> Add a new constrained math intrinsic?
>
>
>
> Both?
>
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>

I'd like to add an explicit FNEG IR operator. We would not need a
constrained FNEG operator if we go this route.


> Andrew Kaylor made a good point here:
>
>    - As I said, all LLVM IR FP instructions are //assumed// to have no
>    side effects. I'm not sure we want an instruction that goes beyond
this to
>    be //defined// as having no side effects. It adds a complication to the
>    language and introduces restrictions on the code generator that
aren't
>    needed in the ordinary case of non-constrained FP.  The target code
>    generators are free to do anything they want with the other FP
>    instructions, including things that introduce new FP status flags being
set
>    that otherwise wouldn't be, and for the normal case the back ends
should be
>    free to do that with fneg as well.
>
>
>
> Personally, I’m not sure I like the idea of having exceptions to the rule
> that FP instructions also have constrained versions. So I lean towards
> having both a regular FNEG and a constrained version.
>
>
>
> But I think I remember pushback. I can’t put my fingers on the message,
> though.
>

We touched on this in the Differential Review and on this thread. To
summarize:

FNEG(X) is not really the same operation as FSUB(-0.0, X), although the
differences are admittedly subtle. I even went as far to say that any
xforms between the two operations should only occur under FastMath
conditions. If we follow those rules, I think emergence guarantees that we
don't have to worry about the side effects of FNEG (please correct me if
I've missed something).

Extending on that, I suspect that we should not be canonicalizing FNEG(X)
as FSUB(-0.0, X), but rather as XOR(X, 1 << (SIZE_OF_TYPE_IN_BITS - 1)). A
utility function could provide us with the sign-bit constant, so it's not
that ugly.

That said, I agree that Andrew's take is compelling. And I would be okay
with adding a constrained FNEG to solve the immediate issue, if that is the
final decision.
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