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Date: Wed, 11 Nov 2009 14:13:13 +0000 (GMT) From: "Maciej W. Rozycki" <macro@...ux-mips.org> To: Ralf Baechle <ralf@...ux-mips.org> cc: Wu Zhangjin <wuzhangjin@...il.com>, David Daney <ddaney@...iumnetworks.com>, linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org, zhangfx@...ote.com, zhouqg@...il.com, rostedt@...dmis.org, Frederic Weisbecker <fweisbec@...il.com>, Ingo Molnar <mingo@...e.hu>, Nicholas Mc Guire <der.herr@...r.at>, Richard Sandiford <rdsandiford@...glemail.com>, Patrik Kluba <kpajko79@...il.com>, Thomas Gleixner <tglx@...utronix.de>, Michal Simek <monstr@...str.eu> Subject: Re: [PATCH v7 04/17] tracing: add static function tracer support for MIPS On Wed, 11 Nov 2009, Ralf Baechle wrote: > 32-bit with -mlong-call: > > lui $25, %hi(foo) > addiu $25, %lo(foo) > jalr $25 [...] > It's time that we get a -G optimization that works for the kernel; it would > allow to cut down the -mlong-calls calling sequence to just: > > lw/ld $25, offset($gp) > jalr $25 Actually this may be no faster than the above. The load produces its result late and the jump needs its data early, so unless a bypass has been implemented in the pipeline, it may well stall for the extra cycle (that's the reason for the load-delay slot in the original MIPS I ISA after all). Of course there is still the benefit of a reduced cache footprint, but the extra load may have to evict a cache line and flush the benefit down the drain. I don't mean it's not to be considered, but it's not at all immediately obvious it would be a win. Maciej -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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