Re: [Libguestfs] [PATCH nbdkit 2/2] common/include/checked-overflow.h: Provide fallback

On Tue, Nov 23, 2021 at 03:25:22PM +0000, Richard W.M. Jones wrote: > On Tue, Nov 23, 2021 at 11:44:06AM +0100, Laszlo Ersek wrote: >> On 11/22/21 23:31, Richard W.M. Jones wrote: >>> My only other thought is that a simple set of tests could be good. >>> However it's not worth having tests that only test if __builtin* >>> functions are correct (hopefully GCC is already testing that). So >>> tests would have to check the fallback macros are correct, even if >>> they are not used on the current platform. >> >> So: the fallbacks need to be available (= built) in the source code >> unconditionally, so they can be directly called by the test suite. The >> actual "falling back" to them must be separate. Is that what you mean? > > I just meant that if we have a test suite which tests the end result > (macros like "ADD_OVERFLOW"), because we will only do CI on modern > systems, it will only ever test that __builtin_add_overflow works. > That's pointless because presumably GCC already tests that, and the > fallback code would never be tested. So the tests would need to > somehow check that the fallback path works. Something along the lines of: #if (...gcc or clang new enough...) && !defined(NBDKIT_FALLBACK) # define ADD_OVERFLOW(a, b, r) __builtin_add_overflow((a), (b), (r)) #else # define ADD_OVERFLOW(a, b, r) ...fallback... #endif so that the testsuite can then #define NBDKIT_FALLBACK for testing the fallback code even on new enough compilers. I'm not sure how best to namespace the witness macro for forcing a fallback if the code is copied between nbdkit and libnbd.