-Original Message-
From: Richard Biener [mailto:richard.guent...@gmail.com]
Sent: Sunday, January 11, 2015 8:05 PM
To: Ajit Kumar Agarwal; vmaka...@redhat.com; l...@redhat.com; gcc@gcc.gnu.org
Cc: Vinod Kathail; Shail Aditya Gupta; Vidhumouli Hunsigida; Nagaraju Mekala
Subject: Re: Allocating some Loop allocno in memory
On January 11, 2015 5:25:23 AM CET, Ajit Kumar Agarwal
wrote:
>
>I was thinking of some of the opportunities with respect to reducing
>spills inside the Loop. If the Live range(allocno) spans through the
>Loop and Live out at the exit of the Loop and there are no references
>or not being touched upon inside the Loop, assign the allocno to the
>memory. This increase the chances of getting the register for other
>allocno which spans through the Loop and have reference inside the Loop
>if the interference graph is not colorable.
>
>Since allocno don't have references inside the Loop there won't be any
>load instructions with respect to restore inside the loops. There will
>be a store instruction with respect to spill which will be outside the
>Loop. This will reduce the conflicting edges of some of the allocno
>increasing the chances of making colorable of the Interference graph
>and reduces the spills and restore inside the Loop.
>
>This heuristics looks reasonable. This heuristics goes side by side
>with the Live range splitting across the Loop boundary.
>On top of this heuristics, after the Live range splitting across the
>Loop boundary there interference graph is not colorable then we can
>assign some of the splitted live ranges in the memory giving chances
>of registers for the Live ranges(allocno) that spans the Loop and have
>reference inside the Loop.
>
>We can change the cost of allocno in memory based on the above
>heuristics and take the above factor into consideration for the cost
>calculation.
>
>Thoughts Please?
>>How can this result in better allocations if you only ever spill at
>>life-range split points? The life-range covering the loop should already be
>>assigned to >>memory if required.
If the live range covering the loop assigned to memory, and there are
references inside the Loops then there are chances
of spills inside the loop which degrades the performance. If live range
covering the loops and there are no references inside
the Loop, then assigning to memory make reasonable as there are no references.
Since there are no references or not
touched inside the Loop, there won't be load instruction to restore which are
required if there are any references. The store
which requires for the def of the Live range will be outside the Loops. This
helps in not degrading the performances as there
are no load and store required for spill and restore inside the Loops.
The above heuristics can also be accompanied with heuristics of the number of
use points in the Live range. Considering both
the heuristics will lead to better allocation if ever spill at the live range
split points and there are chances of getting colorable
the Interference graph without degrading the performance. Also if the Live
range is splitted at the Loop boundary then the spill
at the split points of loop boundary and registers is assigned to this live
range will make it reasonable if there are no references
inside the Loops.
Thanks & Regards
Ajit
Richard.
>Thanks & Regards
>Ajit
