Documentation of fmuladd intrinsic

The fmuladd intrinsic is described as saying that a multiply and
addition sequence can be fused into an fma instruction "if the code
generator determines that the fused expression would be legal and
efficient". (http://llvm.org/docs/LangRef.html#llvm-fma-intrinsic)

I've spent a bit of time puzzling over how a code generator is supposed
to know if it's legal to generate an fma instead of a multiply and add
- surely that's something for the frontend to determine, based on the
FP_CONTRACT setting, and not something for the code generator to work
out?

However, recently I came across
http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20120528/0582
22.html
which explains that "legal" in the above definition doesn't mean legal
from the point of view of the source language, but simply means whether
or not the target architecture has an fma instruction. The thread also
talks about updating the documentation to clarify this, but that
doesn't seem to have happened.

Assuming that the thread I've linked to is correct, would it be
possible to clarify the IR spec accordingly? I think that the current
use of the word "legal" is misleading.

Andrew

Thanks for all the replies - it pretty much confirmed what I think I'd
worked out about how fmuladd is supposed to behave.

But my initial query wasn't asking what fmuladd did - it was asking for
the documentation about it to be clarified. Can I suggest that the
Overview section for fmuladd is changed to say the following (the key
point being the removal of the word "legal" in the description, to
avoid confusion with whether the optimisation is legal from the point
of view of the source language):

The llvm.fmuladd.* intrinsic functions represent multiply-add
expressions that can be fused if the code generator determines that the
target instruction set has support for a fused expression, and that its
use would be efficient.

Andrew

Hi Andrew,

That seems like a useful clarification. I’ve updated the docs with your suggested text (with some tweaks) in r172680.

Thanks for the suggestion.

Cheers,
Lang.