DSE and maskstore trouble

[Andrew Stubbs]

On 03/07/18 11:33, Andrew Stubbs wrote:
> On 03/07/18 11:15, Richard Biener wrote:
>> AVX ones are all UNSPECs I believe - how do your patterns look like?
> 
> AVX has both unspec and vec_merge variants (at least for define_expand, 
> in GCC8), but in any case, AFAICT dse.c only cares about the destination 
> MEM, and all the AVX and SVE patterns appear to use nothing special there.

Sorry, my patterns look something like this:

(set (mem:V64DI (reg:DI)
      (vec_merge:V64DI (reg:V64DI) (unspec ...) (reg:DI)))

Where the unspec just means that the destination remains unchanged. We 
could also use (match_dup 0) there, but we don't, so probably there was 
an issue with that at some point.

>>> I was thinking of solving the problem by adding a target hook to query
>>> the true length of vector types. Any thoughts on that?
>>
>> It isn't about the length but about the mask because there can be mask
>> values that do not affect the length?
> 
> The problem I have right now is that the vector write conflicts with 
> writes to distinct variables, in which case the vector length is what's 
> important, and it's probably(?) safe to assume that if the vector mask 
> is not constant then space for the whole vector has been allocated on 
> the stack.
> 
> But yes, in general it's true that subsequent writes to the same vector 
> could well write distinct elements, in which case the value of the mask 
> is significant to DSE analysis.
> 
> Andrew