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

[Richard Earnshaw <Richard.Earnshaw@foss.arm.com>]

On 15/07/15 16:54, Jim Wilson wrote:
> On Wed, Jul 15, 2015 at 6:04 AM, Michael Matz <matz@suse.de> wrote:
>> Hi,
>>
>> On Tue, 14 Jul 2015, Jim Wilson wrote:
>>
>>> Now that we do have the problem, we can't fix it without an ARM port ABI
>>> change, which is undesirable, so we may have to fix it with a MI change.
>>
>> What's the ABI implication of fixing the inconsistency?
> 

I think that's the wrong question.  We wouldn't change the ABI to fix an
internal problem in GCC.  So the real question is what's the performance
impact of changing PROMOTE_MODE to be the same as the ABI requirements?

There's no easy answer to that since we'd have to consider all the
different architecture variants we currently have to support.  At the
very least we'd have to think about ARMv3 (no signed byte or half-word
loads), ARMv4 (signed byte and half-word loads), ARMv4t (thumb1) and
ARMv7 (thumb2).

R.

> Currently signed chars and signed shorts are passed sign-extended.  If
> we make TARGET_PROMOTE_FUNCTION_MODE work the same as PROMOTE_MODE,
> then they will be passed zero-extended.
> 
> Given the testcase:
> 
> int sub (int) __attribute__ ((noinline));
> int sub2 (signed char) __attribute__ ((noinline));
> int sub (int i) { return sub2 (i); }
> int sub2 (signed char c) { return c & 0xff; }
> 
> Currently sub will do a char sign-extend to convert the int to signed
> char, and sub2 will do a char zero-extend for the and.  With the
> change, sub will do a char zero-extend to convert the int to unsigned
> char, and sub2 will do nothing.  If you compile sub without the change
> and sub2 with the change, then you lose the and operation and get a
> sign-extended char at the end.
> 
>>> There were two MI changes suggested, one was fixing the out-of-ssa pass
>>> to handle SUBREG_PROMOTED_P promotions.  The other was to disallow
>>> creating PHI nodes between parms and locals.  I haven't had a chance to
>>> try implementing the second one yet; I hope to work on that today.
>>
>> Don't bother with the latter, it doesn't have a chance of being accepted.
> 
> I tried looking at it anyways, as I need to learn more about this
> stuff.  It didn't seem feasible without changing a lot of optimization
> passes which doesn't seem reasonable.
> 
>> If the terrible hack in outof-ssa really will be necessary (and I really
>> really hope it won't) then I think I prefer the approach you partly tried
>> in comment #12 of PR 65932 already.  Let partition_to_pseudo[] refer to
>> the promoted subreg and deal with that situation in emit_partition_copy;
>> I'd then hope that the unsignedsrcp parameter could go away (unfortunately
>> the sizeexp will have to stay).
> 
> Yes, I think that is a cleaner way to do it, but I had trouble getting
> that to work as I don't know enough about the code yet.  Doing it
> directly in emit_partition_copy was easier, just to prove it could
> work.  I can go back and try to make this work again.
> 
> Jim
>