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Re: PATCH: reorg branch displacement fix
- From: Jeff Law <law at porcupine dot cygnus dot com>
- To: tm <tm at mail dot kloo dot net>
- Cc: Joern Rennecke <joern dot rennecke at superh dot com>, kkojima at gcc dot gnu dot org, gcc-patches at gcc dot gnu dot org
- Date: Fri, 01 Nov 2002 16:15:34 -0700
- Subject: Re: PATCH: reorg branch displacement fix
- Reply-to: law at redhat dot com
In message <Pine.LNX.firstname.lastname@example.org>, tm writes:
>On Fri, 1 Nov 2002, Joern Rennecke wrote:
>> > > * reorg.c (relax_delay_slots): Don't thread conditional jump
>> > > through unconditional jump if the conditional jump can't reach
>> > > the branch target on this processor target.
>> > This is wrong. reorg does not and should not be checking branch displace
>> > That is a problem for shorten-branches and the backend.
>> The SH does some early branch shortening in machine_dependent_reorg because
>> a long conditional branch has to be split into a short conditional branch
>> around an unconditional branch, or a short conditional branch to an uncondi
>> branch (that might have been inserted elsewhere after an inverted condbranc
>> Unconditional branches have mandatory delay slots, so we get additional
>> delay slots exposed by doing this splitting before reorg, and the condition
>> branches have optional delay slots, which obviously can't be used in the sa
>> way when the branch is reversed or redirected.
>> Thus, machine_dependent_reorg already makes sure all the conditional branch
>> that need splitting are split. If reorg redirects conditional branches
>> willy-nilly, it destroys the very data it is supposed to operate on.
>It sounds like there should be a machine-independent branch-lengthening
>pass for machines which have limited branch displacements. There must be
>other processors which have this same problem?
Or the SH port should use the model that all the other ports with
variable length insns + limited branch displacements + delay slots use.
That model is we don't worry about such things in reorg and deal with
branch shortening entirely in the branch shortening pass.