RTL cprop vs. fixed hard regs

[Jeff Law]

On 01/16/15 02:42, Alan Modra wrote:
> https://lists.ozlabs.org/pipermail/linuxppc-dev/2014-December/123776.html
> shows gcc-5 miscompiling a powerpc64 linux kernel.  The executive
> summary is that the rs6000 backend has a bug in its RTL description of
> indirect calls.  We specify a parallel containing both the actual call
> and an action that happens after the call, the restore of r2.  The
> restore is simply a memory load:
>              (set (reg:DI 2 2)
>                  (mem/v/c:DI (plus:DI (reg/f:DI 1 1)
>                          (const_int 40 [0x28])) [0  S8 A8]))
> This leads to cprop concluding that it is valid to replace the
> reference to r1 with another register having the same value before the
> call.  Unfortunately, sometimes a call-clobbered register is chosen.
>
> OK, so we need to fix this in the rs6000 backend, but it occurs to me
> that cprop also has a bug here.  It shouldn't be touching fixed hard
> registers.  Bootstrapped and regression tested powerpc64-linux.  OK
> for mainline?
>
> 	* cprop.c (do_local_cprop): Disallow replacement of fixed
> 	hard registers.
OK.  Extra credit for a testcase, ppc specific is obviously OK.

jeff