On Fri, Jun 05, 2020 at 03:41:27PM +0100, Robin Murphy wrote:

> Ha, and in fact having checked a build out of curiosity, this patch as-is
> actually stands to make things considerably worse. At least with GCC 8.3 and
> bcm2835_defconfig, bcm2835_spi_interrupt_common() doesn't get inlined, which
> means bcm2835_spi_interrupt() pushes/pops a stack frame and makes an
> out-of-line call to bcm2835_spi_interrupt_common(), resulting in massively
> *more* work than the extra two instructions of simply inlining the test.

Whichever compiler I was using (clang-11 probably) did manage to inline
the tail call so it really was the two instructions but yeah, in general
this approach is going to be fragile.

> So yes, the overhead of inlining the test vs. the alternative is indeed
> non-zero. It's just also negative :D

And variable!