[PATCH v2] elf: Add optimization barrier for __ehdr_start and _end

[Jakub Jelinek]

On Mon, Jun 16, 2025 at 08:31:24AM +0200, Florian Weimer wrote:
> >> It doesn't say why "g" is the appropriate constraint here.
> >
> > "+g" is the least restricted constraint.  It tells compilers that the
> > operand is both input and output.
> 
> If it's least restricted, why can't this trigger the use of a vector
> register?  It seems to increase the risk of the bug happening, not
> suppress it.

The question is on what it is used.  Either "+r" is used on the value of the
address before storing it into memory, that ensures the address is loaded
into a register first and then the register is later stored into memory,
possibly vectorized.
The other option is to break the vectorization by adding a read/write
optimization barrier in between the two stores into memory, what the patch
does (though because _dl_rtld_map.l_whatever is certainly in memory, I think
using "=m" (_dl_rtld_map.l_whatever) : "m" (_dl_rtld_map.l_whatever)
might be clearer; or even the whole _dl_rtld_map).  That will prevent the
vectorization, each store has to be done separately, so no vectorization
of the two adjacent stores is possible.

	Jakub