[cfe-dev] case-insensitive #include warning
Eric Niebler via cfe-dev
cfe-dev at lists.llvm.org
Mon Apr 11 16:45:09 PDT 2016
On 4/11/16, 4:12 PM, "Jason Haslam" <jason.haslam at gmail.com<mailto:jason.haslam at gmail.com>> wrote:
On Apr 7, 2016, at 5:44 PM, Eric Niebler via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
On 4/7/16, 11:37 AM, "clattner at apple.com<mailto:clattner at apple.com> on behalf of Chris Lattner" <clattner at apple.com<mailto:clattner at apple.com>> wrote:
On Apr 5, 2016, at 4:03 PM, Eric Niebler via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
I have an initial cut at patch that issues a warning when a file is #included on a case-insensitive file system that would not be found on a case-sensitive system. Is there interest?
Since this is a hard problem to solve perfectly, I have opted for a strategy that is efficient and conservative about issuing diagnostics. That is, it shouldn't give false positives, but it will fail to diagnose some non-portable paths. On *nix systems, the low-level APIs that stat and open files get an extra call to ::realpath, and the result is cached along with the rest of the file metadata. […]
I was working on a similar patch a few years ago (http://lists.llvm.org/pipermail/cfe-dev/2012-July/023090.html<https://urldefense.proofpoint.com/v2/url?u=http-3A__lists.llvm.org_pipermail_cfe-2Ddev_2012-2DJuly_023090.html&d=CwMFaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=t4UJXwtMcIwGvgOaMqGzrQ&m=c8D9-z1QyfRWobwgBlnojB0yAu4P3Ve2-Wk9Z4vAxjg&s=5yrhYcvssUCiHzENOa2jFtUbhJNW1jTrGLKAnlJLkuQ&e=> is the start of a very long thread). Does realpath really give the real case on unix? My understanding is that it doesn’t. I had to resort to reading the directory listings and comparing entries. And of course readdir is even slower. I could only ever get decent performance by aggressively caching the directory listings. In the end it got too complicated and wasn’t entirely convinced that performance would ever be good enough to warrant the addition. I was mostly interested in Mac though. Maybe it’s a different story on Windows.
I haven't been able to successfully mount a case-insensitive file system on my Linux VM, so I haven't specifically tested this. On Mac, realpath gets the case right, but it's not needed since it has fcntl/F_GETPATH, which I've discovered is faster. In my tests, realpath does not get the right case on Cygwin. On *nixes that have /proc, it's possible to readlink on /proc/self/fd/$FD once the file is opened, and that should give the properly cased filename. I haven't done extensive benchmarking, but that is certain to be faster than recursive directory traversal. But not all *nixes have /proc.
TL;DR this is going to be an unreliable warning. Personally, I think that's fine so long as it doesn't give false positives and doesn't slow things down too badly. I'm busy reworking the patch to that end.
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