Re: [PATCH] tree-optimization/108724 - vectorized code getting piecewise expanded

[Jeff Law <jeffreyalaw@gmail.com>]

On 2/10/23 04:02, Richard Biener via Gcc-patches wrote:
> This fixes an oversight to when removing the hard limits on using
> generic vectors for the vectorizer to enable both SLP and BB
> vectorization to use those.  The vectorizer relies on vector lowering
> to expand plus, minus and negate to bit operations but vector
> lowering has a hard limit on the minimum number of elements per
> work item.  Vectorizer costs for the testcase at hand work out
> to vectorize a loop with just two work items per vector and that
> causes element wise expansion and spilling.
> 
> The fix for now is to re-instantiate the hard limit, matching what
> vector lowering does.  For the future the way to go is to emit the
> lowered sequence directly from the vectorizer instead.
> 
> Bootstrapped and tested on x86_64-unknown-linux-gnu, OK?
> 
> Thanks,
> Richard.
> 
> 	PR tree-optimization/108724
> 	* tree-vect-stmts.cc (vectorizable_operation): Avoid
> 	using word_mode vectors when vector lowering will
> 	decompose them to elementwise operations.
> 
> 	* gcc.target/i386/pr108724.c: New testcase.
OK.  Though can't this be a problem with logicals too?  Or is there 
something special about +- going on here?


jeff