[Bug c/113134] gcc does not version loops with early break conditions that don't have side-effects

[tnfchris at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113134

--- Comment #11 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
(In reply to JuzheZhong from comment #10)
> (In reply to Tamar Christina from comment #9)
> > (In reply to JuzheZhong from comment #8)
> > > Suppose the loop mask is generated by whilelo instruction of ARM SVE.
> > > 
> > > Suppose we have 8 elements in a single whole vector.
> > > 
> > > mask = whilo (0, res) if res = 6, then mask = 11111000.
> > > data = 12345678
> > > 
> > > Then if it is early break. You are reversing both data and mask as follows:
> > > 
> > > new_mask = 00011111
> > > new_data = 87654321
> > > 
> > > Then use the EXTRACT_LAST, we will get value = 1 for early break.
> > > 
> > > Am I right ?
> > 
> > Yeah, the idea being the scalar loop will then run from 1 to 6 to do any
> > side effects that we couldn't apply.
> > 
> > We went with this approach first because it works for non-masked
> > architectures too. In GCC-15 we'll try to implement staying entirely inside
> > a vector loop by splitting the mask in elements until first active and
> > element from first active so we can correctly mask the operations.
> 
> Ok. For the current approach. Isn't it the first element is always element 0
> ?
> 
> Since for ARM SVE loop mask is generated by whilelo instructions, it always
> set
> mask bit from 0 to the last active element - 1.

sure, but you can't use BIT_FIELD_REF on VLA vectors.