[LLVMdev] 65bit integer math

[Jeffrey Yasskin]

On Mon, Oct 12, 2009 at 8:22 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
> 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.

It's -indvars. I ran mem2reg, instcombine, and simplifycfg on your
input and got the attached file. I thought I could add "nuw nsw" to
the arithmetic inside the loop to say that i65 is unnecessary, but
that doesn't cause indvars to leave the arithmetic at i64. Is that a
bug in indvars, or in SCEV, or is this just a natural limitation of
SCEV?
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