Re: [PATCH, ARM] stop changing signedness in PROMOTE_MODE

[Jeff Law <law@redhat.com>]

On 06/29/2015 07:15 PM, Jim Wilson wrote:
> This is my suggested fix for PR 65932, which is a linux kernel
> miscompile with gcc-5.1.
>
> The problem here is caused by a chain of events.  The first is that
> the relatively new eipa_sra pass creates fake parameters that behave
> slightly differently than normal parameters.  The second is that the
> optimizer creates phi nodes that copy local variables to fake
> parameters and/or vice versa.  The third is that the ouf-of-ssa pass
> assumes that it can emit simple move instructions for these phi nodes.
> And the fourth is that the ARM port has a PROMOTE_MODE macro that
> forces QImode and HImode to unsigned, but a
> TARGET_PROMOTE_FUNCTION_MODE hook that does not.  So signed char and
> short parameters have different in register representations than local
> variables, and require a conversion when copying between them, a
> conversion that the out-of-ssa pass can't easily emit.
So if these "copies" require a  conversion, then isn't it fundamentally 
wrong to have a PHI node which copies between them?  That would seem to 
implicate the eipa_sra pass as needing to be aware of these promotions 
and avoid having these objects with different representations appearing 
on the lhs/rhs of a PHI node.

Jeff