Implement link-time optimization safe explicit_bzero()?

[Sebastian Huber]

----- Manuel López-Ibáñez <lopezibanez@gmail.com> schrieb:
> On 12/02/16 06:21, Sebastian Huber wrote:
> >> If I were to tackle this problem, I would try first to implement a
> >> __builtin_explicit_bzero() directly in GCC such that GCC will know exactly how
> >> to handle it. This would be faster, safer and more portable than any trick you
> >> may get working now (since LTO keeps improving), and it will provide the basis
> >> for a gnu libc implementation.
> >
> > Ok, since I am not a compiler expert, I need an example for this.  My first try was to search for the memset_s() builtin. However, this doesn't seem to exist.  Is memset_s() supported at all by GCC currently?
> 
> See https://gcc.gnu.org/git/?p=gcc.git;a=blob_plain;f=gcc/builtins.def;hb=HEAD
> 
> memset_s is not there, so I guess it is not provided as a builtin, but 
> implemented by the C library. Probably it is not LTO-safe either.
> 
> But there is:
> 
> DEF_EXT_LIB_BUILTIN (BUILT_IN_BZERO, "bzero", BT_FN_VOID_PTR_SIZE, 
> ATTR_NOTHROW_LEAF_LIST)
> 
> No need to be a compiler expert to hack on GCC. And I'm sure you are more 
> qualified than I am.

Yes, this is the easy part.  However, the expand_builtin_memset_args() in builtins.c doesn't look so easy to me.  I really have no idea how to tell GCC that it should stop to optimize this particular memset() variant away.  This is why I asked for the memset_s().

> 
> In any case, these questions are more appropriate for gcc@ and gcc-patches@. 
> Few devs read gcc-help@
> 
> >> Of course, there is the question of whether explicit_bzero() or similar
> >> functions are a solution at all to any problem
> >> (https://sourceware.org/ml/libc-alpha/2016-01/msg00208.html)
> >
> > Would a builtin not fix this issue? I am not sure if I understood this post at all.
> 
> Sorry, wrong link. Correct one: 
> https://gcc.gnu.org/ml/gcc-help/2014-10/msg00061.html

Thanks for all the useful links.  A best-effort approach is probably all what we can get at the moment.

-- 
Sebastian Huber, embedded brains GmbH

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