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Re: [PATCH][AArch64] Improve aarch64_case_values_threshold setting
- From: Kyrill Tkachov <kyrylo dot tkachov at foss dot arm dot com>
- To: Wilco Dijkstra <Wilco dot Dijkstra at arm dot com>, Evandro Menezes <e dot menezes at samsung dot com>, "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>
- Cc: nd <nd at arm dot com>
- Date: Wed, 27 Apr 2016 11:29:10 +0100
- Subject: Re: [PATCH][AArch64] Improve aarch64_case_values_threshold setting
- Authentication-results: sourceware.org; auth=none
- References: <AM3PR08MB008846A786228571DC6E0731836F0 at AM3PR08MB0088 dot eurprd08 dot prod dot outlook dot com>,<571E68BA dot 8060406 at samsung dot com> <AM3PR08MB0088732139A7C9954D8554A683620 at AM3PR08MB0088 dot eurprd08 dot prod dot outlook dot com>
Hi Wilco,
On 25/04/16 20:21, Wilco Dijkstra wrote:
Evandro Menezes wrote:
I assume that you mean that such improvements are true for
-mcpu=generic, yes? On which target, A53 or A57 or other?
It's true for any CPU setting. The SPEC results are for Cortex-A57
however I wrote a microbenchmark that shows improvements on
all targets I have access to. The GCC switch expansion is awful, so
even with a good indirect predictor it is better to use conditional
branches.
In what way is it awful? If there's something we can do better at
can you file a bug report with a testcase so that we can work on
improving it rather than tweaking a heuristic in the backend.
(Not that I'm against your patch ;)
Cheers,
Kyrill
Wilco