On Wed, Sep 16, 2020 at 4:34 AM Borislav Petkov <b...@alien8.de> wrote: > > On Tue, Sep 15, 2020 at 05:50:54PM -0400, Arvind Sankar wrote: > > On Tue, Sep 15, 2020 at 10:49:12PM +0200, Borislav Petkov wrote: > > > > > > (Btw, clang doesn't need to add that "xor %eax,%eax" - panic() should not > > > be > > > returning, ever. :-)) > > > > > > > I think this is because panic() is varargs, and clang doesn't support > > gcc's -mskip-rax-setup. The normal ABI requires the caller to set RAX to > > the number of arguments in vector registers. > > > > https://patchwork.ozlabs.org/project/gcc/patch/20141218131150.ga32...@intel.com/ > > Ah, good point. Found this in the ABI doc: > > "For calls that may call functions that use varargs or stdargs > (prototype-less calls or calls to functions containing ellipsis (...) in > the declaration) %al is used as hidden argument to specify the number of > vector registers used. The contents of %al do not need to match exactly > the number of registers, but must be an upper bound on the number of > vector registers used and is in the range 0–8 inclusive."
pt. 3 https://nickdesaulniers.github.io/blog/2014/04/18/lets-write-some-x86-64/ ...2014, jesus... "But you never can tell exactly what the compiler is doing. It doesn’t always come out the way you want. Like a dusty pane or warped mirror." Seems I'd learn more about that than I'd could have ever envisioned... -- Thanks, ~Nick Desaulniers
