Re: DSE and maskstore trouble

[Andrew Stubbs <andrew_stubbs at mentor dot com>]

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.

> > 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