[LLVMdev] some thoughts on the semantics of !fpmath

[Eli Friedman]

On Tue, Apr 17, 2012 at 11:29 AM, Rafael Espíndola
<rafael.espindola at gmail.com> wrote:
>> The point isn't whether it's a good idea to discard FP precision data
>> (or vectorization data, or debug data), but rather whether IR
>> transformations are allowed to treat a meta-data annotated instruction
>> as having the same semantics as an unannotated instruction.  If
>> transformation passes which aren't metadata aware aren't allowed to
>> reason about an annotated instruction, there's no point to metadata in
>> the first place: we can just introduce new intrinsics/instructions, or
>> change the definition of an existing instruction/intrinsic.
>
> I am also not sure if it is at all possible to design a metadata
> system that allows passes to ignore it. I don't think that is true for
> any metadata we have other than debug info (if you don't consider
> broken debug info).
>
> IMHO the advantage that is still left in using metadata is that it is
> easier to implement gradually, since passes are allowed to drop it.
>
> Without metadata, changing the IL is an atomic operation where every
> pass has to be audited for correctness. If we insist that passes must
> drop metadata that they don't know about, support can be added one
> pass at a time.

Okay... that could make sense.  So under this system, each pass would
declare what kinds of metadata it supports, and the PassManager would
automatically drop all other instruction-level metadata from the
current module/function/loop?

-Eli