allocateBranchIsland is slow

[rentzsch]

See rentzsch/mach_star#38 for an attempt to address this.

Alexander Potapenko has enhancements that's part of llvm's AddressSanitizer, but unfortunately the link he gave me is now broken.

[ramosian-glider]

Citing my own letter:

Next, allocateBranchIsland() is very slow on 64 bits -- mainly because
of the necessity to scan the memory page by page with vm_allocate().
Some information about that is at
http://code.google.com/p/address-sanitizer/issues/detail?id=24,
basically our tests made 3M calls to vm_allocate in order to override
26 functions.
In order to fix that, we need to:
-- search for empty memory in a more efficient way (e.g. look for
holes in the process address space)
-- map several pages with a single mmap/vm_allocate call, so that
further mach_override_ptr() invocations can avoid searching and
mapping memory.
Because AddressSanitizer adds some limitations on the placement of the
mappings, I chose to externalize the branch island allocator to let
ASan decide where to place the islands.
(see http://llvm.org/viewvc/llvm-project?view=rev&revision=148115)
I'm a bit dissatisfied with the resulting code, mainly because of
casts between BranchIsland* and void_.
To cope with that it's better to pass the islands as void_ everywhere
and modify allocateBranchIsland so that it keeps track of additional
flags itself (it may still store them in the same location)

The LLVM version of mach_override resides at http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/interception/mach_override/
I'll try to find some time and prepare a patch.

[rentzsch]

@ramosian-glider thanks for the link, I'll take a look

[Fl0ri4n]

The link above is dead, but I assume this one is the same:
https://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/asan/mach_override/?diff_format=l&sortby=log&pathrev=146166

I had to use the linked version to successfully override functions (in my case: CGLFlushDrawable) on 10.8 using 32bit clang, the mach_override in the stable branch didn't work for me.
I think it should be merged into this project, at least in the unstable branch, it took me quite a while to find that patch.

[rentzsch]

@Fl0ri4n unfortunately I'm too busy with other projects to look into this. How does the stable branch not work for you? Crash? Just doesn't override?

[Fl0ri4n]

It just didn't override. I didn't debug it in more detail because I was overriding a function in a foreign precess using mach_inject.
The last working compile using the stable branch was on 10.7 using gcc 4.2 (not LLVM), but I‘ve got neither of them installed at the moment, so I can't test if a 32bit gcc-compile using the stable branch would work on Mountain Lion.