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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/59644] [4.9 Regression] r206243 miscompiles Linux kernel
Date: Mon, 06 Jan 2014 11:28:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59644

--- Comment #21 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Markus Trippelsdorf from comment #20)
> (In reply to Jakub Jelinek from comment #19)
> > (In reply to Markus Trippelsdorf from comment #17)
> > > If I add "pushq   %r10" and "popq    %r10" by hand, the kernel boots fine.
> > 
> > Thanks.  Now, can you please try to use pushq %r9 and popq %r9 instead?
> 
> Adding "pushq %r9" and "popq %r9" also results in a bootable kernel...

Ah, I see what is going wrong, sorry for your wasted time.

The problem is that ix86_save_reg is called already with
crtl->stack_realign_finalized == false, e.g. in
ix86_initial_elimination_offset,
at that point ix86_save_reg will return true for the drap register and thus it
is accounted for during RA, but if we don't count it later on,
ix86_compute_frame_layout will compute a different layout.