Messages in this thread | | | Date | Wed, 17 Jan 2018 09:03:29 -0500 (EST) | From | Nicolas Pitre <> | Subject | Re: [PATCH] ARM: make memzero optimization smarter |
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On Wed, 17 Jan 2018, Russell King - ARM Linux wrote:
> On Tue, Jan 16, 2018 at 11:07:34PM -0500, Nicolas Pitre wrote: > > On Tue, 16 Jan 2018, Arnd Bergmann wrote: > > > > > On Tue, Jan 16, 2018 at 6:10 PM, Nicolas Pitre <nicolas.pitre@linaro.org> wrote: > > > > On Tue, 16 Jan 2018, Arnd Bergmann wrote: > > > > > > > >> However, we can avoid this class of bogus warnings for the memset() macro > > > >> by only doing the micro-optimization for zero-length arguments when the > > > >> length is a compile-time constant. This should also reduce code size by > > > >> a few bytes, and avoid an extra branch for the cases that a variable-length > > > >> argument is always nonzero, which is probably the common case anyway. > > > >> > > > >> I have made sure that the __memzero implementation can safely handle > > > >> a zero length argument. > > > > > > > > Why not simply drop the test on (__n) != 0 then? I fail to see what the > > > > advantage is in that case. > > > > > > Good point. We might actually get even better results by dropping the > > > __memzero path entirely, since gcc has can optimize trivial memset() > > > operations and inline them. > > > > > > If I read arch/arm/lib/memzero.S correctly, it saves exactly two 'orr' > > > instructions compared to the memset.S implementation, but calling > > > memset() rather than __memzero() from C code ends up saving a > > > function call at least some of the time. > > > > > > Building a defconfig kernel with gcc-7.2.1, I see 1919 calls to __memzero() > > > and 636 calls to memset() in vmlinux. If I remove the macro entirely, > > > I get 1775 calls to memset() instead, so 780 memzero instances got > > > inlined, and kernel shrinks by 5488 bytes (0.03%), not counting the > > > __memzero implementation that we could possibly also drop. > > > > I get 3668 fewer bytes just by removing the test against 0 in the macro. > > > > And an additional 5092 fewer bytes by removing the call-to-__memzero > > optimization. > > However, __memzero is not safe against being called with a zero length > so it's not something we can simply remove.
The idea is about the possibility of removing __memzero altogether. It is not clear that the tiny performance gain from a dedicated memzero implementation is worth the current overhead around it.
Nicolas
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