On Wed, May 04, 2016 at 12:41:25AM +0200, Matthias Klose wrote: > On 03.05.2016 22:50, Josh Triplett wrote: > > Debian Policy requires the use of -fPIC for shared libraries, but > > documents potential exceptions for libraries with position-dependent > > assembly, and for libraries that would incur a significant performance > > hit. We can't do anything automated about the former, only fix the > > assembly. But regarding the latter, conventional wisdom advises that > > -fPIC incurs a particularly high cost on x86-32, since it burns a > > register across the whole program, of which the architecture has > > precious few. > > > > However, since GCC 5, that advice doesn't actually hold true anymore. > > Quoting https://gcc.gnu.org/gcc-5/changes.html: > > > Reuse of the PIC hard register, instead of using a fixed register, was > > > implemented on x86/x86-64 targets. This improves generated PIC code > > > performance as more hard registers can be used. Shared libraries can > > > significantly benefit from this optimization. > > > > While this doesn't make PIC absolutely free, it does eliminate almost > > all of the cost, to the point that it no longer seems worthwhile to > > build without -fPIC. Apart from that, building *all* code with -fPIC > > (including both programs and libraries) helps with hardening. > > > > With GCC 5 already the default as of the last stable release, I think > > we may want to reevaluate the conventional wisdom. Perhaps this > > information might help support the current iteration of any proposals to > > adopt more hardening flags by default, for instance. > > No, GCC 5 is not part of jessie.
Misreading of https://packages.debian.org/gcc; skimmed through the versions too quickly. Nonetheless, GCC 5 is available in current testing and unstable, and this optimization will apply to the next stable release, so I think the point still holds. > Did you intend to speak about -fPIE instead of -fPIC? No, I was referring to -fPIC, though for an executable -fPIE would be more appropriate. I would hope the same optimization applies there too. - Josh Triplett
