Andy Wingo <wi...@pobox.com> skribis: > On Wed 22 May 2013 23:06, l...@gnu.org (Ludovic Courtès) writes: > >>> +/* This function leaks the memory that it allocates. */ >>> +static char* >>> +alloc_aligned (size_t len, unsigned alignment) >> >> What about using posix_memalign or similar? > > Not present on many systems, and gnulib can't make a useful wrapper. If > you do the normal trick of allocating more with malloc then aligning it, > you make it impossible to free().
OK. >> Alternatively, using scm_gc_malloc_pointerless, which is known to return >> 8-byte-aligned boundary? > > For two reasons. One is that this memory is not collectable. Static > allocation of constants means that you can get pointers into the middle > of the image from the heap. I wanted to avoid pressuring the GC. > > The other reason is that you can get pointers out of the image *to* the > heap -- for data that is statically allocated, but fixed up at runtime. > For example, keywords. Their stringbuf is allocated statically in > read-only memory, but there is also a cell allocated for the keyword > itself, and that lives on the heap. Right. > The best is to use mmap. But if you fall back, malloc is OK, and we can > manually align it. In any case the loader ends up adding part of the > image to the GC roots, so it's probably best that the memory not be > managed by the GC itself. But malloc’d and mmap’d regions are not scanned by default, so here they would need to be scanned, or the GC-managed data they refer to would need to be registered as root, no? (This reminds me of <http://thread.gmane.org/gmane.comp.programming.garbage-collection.boehmgc/2576>, except that with Guile’s loader GNU_RELRO regions could refer to GC-managed data.) >>> + /* If our optimism failed, fall back. */ >>> + { >>> + unsigned alignment = sniff_elf_alignment (data, end); >>> + >>> + if (alignment != 8) >> >> Since .go are only produced by Guile, can it really happen to have .go >> files without 8-byte alignment? > > Yes! If the system supports mmap, .go files should be produced with > 4096-byte alignment so that the whole thing can be mapped read-only, and > write permissions only need be given to the writable pages. (Mprotect > only works on a page level.) Oh, I’m lost. Is it 8B or 4KiB? Can’t .go files always be page-aligned? Ludo’.
