-----Original Message-----
From: Bernd Schmidt [mailto:bschm...@redhat.com]
Sent: Wednesday, December 09, 2015 7:34 PM
To: Ajit Kumar Agarwal; Richard Biener
Cc: Jeff Law; GCC Patches; Vinod Kathail; Shail Aditya Gupta; Vidhumouli
Hunsigida; Nagaraju Mekala
Subject: Re: [Patch,rtl Optimization]: Better register pressure estimate for
Loop Invariant Code Motion.
On 12/09/2015 12:22 PM, Ajit Kumar Agarwal wrote:
>
> This is because the available_regs = 6 and the regs_needed = 1 and
> new_regs = 0 and the regs_used = 10. As the reg_used that are based
> on the Liveness given above is greater than the available_regs, then
> it's candidate of spill and the estimate_register_pressure calculates > the
> spill cost. This spill cost is greater than inv_cost and gain > comes to be
> negative. The disables the loop invariant for the above > testcase.
>>As far as I can tell this loop does not lead to a spill. Hence, failure of
>>this testcase would suggest there is something wrong with your idea.
As far as I can see there is nothing wrong with the idea, the existing
implementation of the calculation of available_regs results to 6 which is not
the case for this
testcase. The estimate of regs_used based on the Liveness ( idea behind this
patch) is correct, but the incorrect estimate of the available_regs (6)
results in the gain to be negative.
>> + FOR_EACH_LOOP (loop, LI_FROM_INNERMOST) {
>>Formatting.
>> + /* Loop Liveness is based on the following proprties.
>>"properties"
I will incorporate the change.
>> + we only require to calculate the set of objects that are live at
>> + the birth or the header of the loop.
>> + We don't need to calculate the live through the Loop considering
>> + Live in and Live out of all the basic blocks of the Loop. This is
>> + because the set of objects. That are live-in at the birth or header
>> + of the loop will be live-in at every node in the Loop.
>> + If a v live out at the header of the loop then the variable is
>> live-in
>> + at every node in the Loop. To prove this, Consider a Loop L with
>> header
>> + h such that The variable v defined at d is live-in at h. Since v is
>> live
>> + at h, d is not part of L. This follows from the dominance property,
>> i.e.
>> + h is strictly dominated by d. Furthermore, there exists a path from
>> h to
>> + a use of v which does not go through d. For every node of the loop,
>> p,
>> + since the loop is strongly connected Component of the CFG, there
>> exists
>> + a path, consisting only of nodes of L from p to h. Concatenating
>> those
>> + two paths prove that v is live-in and live-out Of p. */
>>Please Refrain From Randomly Capitalizing Words, and please also fix the
>>grammar ("set of objects. That are live-in"). These problems make this
>>comment (and also your emails) >>extremely hard to read.
>>Partly for that reason, I can't quite make up my mind whether this patch
>>makes things better or just different. The testcase failure makes me think
>>it's probably not an improvement.
I'll correct it. I am seeing the gains for Mibench/EEMBC benchmarks for
Microblaze target with this patch. I will run SPEC CPU 2000 benchmarks for
i386 with this patch.
>> + bitmap_ior_into (&LOOP_DATA (loop)->regs_live, DF_LR_IN (loop->header));
>> + bitmap_ior_into (&LOOP_DATA (loop)->regs_live, DF_LR_OUT
>> + (loop->header));
>>Formatting.
I'll incorporate the change.
Thanks & Regards
Ajit
Bernd
