On Fri, Sep 6, 2024 at 5:49 PM Jeremy Rifkin <jer...@rifkin.dev> wrote: > > Thanks Andrew, I appreciate the context and links. It looks like the > prior implementation failed to handle links due to being based on file > path, given cpp_simplify_pathname. Do you have thoughts on the use if > device ID + inode as a way to also accommodate symbolic links and hard > links without the fickleness of mtime?
Not always. because inodes are not always stable on some file systems. And also does not work with multi-mounted devices too. The whole definition of what is the same file is really up for debate here. I say if the file has the same content, then it is the same file and GCC uses that definition. While clang says it is based on if it is the same inode which is not always true because of file systems which don't use an inode number. While MSVC says it is based on the path but what is the canonical path to a file, is a hard link to the same file the same file or not; what about symbolic links? How about overlays and mounted directories are they the same then? GCC definition is the only one which supports all issues described here dealing with inodes (sometimes being non-stable), canonical paths and both kinds of links and even re-mounted file systems. What does the other implementations say about changing their definition of what "the same file is"? Have you asked clang and MSVC folks? Anyways GCC has an optimization already for #ifdef/#define/#endif (and that is documented here: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html) so does it make sense to really standardize `#pramga once` here or just push other implementations to add a similar optimization instead? Thanks, Andrew Pinski > > Cheers, > Jeremy > > On Sep 6 2024, at 12:25 am, Andrew Pinski <pins...@gmail.com> wrote: > > > On Thu, Sep 5, 2024 at 10:04 PM Jeremy Rifkin <jer...@rifkin.dev> wrote: > >> > >> Hello, > >> > >> I'm looking at #pragma once behavior among the major C/C++ compilers as > >> part of a proposal paper for standardizing #pragma once. (This is > >> apparently a very controversial topic) > >> > >> To put my question up-front: Would GCC ever be open to altering its > >> #pragma once behavior to bring it more in-line with behavior from other > >> compilers and possibly more in-line with what users expect? > >> > >> To elaborate more: > >> > >> Design decisions for #pragma once essentially boil down to a file-based > >> definitions vs a content-based definition of "same file". > >> > >> A file-based definition is easier to reason about and more in-line with > >> what users expect, however, distinct copies of headers can't be handled > >> and multiple mount points are problematic. > >> > >> A content-based definition works for distinct copies, multiple mount > >> points, and is completely sufficient 99% of the time, however, it could > >> potentially break in hard-to-debug ways in a few notable cases (more > >> information later). > >> > >> Currently the three major C/C++ compilers treat #pragma once very > >> differently: > >> - GCC uses file mtime + file contents > >> - Clang uses inodes > >> - MSVC uses file path > > > > See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52566#c2 . > > Note this was changed specifically in GCC 3.4 to fix the issue around > > symlinks and hard links. > > See https://gcc.gnu.org/pipermail/gcc-patches/2003-July/111203.html > > for more information on the fixes. > > > > In fact `#pragma once` was deprecated before GCC 3.4 because it would > > do incorrectly what clang and MSVC are doing and that was considered > > wrong. > > So GCC behavior has been this way before clang was even written. > > > > Thanks, > > Andrew > > > >> > >> None of the major compilers have documented their #pragma once semantics. > >> > >> In practice all three of these approaches work pretty well most of the > >> time (which is why people feel comfortable using #pragma once). However, > >> they can each break in their own ways. > >> > >> As mentioned earlier, clang and MSVC's file-based definitions of "same > >> file" break for multiple mount points and multiple copies of the same > >> header. MSVC's approach breaks for symbolic links and hard links. > >> > >> GCC's hybrid approach can break in surprising ways. I have three > >> examples to share: > >> > >> Example 1: > >> > >> Consider a scenario such as: > >> > >> usr/ > >> include/ > >> library_a/ > >> library_main.hpp > >> foo.hpp > >> library_b/ > >> library_main.hpp > >> foo.hpp > >> src/ > >> main.cpp > >> > >> main.cpp: > >> #include "library_a/library_main.hpp" > >> #include "library_b/library_main.hpp" > >> > >> And both library_main.hpp's have: > >> #pragma once > >> #include "foo.hpp" > >> > >> Example 2: > >> > >> namespace v1 { > >> #include "library_v1.hpp" > >> } > >> namespace v2 { > >> #include "library_v2.hpp" > >> } > >> > >> Where both library headers include their own copy of a shared header > >> using #pragma once. > >> > >> Example 3: > >> > >> usr/ > >> include/ > >> library/ > >> library.hpp > >> vendored-dependency.hpp > >> src/ > >> main.cpp > >> vendored-dependency.hpp > >> > >> main.cpp: > >> #include "vendored-dependency.hpp" > >> #include <library/library.hpp> > >> > >> library.hpp: > >> #pragma once > >> #include "vendored-dependency.hpp" > >> > >> Assuming the same contents byte-for-byte of vendored-dependency.hpp, and > >> it uses #pragma once. > >> > >> Each of these examples are plausible scenarios where two files with the > >> same contents could be #included. In each example, on GCC, the code can > >> work or break based on mtime: > >> - Example 1: Breaks if mtimes for library_main.hpp happen to be the same > >> - Example 2: Breaks if mtimes for the shared dependency copies happen to > >> be the same > >> - Example 3: Only works if mtimes are the same > >> > >> File mtimes can happen to match sometimes, e.g. in a fresh git clone. > >> However, this is a rather fickle criteria to rely on and could easily > >> diverge in the middle of development. Notably, Example 2 was shared with > >> me as an example where #pragma once worked great in development and > >> broke in CI. > >> > >> Additionally, while GCC's approach might be able to handle multiple > >> mounts better than other approaches, it can still break under multiple > >> mounts if mtime resolution differs. > >> > >> Obviously there is no silver bullet for making #pragma once work > >> perfectly all the time, however, I think it's easier to provide clear > >> guarantees for #pragma once behavior when the definition of "same file" > >> is based on file identity on device, i.e. device id + inode. > >> > >> Would GCC ever consider using device id + inode instead of mtime + > >> contents for #pragma once? > >> > >> I presume the primary reason against changing the mtime + file contents > >> approach in GCC would be caution over breaking any existing use. While > >> the three examples above are cases where fickle mtime can be > >> problematic, and I can't imagine any situations where mtime could > >> reliably be relied upon, I do understand the degree of caution required > >> for changes like this. > >> > >> > >> Cheers, > >> Jeremy > >
