Re: [patch] combine ICE fix

[Cesar Philippidis <cesar at codesourcery dot com>]

On 10/15/13 12:16 PM, Jeff Law wrote:
> On 10/10/13 10:25, Jakub Jelinek wrote:
>> On Thu, Oct 10, 2013 at 07:26:43AM -0700, Cesar Philippidis wrote:
>>> This patch addresses an ICE when building qemu for a Mips target in
>>> Yocto. Both gcc-trunk, gcc-4.8 and all of the targets are potentially
>>> affected. The problem occurs because the instruction combine phase uses
>>> two data structures to keep track of registers, reg_stat and
>>> regstat_n_sets_and_refs, but they potentially end up out of sync; when
>>> combine inserts a new register into reg_stat it does not update
>>> regstat_n_sets_and_refs. Failing to update the latter results in an
>>> occasional segmentation fault.
>>>
>>> Is this OK for trunk and gcc-4.8? If so, please check it in. I tested it
>>> on Mips and x86-64 and no regressions showed up.
>>
>> That looks broken.  You leave everything from the last size till the
>> current
>> one uninitialized, so it would only work if combine_split_insns
>> always increments max_reg_num () by at most one.
> I don't think that assumption is safe.  Consider a parallel with a bunch
> of (clobber (match_scratch)) expressions.

I address that in the patch posted here
<http://gcc.gnu.org/ml/gcc-patches/2013-10/msg00802.html>. Is that still
insufficient?

>> Furthermore, there are macros which should be used to access
>> the fields, and, if the vector is ever going to be resized, supposedly
>> it should be vec.h vector rather than just array.
>> Or perhaps take into account:
>> /* If a pass need to change these values in some magical way or the
>>     pass needs to have accurate values for these and is not using
>>     incremental df scanning, then it should use REG_N_SETS and
>>     REG_N_USES.  If the pass is doing incremental scanning then it
>>     should be getting the info from DF_REG_DEF_COUNT and
>>     DF_REG_USE_COUNT.  */
>> and not use REG_N_SETS etc. but instead the df stuff.
> Which begs the question, how exactly is combine utilizing the
> regstat_n_* structures and is that use even valid for combine?

I'll take a look at that.

Cesar