question on find_if_case_2 in ifcvt.c

question on find_if_case_2 in ifcvt.c

[Amker.Cheng]

Hi,
In ifcvt.c's function find_if_case_2, it uses cheap_bb_rtx_cost_p to
judge the conversion.

Function cheap_bb_rtx_cost_p checks whether the total insn_rtx_cost on
non-jump insns in
basic block BB is less than MAX_COST.

So the question is why uses cheap_bb_rtx_cost_p, even when we know the
ELSE is predicted,
which means there is benefit from this conversion anyway.
Second, should cheap_bb_rtx_cost_p be tuned as "checks whether the
total insn_rtx_cost on
non-jump insns in basic block BB is no larger than MAX_COST." to
prefer normal instructions
than branch even there have same costs.

Any suggestions? Thanks in advance.
-- 
Best Regards.

Re: question on find_if_case_2 in ifcvt.c

[Jeff Law]

On 09/08/2011 08:20 PM, Amker.Cheng wrote:
> Hi,
> In ifcvt.c's function find_if_case_2, it uses cheap_bb_rtx_cost_p to
> judge the conversion.
>
> Function cheap_bb_rtx_cost_p checks whether the total insn_rtx_cost on
> non-jump insns in
> basic block BB is less than MAX_COST.
>
> So the question is why uses cheap_bb_rtx_cost_p, even when we know the
> ELSE is predicted,
> which means there is benefit from this conversion anyway.
Not necessarily.  This transformation is speculating  insns from the 
ELSE path.  So there's a cost every time we mispredict the branch.


> Second, should cheap_bb_rtx_cost_p be tuned as "checks whether the
> total insn_rtx_cost on
> non-jump insns in basic block BB is no larger than MAX_COST." to
> prefer normal instructions
> than branch even there have same costs.
Perhaps, it's a corner case and I doubt it matters too much.

I have a pending patch which twiddles this code so that it takes into 
account the weight of the prediction.  This is important, particularly 
in the case where the ELSE is not predicted -- we're paying an awful 
cost for the speculation in that case.
jeff