Created attachment 29460 [details] (Generated assembly code) As recently discussed in bug 56334, the documentation for __attribute__((aligned(N))) notes that it does not necessarily produce the requested alignment for static variables: "On many systems, the linker is only able to arrange for variables to be aligned up to a certain maximum alignment. (For some linkers, the maximum supported alignment may be very very small.)" However, it appears that GCC itself has not read this documentation! Consider this trivial .c file: #define N (1<<27) static float __attribute__((aligned(N))) a[128]; void foo() { if ((unsigned long) a % N == 0) bar(a); else bar_unaligned(a); } We are not actually going to get this static array aligned to a 128-megabyte alignment (especially if this goes into a shared library), but GCC nonetheless eliminates the branch and possible call to bar_unaligned. See, for instance, the output of this command line (where align5.c is the above file): i686-pc-linux-gnu-gcc -O2 -S -fpic align5.c -o align5.s There is clearly no reference to bar_unaligned in the generated assembly, indicating that it has been optimized out.
That's a bug in alignment attribute processing then. It should not communicate alignments that can not be reached to the middle-end. Language lawyer question: Is __alignof__ then allowed to report a lower alignment? Or do we have to reject a testcase with a too large alignment specification as invalid?
See <http://gcc.gnu.org/ml/gcc-patches/2011-11/msg00841.html>, where I discuss what I think would be the appropriate checks for supported alignment (which I deferred for the initial implementation of _Alignas / _Alignof). I think the checks should be errors for both the C11 _Alignas syntax and the __attribute__ syntax.
> .comm a,512,134217728 The linker should fail to link if it can't link to 128 MB here. From JSM's email: > MAX_OFILE_ALIGNMENT (presently just a warning), That was PR 87795 and was fixed in r9-3979-g4c7bd36194e13c . > an object with automatic storage duration has an alignment greater than MAX_STACK_ALIGNMENT See PR 89357 which removed the constraint for C++ _Alignas as the middle-end supports huge alignments now. So closing as fixed for GCC 9.3.0.