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Re: PATCH: Fix ll/sc for mips (take 3)
- From: "H . J . Lu" <hjl at lucon dot org>
- To: Justin Carlson <justinca at ri dot cmu dot edu>
- Cc: Daniel Jacobowitz <dan at debian dot org>,"Maciej W. Rozycki" <macro at ds2 dot pg dot gda dot pl>,Hiroyuki Machida <machida at sm dot sony dot co dot jp>,libc-alpha at sources dot redhat dot com, linux-mips at oss dot sgi dot com,gcc at gcc dot gnu dot org
- Date: Sat, 2 Feb 2002 12:03:54 -0800
- Subject: Re: PATCH: Fix ll/sc for mips (take 3)
- References: <20020131231714.E32690@lucon.org> <Pine.GSO.3.96.1020201124328.26449Afirstname.lastname@example.org> <20020201102943.A11146@lucon.org> <20020201180126.A23740@nevyn.them.org> <20020201151513.A15913@lucon.org> <20020201222657.A13339@nevyn.them.org> <email@example.com>
On Sat, Feb 02, 2002 at 01:53:23PM -0500, Justin Carlson wrote:
> Actually, regardless of whether modern cpus implement it, I'd argue that
> avoiding the branch likely is a good idea for 2 reasons:
> 1) In the latest MIPS specs (mips32 and mips64) branch likelies have
> officially been deprecated as probable removals from the architecture in
> the not-too-distant future.
> 2) More importantly, most implementations don't use any sort of dynamic
> branch prediction on branch likelies. They predict taken, always, since
> that's the specified intent (it's a branch *likely* to be taken). For
> most spin locks, the normal behaviour is a fall through, not taking that
> branch, so you're inflicting a branch mispredict penalty on every lock
> grabbed without contention. Even for locks which the general case is
> contention, giving the processor branch predictor a chance to learn that
> is a Good Idea.
Does everyone agree with this? If yes, I can make a patch not to use
branch likely. But on the other hand, "gcc -mips2" will generate code
using branch likely. If branch likely doesn't buy you anything,
shouldn't we change gcc not to generate branch likely instructions?