[LLVMdev] 65bit integer math

[Eli Friedman]

On Mon, Oct 12, 2009 at 6:15 PM, Villmow, Micah <Micah.Villmow at amd.com> wrote:
> I have a test case(attached as fc_long.ll) that when run through the
> optimizer produces 65bit integer math(fc_long-opt.ll).
>
>
>
> Now I understand that llvm can have any length integer, but I consider
> turning a 64bit mul into multiple 65 bit instructions to be a ‘bad’
> optimization. This eventually expands to a 128bit multiply call(__multi3)
> which I have absolutely no interest in supporting. So I’m wondering what
> optimization might be the culprit here so I can disable it in this
> situation.

I'm pretty sure this is the fault of one of the loop passes (I forget
which one off the top of my head).  It's worth noting that the
optimizer manages to eliminate the loop; computing the exit value of
one of the loop variables uses the wide multiply.

-Eli