[LLVMdev] Adding NonTemporal

[Jeffrey Yasskin]

On Fri, Feb 12, 2010 at 1:43 PM, David Greene <dag at cray.com> wrote:
> On Thursday 11 February 2010 17:40:24 David Greene wrote:
>> While hacking around in the SelectionDAG build code, I've made the
>> isVolatile, (new) isNonTemporal and Alignment parameters to
>> SelectionDAG::getLoad/getStore and friends non-default.
>>
>> I've already caught one bug in the XCore backend by doing this:
>>
>>     if (Offset % 4 == 0) {
>>       // We've managed to infer better alignment information than the load
>>       // already has. Use an aligned load.
>>       return DAG.getLoad(getPointerTy(), dl, Chain, BasePtr, NULL, 4,
>>                          false, false, 0);
>>     }
>>
>> Whoops!  There's no alignment info being set here!
>>
>> Is there any reason to keep these as default arguments?  It invites silent
>> coding errors.  I did this because I have to propagate the non-temporal
>> information through various phases of lowering and making these non-default
>> helps me catch missing cases.
>>
>> We've done the same in our local sources and we've never found
>> specifying the extra arguments to be burdensome.
>
> I have a patch ready to track nontemporal semantics but I'm waiting for an
> opinion on this.  I'd really like to get this integrated for 2.7.  In my
> opinion default arguments are a bad idea in this context.  It's very easy
> to make coding errors.  I found a couple more today.

FWIW, I'm happy with removing default arguments. I'm not happy with
long strings of simply-typed arguments like "NULL, 4, false, false, 0"
that don't give a reader any indication of what the arguments mean,
but that's trickier to fix.