On Tue, Apr 17, 2018 at 11:55 AM, Uros Bizjak <ubiz...@gmail.com> wrote: > On Tue, Apr 17, 2018 at 8:42 PM, H.J. Lu <hongjiu...@intel.com> wrote: >> -fcf-protection -mcet can't be used with IFUNC features, like symbol >> multiversioning or target clone, since IBT/SHSTK are applied to the whole >> program and they may be disabled in some functions. But -fcf-protection >> is implemented with multi-byte NOPs on all 64-bit processors as well as >> 32-bit processors starting with Pentium Pro. If -fcf-protection requires >> -mcet, IFUNC features can't be used on Linux when -fcf-protection is >> enabled by default. >> >> This patch changes -fcf-protection to to enable the NOP portion of CET >> ISAs unless IBT and/or SHSTK are disabled explicitly. The rest of CET >> ISAs, including intrinsics, still requires -mcet, -mibt or -mshstk. >> >> OK for trunk? > > As said in the PR, NOP sequences have non-zero cost in the executable > (they enlarge the executable), so I don't think this feature should be > enabled by default. > > There is always a configure option if someone wants their compiler to > always emit relevant multi-byte nops.
What we need is an option to enable -fcf-function with multi-byte NOPs without -mcet which enables the full CET ISAs. A configure option without the corresponding the command-line option makes test and debug difficult. I can add --enable-cf-function-nop or --with-cf-function-nop with -fct-function-nop -- H.J.
