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 <[email protected]> > > 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. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 8.8Mbps down 630kbps up According to speedtest.net: 8.21Mbps down 510kbps up
