Re: [PATCH] Compile elf/rtld.c with -fno-tree-loop-distribute-patterns.

[Sandra Loosemore <sandra at codesourcery dot com>]

On 11/23/19 6:25 AM, Florian Weimer wrote:
> * Sandra Loosemore:
>
> > In GCC 10, the default at -O2 is now -ftree-loop-distribute-patterns.
> > This optimization causes GCC to "helpfully" convert the hand-written
> > loop in _dl_start into a call to memset, which is not available that
> > early in program startup.  Similar problems in other places in GLIBC
> > have been addressed by explicitly building with
> > -fno-tree-loop-distribute-patterns, but this one may have been
> > overlooked previously because it only affects targets where
> > HAVE_BUILTIN_MEMSET is not defined.
>
> Thanks for pointing out this problem.
>
> > +# On targets without __builtin_memset, rtld.c uses a hand-coded loop
> > +# in _dl_start.  Make sure this isn't turned into a call to regular memset.
> > +ifeq (yes,$(have-loop-to-function))
> > +CFLAGS-rtld.c += -fno-tree-loop-distribute-patterns
> > +endif
>
> Is it possible to do this via a pragma?  If I understand things
> correctly, this is not necessary for PI_STATIC_AND_HIDDEN targets
> (where initialization of the dynamic loader is simpler).

I see that the definitions of memset and memmove use 
"inhibit_loop_to_libcall" (which expands into an optimize attribute) to 
prevent recursion, but I didn't think the header where that is defined 
(include/libc-symbols.h) is supposed to be included in the dynamic 
linker?  Also, already in elf/Makefile there is another instance where 
it adds -fno-tree-loop-distribute-patterns to the CFLAGS, so I just 
copied that.  I don't work with glibc internals enough to have a good 
feel for what the preferred solution is but I'll test a different 
solution if this one isn't good enough.

-Sandra