Laszlo Ersek <ler...@redhat.com> writes: > On 02/02/16 21:03, John Snow wrote: >> Recently, qemu iotest 013 has started to fail for me: >> >> Fedora release 22 (Twenty Two) >> >> 3.5.0-9.fc22 >> clang version 3.5.0 (tags/RELEASE_350/final) >> Target: x86_64-redhat-linux-gnu >> Thread model: posix >> >> >> +4 KiB/home/jsnow/src/qemu/qemu-io-cmds.c:230:18: runtime error: >> division by zero >> >> >> The problem is that in the print report for read_f, t2 and t1 can >> actually be the same exact timestamp, and tdiv will try to divide by 0.0. >> >> Normally this is not a problem as this is defined to be INFINITY in C99 >> Annex F. >> >> Clang, however, has once again decided to take the pedantic road and >> state that Annex F is optional, and therefore division by 0.0 is >> actually undefined when using -fsanitize=undefined. >> >> Groan. >> >> Two workarounds: >> >> (1) Modify the tdiv() function to just return INFINITY manually if the >> timestamp provided is 0 >> >> (2) Modify tester scripts to also use -fno-sanitize=float-divide-by-zero >> >> >> I prepared a patch to do the first workaround [1] so I could test >> patches with clang in peace as I need to test my pull requests under >> clang to make sure I don't break OSX, but it seems so absurd to have to >> do this, so I have copied our resident language lawyers (and language >> pragmatists) so that they can have a say. >> >> Relevant upstream BZ: https://llvm.org/bugs/show_bug.cgi?id=17000 >> >> --js >> >> [1] >> https://github.com/jnsnow/qemu/commit/af93977dd2bc7ea936b8064c41c5a0f9d25ae2d1 >> > > Apologies in advance for the knee-jerk reaction: > > I don't use double, ever. The last time I did anything resembling > numerical analysis was in college (now gracefully veiled by time). > > If I need decimals after the point, I opt for fixed point math, done > with integers. Surely uint64_t suffices for the purposes of > "qemu-io-cmds.c"; it just forces the programmer to think about those > issues explicitly that "double" promises, but fails, to solve.
I doubt integer types can "force" someone who uses double unthinkingly not to use uint64_t equally unthinkingly. People who manage to misuse floating-point for trivial computations like "given an amount and a time, compute rate per second" are likely to misuse integers as well. With integers, they get a crash when computing a rate for a time that got flushed to zero, which has a chance to do some forcing after the fact (whatever good that may do). With double, they get an infinity, which has a real chance to just work (it does in qemu-io). > I doubt microsecond resolution is necessary here, but even if it is, I'd > assume that approx. 584,942 years sufficed as an upper limit on time > differences. > > To frobnicate the saying about regular expressions, "when people want to > print decimals, they reach for floating point -- now they have two > problems". Fixed point has its own set of problems. Sometimes they're preferrable to floating-point problems, sometimes they aren't.
