[LLVMdev] Predicate registers/condition codes question

[Ivan Llopard]

Hi Sebastian,

Le 25/05/2012 18:54, Sebastian Pop a écrit :
> On Thu, May 24, 2012 at 5:40 PM, Sebastian Pop<spop at codeaurora.org>  wrote:
>> On Thu, May 24, 2012 at 5:06 PM, Hal Finkel<hfinkel at anl.gov>  wrote:
>>> Sebastian,
>>>
>>> First, it might be useful to look at what is done in the PowerPC
>>> backend. PPC also has condition registers that are larger than the
>>> 1-bit conditional results, and it defines 1-bit subregisters in
>>> addition to the larger condition registers. The spill-restore code ends
>>> up being more complicated, but that, perhaps, is a separate issue. [To
>>> be clear, I am not advocating for (or against) this solution even if it
>>> would work for you].
>> Ok, thanks for the pointer, I'll go read in the PPC bits.
> I see that PPC has its condition registers CRRC as i32, and that PPC
> also has general purpose i32 registers GPRC, so the situation is slightly
> different than on Hexagon, where there are no general purpose registers
> of the same size as the predicate registers: i8.
>
> So on PPC it is "safe" to promote from i1 to i32 and to "allow confusion"
> between the promoted i32 and the existing operations that were using i32:
> as we can always select between a CR and a GPR following the op type.
>
> On Hexagon, if type legalization promotes i1 into i8, that would create
> this confusion between the i8 ops existing before legalization and the
> newly promoted ones.  Then as Ivan was suggesting, we will have to
> provide custom expansion to promote the "illegal" ops on i8 on almost
> all the operations, except logical ops.

I think there is also another (and cleaner) workaround, a kind of 
operation-based type promotion of __illegal__ types.
This can be done by simply setting the operation with illegal type 
result to have a custom expander, for example:

setOperationAction(ISD::AND, MVT::i1, Custom)

See LowerOperationWrapper() & ReplaceNodeResults() hooks in 
TargetLowering. If you make this work only on logical ops, the rest will 
get automatically promoted by setting the promotion of i1 to be i32 by 
default. The latter will require a little hack though...
I hope this helps.

Ivan

>
> Sebastian
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