[PATCH PR62631]Return shiftadd cost only when it's smaller than normal one

Richard Biener richard.guenther@gmail.com
Thu Sep 4 13:17:00 GMT 2014


On Thu, Sep 4, 2014 at 10:59 AM, Bin Cheng <bin.cheng@arm.com> wrote:
> Hi,
> As reported by pr62631, case ivopts-lt-2.c is failed on sparc64.  The root
> cause is shiftadd_cost calculated on sparc64 is huge which prevents gcc from
> choosing the wanted candidate.  The problem with IVOPT is it always assumes
> that shiftadd_cost is smaller than the normal cost, while it's not true in
> this extreme case.  This patch checks if the shiftadd_cost is smaller before
> returning it.  The patch also cleans code indent in context.
>
> One question is left here.
> Why >100 cost is calculated for shiftadd on sparc64?  In final assembly
> code, the computation is implemented by separated shift and add
> instructions.  Probably sparc64 should return cost no larger than
> cost(shift)+cost(add) in the first place?.
>
> Bootstrap and tested on x86_64/cortex-m3, the case is fixed on sparc64 now.
> Is it OK?

So is the IVOPTs code just checking if the target returned
sth > cost(shift) + cost(add)?  Then I think we should fix the target
instead.

Richard.

> Thanks,
> bin
>
>
> 2014-09-04  Bin Cheng  <bin.cheng@arm.com>
>
>         * tree-ssa-loop-ivopts.c (force_expr_to_var_cost): Return cost
>         calculated by get_shiftadd_cost only when it is smaller.  Fix
>         code indent.



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