[llvm-dev] How to handle UMULO?

[Bruce Hoult via llvm-dev]

I think your users will be very upset if you don't set the boolean return
value correctly :-)

Whatever work it takes to determine the correct value for it, if the user
code doesn't need/use that value then the dead code will be eliminated
later. But if they need that return flag then they will want it to be
correct!

You may need to use a multiply instruction that returns a double-register
result, or an instruction that returns only the upper half of the result.
Or you might need to widen the operands and do a full double-width
multiply. Or you might need to narrow the operands into two halves, do four
(or three) multiplies and some shifts and adds (again detecting
carry/overflow, but that's easier for addition).

It all depends on what instructions your target has.

On Wed, Feb 28, 2018 at 4:31 PM, 陳韋任 via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

> Hi All,
>
>   While compiling libgcc, I find I have to deal with UMULO (overflow-aware
> unsigned multiplication) SDNode. UMULO returns the result of
> multiplication, and a boolean indicating overflow occurred or not. Our
> target's multiply instruction doesn't care (detect) overflow. I am
> wondering if I can always set the boolean to false. I am not sure about
> this as I see AArch64 [1] seems trying to emulate the overflow behavior.
>
>   Thanks.
>
> [1] https://github.com/llvm-mirror/llvm/blob/master/lib/Target/AArch64/
> AArch64ISelLowering.cpp
>
> ​Regards,
> chenwj​
>
> --
> Wei-Ren Chen (陳韋任)
> Homepage: https://people.cs.nctu.edu.tw/~chenwj
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180228/7c3cd70b/attachment.html>