Re: [PATCH 13/13] jump label v9: add docs

From: Peter Zijlstra
Date: Thu Jun 10 2010 - 08:49:26 EST

On Wed, 2010-06-09 at 17:39 -0400, Jason Baron wrote:
> +The optimization depends on !CC_OPTIMIZE_FOR_SIZE. When CC_OPTIMIZE_FOR_SIZE is
> +set, gcc does not always out of line the not taken label path in the same way
> +that the "if unlikely()" paths are made out of line. Thus, with
> +CC_OPTIMIZE_FOR_SIZE set, this optimization is not always optimal. This may be
> +solved in subsequent gcc versions, that allow us to move labels out of line,
> +while still optimizing for size. In the case of !CC_OPTIMIZE_FOR_SIZE this
> +optimization is seen on high level benchmarks such as tbench where I can get
> +~1-2% higher throughput. In addition we are within .5% of the performance of no
> +tracepoints compiled in at all.

But does it generate invalid code for whatever -f flag triggers this
(-fno-reorder-blocks ?)

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