Re: [4/7] Use correct promoted mode sign for result of GIMPLE_CALL

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, Sep 8, 2015 at 11:50 PM, Jim Wilson <jim.wilson@linaro.org> wrote:
> On 09/08/2015 08:39 AM, Jeff Law wrote:
>> Is this another instance of the PROMOTE_MODE issue that was raised by
>> Jim Wilson a couple months ago?
>
> It looks like a closely related problem.  The one I am looking at has
> confusion with a function arg and a local variable as they have
> different sign extension promotion rules.  Kugan's is with a function
> return value and a local variable as they have different sign extension
> promotion rules.
>
> The bug report is
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65932
>
> The gcc-patches thread spans a month end boundary, so it has multiple heads
> https://gcc.gnu.org/ml/gcc-patches/2015-06/msg02132.html
> https://gcc.gnu.org/ml/gcc-patches/2015-07/msg00112.html
> https://gcc.gnu.org/ml/gcc-patches/2015-07/msg00524.html
>
> Function args and function return values get the same sign extension
> treatment when promoted, this is handled by
> TARGET_PROMOTE_FUNCTION_MODE. Local variables are treated differently,
> via PROMOTE_MODE. I think the function arg/return treatment is wrong,
> but changing that is an ABI change which is undesirable.  I suppose we
> could change local variables to match function args and return values,
> but I think that is moving in the wrong direction.  Though Kugan's new
> optimization pass will remove some of the extra unnecessary sign/zero
> extensions added by the arm TARGET_PROMOTE_FUNCTION_MODE definition, so
> maybe it won't matter enough to worry about any more.
>
> If we can't fix this in the arm backend, then we may need different
> middle fixes for these two cases.  I was looking at ways to fix this in
> the tree-out-of-ssa pass.  I don't know if this will work for Kugan's
> testcase, I'd need time to look at it.

I think the function return value should have been "promoted" according to
the ABI by the lowering pass.  Thus the call stmt return type be changed,
exposing the "mismatch" and compensating the IL with a sign-conversion.

As for your original issue with function arguments they should really
get similar
treatment, eventually in function arg gimplification already, by making
the PARM_DECLs promoted and using a local variable for further uses
with the "local" type.  Eventually one can use DECL_VALUE_EXPR to fixup
the IL, not sure.  Or we can do this in the promotion pass as well.

Richard.

> Jim
>