3 ms·

IIRC, there are passes that also detect memcpy-"equivalent" loops and replace them with the builtin to memcpy (mostly to get the vectorization benefit). And of

by boulos 11y ago

IIRC, there are passes that also detect memcpy-"equivalent" loops and replace them with the builtin to memcpy (mostly to get the vectorization benefit). And of course there's SimplifyLibCalls (or whatever it's named) that undoes this or any other "you called a standard library function we know how to inline in this case".

majewsky 11y ago
The second one sounds like big fun when you LD_PRELOAD a different memcpy (say, for debugging purposes) and spend hours and hours trying to figure out why it is not being called.
- boulos 11y ago
  Yes, it's a nightmare, particularly on OS X with its funny loader semantics that make replacing malloc and free correctly very painful. For example, you can easily LD_PRELOAD your way out of malloc() but did you remember that asprintf also calls an allocation routine and won't use your new malloc? Enjoy.
  - ktRolster 11y ago
    For debugging purposes, it can sometimes be easier to include a header file "mem_debug.h" or whatever, and redefine malloc: >#define malloc(a) debug_malloc(a, __FILE__, __LINE__) >#define free(a) debug_free(a, __FILE__, __LINE__) >. >. >. Then in your debug_malloc() function you can track where every allocation took place. Not always applicable but nice when it works.
  - comex 11y ago
    Hmm, why won't it? Maybe in the past when asprintf and malloc were located in the same dylib, since OS X dylibs generate direct calls to other public functions in the same image (rather than going through the PLT as GNU does by default), but for many years /usr/lib/libSystem.B.dylib has been split into several reexported libraries in /usr/lib/system, so there shouldn't be an issue there. Probably better to use this technique though (should still work in the latest version but needs mprotect): http://eatmyrandom.blogspot.com/2010/03/mallocfree-interception-on-mac-os-x.html http://eatmyrandom.blogspot.com/2010/03/mallocfree-intercept...
  - astrange 11y ago
    malloc is set up to be specifically overridable so that guardmalloc(3)* will work. Are you saying asprintf doesn't work with guardmalloc? * check it out, I used a man reference, am I a professional UNIX developer yet?
- achamayou 11y ago
  GCC does the exact same thing, and it can be turned off with -fno-builtin.

by boulos 11y ago

majewsky 11y ago

The second one sounds like big fun when you LD_PRELOAD a different memcpy (say, for debugging purposes) and spend hours and hours trying to figure out why it is not being called.

boulos 11y ago
Yes, it's a nightmare, particularly on OS X with its funny loader semantics that make replacing malloc and free correctly very painful. For example, you can easily LD_PRELOAD your way out of malloc() but did you remember that asprintf also calls an allocation routine and won't use your new malloc? Enjoy.
- ktRolster 11y ago
  For debugging purposes, it can sometimes be easier to include a header file "mem_debug.h" or whatever, and redefine malloc: >#define malloc(a) debug_malloc(a, __FILE__, __LINE__) >#define free(a) debug_free(a, __FILE__, __LINE__) >. >. >. Then in your debug_malloc() function you can track where every allocation took place. Not always applicable but nice when it works.
- comex 11y ago
  Hmm, why won't it? Maybe in the past when asprintf and malloc were located in the same dylib, since OS X dylibs generate direct calls to other public functions in the same image (rather than going through the PLT as GNU does by default), but for many years /usr/lib/libSystem.B.dylib has been split into several reexported libraries in /usr/lib/system, so there shouldn't be an issue there. Probably better to use this technique though (should still work in the latest version but needs mprotect): http://eatmyrandom.blogspot.com/2010/03/mallocfree-interception-on-mac-os-x.html http://eatmyrandom.blogspot.com/2010/03/mallocfree-intercept...
- astrange 11y ago
  malloc is set up to be specifically overridable so that guardmalloc(3)* will work. Are you saying asprintf doesn't work with guardmalloc? * check it out, I used a man reference, am I a professional UNIX developer yet?
achamayou 11y ago
GCC does the exact same thing, and it can be turned off with -fno-builtin.