[Bug target/83994] New: %ebx is clobbered by stack-clash probing for regparm-3 function in PIC mode

["fw at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83994

            Bug ID: 83994
           Summary: %ebx is clobbered by stack-clash probing for regparm-3
                    function in PIC mode
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fw at gcc dot gnu.org
  Target Milestone: ---
            Target: i686

Created attachment 43219
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43219&action=edit
regparm-3-pic-probe.c

The attached reproducer, when compiled with -O2 -m32 -march=i686 -fpic
-fstack-clash-protection, clobbers %ebx in PIC mode due to the probing at the
start of the function.  (Seen on trunk@25693.)

The comment on get_scratch_register_on_entry says:

/* Return a short-lived scratch register for use on function entry.
   In 32-bit mode, it is valid only after the registers are saved
   in the prologue.  This register must be released by means of
   release_scratch_register_on_entry once it is dead.  */

This is called from ix86_adjust_stack_and_probe_stack_clash, but in the
reproducer, this happens when int_registers_saved is still false in
ix86_expand_prologue.  It seems that in this case, ix86_save_reg cannot be used
to select a register which is save to clobber.