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Re: [PATCH] x86: Disable jump tables when retpolines are used (PR target/86952).
- From: Martin Liška <mliska at suse dot cz>
- To: Jan Hubicka <hubicka at ucw dot cz>
- Cc: gcc-patches at gcc dot gnu dot org, daniel at iogearbox dot net, "H.J. Lu" <hjl dot tools at gmail dot com>, Uros Bizjak <ubizjak at gmail dot com>
- Date: Fri, 8 Mar 2019 14:50:26 +0100
- Subject: Re: [PATCH] x86: Disable jump tables when retpolines are used (PR target/86952).
- References: <f8105d40-90bd-725e-30d2-8921fa6a2c5f@suse.cz> <20190308124425.at6mngxkxjtmjger@kam.mff.cuni.cz>
On 3/8/19 1:44 PM, Jan Hubicka wrote:
>> Hi.
>>
>> Thanks to Intel guys, we've done some re-measurement in PR86952
>> about usage of jump tables when retpolines are used.
>> Numbers prove that disabling of JT should be the best for now.
>>
>> Patch can bootstrap on x86_64-linux-gnu and survives regression tests.
>>
>> Ready to be installed?
>
> OK, thanks!
> I wonder if there is some threshold for extremely large jumptables where
> branchy sequence will loose, but I think it is better to disable them
> than what we have right now.
I tested switch statements up to 4096 and it was still slower ;)
Martin
>
> Honza
>