[RFC] Don't inline builtin memory functions when ASan is enabled.

[Maxim Ostapenko <m dot ostapenko at partner dot samsung dot com>]

Hi,

At this moment, most of GCC builtin memory functions (for example 
strcpy, stpcpy, wcpcpy, strdup, etc) are not instrumented by GCC, 
however some of them are rather dangerous. If GCC inlines these builtin 
functions, we will miss important checks for arguments, and possible 
overflow won't be detected. I know, that Clang ASan team simply disable 
inlining of builtin functions in Clang if -fsanitize=address is enabled 
and rely on libsanitizer's hooks.

The main benefit of this approach is that we won't miss overflow in 
builtins, that can significantly increase target programs safety. Also, 
some redundant checks will be removed for builtin functions, that are 
instrumented and are not inlined for some reasons.

The potential disadvantage of this approach is performance decreasing 
for sanitized programs.

Does disabling of builtin functions inlining look sane in this case? If 
yes, I can provide performance investigation and prepare the patch.

What do you think?

-Maxim