[Webkit-unassigned] [Bug 54456] Optimizing lightning filter to ARM-neon SIMD instruction set

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=54456





--- Comment #26 from Zoltan Herczeg <zherczeg at webkit.org>  2011-03-01 04:45:15 PST ---
> Stricto sensu, this is not portable.
> You can compile with Neon because you target Cortex CPUs, while at the same time you might deploy on some target without the Neon extension. A common Cortex CPU without Neon is the tegra.
> 
> What we do in Qt is using the hardware capabilities from the kernel to detect Neon at runtime (HWCAP_NEON).

I don't understand you. If you tell -mfpu=neon to the compiler, it will generate some neon instructions. If you deploy it on a non-neon architecture, it will stop with a illegal instruction.

If you say -mfpu=vfp, it will not generate neon instructions, and the optimization will be disabled as well.

Runtime detection will not work here, since if you enable neon support, the compiler WILL generate neon instructions (independently from this patch), or if you disable it, it will NOT compile the assembly source code (because they are undefined instructions in that case). (Note: currently it is not supported to have different compiler optimizations for individual files)

If you unsure that the target platform have neon support, just disable it. This is my best advice for you.

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