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Re: RTL cprop vs. fixed hard regs
- From: Jeff Law <law at redhat dot com>
- To: gcc-patches at gcc dot gnu dot org
- Date: Fri, 16 Jan 2015 09:35:16 -0700
- Subject: Re: RTL cprop vs. fixed hard regs
- Authentication-results: sourceware.org; auth=none
- References: <20150116094227 dot GV23768 at bubble dot grove dot modra dot org>
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