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Re: Optimal Coalescing with respect to move instruction for Live range splitting

From: Vladimir Makarov <vmakarov at redhat dot com>
To: Ajit Kumar Agarwal <ajit dot kumar dot agarwal at xilinx dot com>, "law at redhat dot com" <law at redhat dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
Cc: Vinod Kathail <vinodk at xilinx dot com>, Shail Aditya Gupta <shailadi at xilinx dot com>, Vidhumouli Hunsigida <vidhum at xilinx dot com>, Nagaraju Mekala <nmekala at xilinx dot com>
Date: Mon, 26 Jan 2015 14:47:27 -0500
Subject: Re: Optimal Coalescing with respect to move instruction for Live range splitting
Authentication-results: sourceware.org; auth=none
References: <a1a3f5a1e3bf419cab570b2a6f61333d at BY2FFO11FD035 dot protection dot gbl>


On 2015-01-18 12:37 AM, Ajit Kumar Agarwal wrote:

Register allocation with two phase approach does optimal coalescing after the spilling. Sometime Live range splitting makes
the coalescing non optimal. The splitted Live range are connected by move instruction. Thus the Live range splitting and more
specifically aggressive Live range splitting  increases the number of move instruction and making the coalescing
non optimal.

The optimal coalescing should take the frequency of the regions into consideration. The more the frequency of the region the cost
associated with the splitted live ranges will increase. If such cost increasing then we should do aggressive coalescing in this case and
remove the move instruction by associating the same color or hard registers associated with move instruction for the splitting live
ranges.

I am think of adding such heuristics in the GCC Top Down Region based register allocator for the optimal coalescing, thereby reducing
the move instruction connected with the splitted Live ranges in the more frequently code.

Thoughts please ?

It is difficult for me to understand what and how you will do thisexactly. If it is not a big work to implement this heuristic, I guessyou should try and see the result. If it works we can add it to the RA.

References:
- Optimal Coalescing with respect to move instruction for Live range splitting
  - From: Ajit Kumar Agarwal

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