Re: [PATCH] Always enable LRA

[Jeff Law <jeffreyalaw@gmail.com>]

On 10/14/22 15:21, Koning, Paul wrote:
>
>> On Oct 14, 2022, at 5:15 PM, Jeff Law via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
>>
>>
>> On 10/14/22 11:36, Koning, Paul wrote:
>>>> On Oct 14, 2022, at 1:10 PM, Jeff Law <jeffreyalaw@gmail.com> wrote:
>>>>
>>>> On 10/14/22 10:37, Koning, Paul wrote:
>>>>>> ...
>>>>>> But that approach falls down with reload/lra doing substitutions without validating the result.  I guess it might be possible to cobble together something with secondary reloads, but it's way way way down on my todo list.
>>>>> Aren't the constraints enforced?  My experience is that I was getting these bad addressing modes in some test programs, and that the constraints I created to make the requirement explicit cured that.  Maybe I'm expecting too much from constraints, but my (admittedly inexperienced) understanding of them is that they inform reload what sort of things it can construct, and what it cannot.
>>>> It's not really a constraint issue -- the pattern's condition would cause this not to recognize, but LRA doesn't re-recognize the insn.  We might be able to hack something in the constraints to force a reload of the source operand in this case.   Ugly, but a possibility.
>>> I find it hard to cope with constraints that don't constrain.  Minimally it should be clearly documented exactly what cases fail to obey the constraints and what a target writer can do to deal with those failures.
>> Constraints have a purpose, but as I've noted, they really don't come into play here.   Had LRA tried to see if what it created as a valid move insn, the backend would have said "nope, that's not valid".  That's a stronger test than checking the constraints.  If the insn is not valid according to its condition, then the constraints simply don't matter.
>>
>> I'm not aware of a case where constraints are failing to be obeyed and constraints simply aren't a viable solution here other than to paper over the problem and hope it doesn't show up elsewhere.
>>
>> Right now operand 0's constraint is "<" meaning pre-inc operand, operand 1 is "r".  How would you define a new constraint for operand 1 that disallows overlap with operand 0 given that the H8 allows autoinc on any register operand?   You can't look at operand 0 while processing the constraint for operand 1. Similarly if you try to define a new constraint for operand0 without looking at operand1.
> Easy but cumbersome: define constraints for "register N" (for each N) and another set for "autoinc on any register other than N".  In pdp11, I called these Z0, Z1... and Za, Zb... respectively.  Then the insn gets constraints that look like "Z0,Z1,Z2..." and "Za, Zb, Zc..." for the two operands.  As I said, see pdp11.md, the mov insn.

It generally looks sound, but golly gee, this runs into the "reload 
doesn't validate insns problem"  if it's done in a reload tree rather 
than an lra tree.  We've got an insn with a pre-inc destination and a 
reg source.  The source pseudo doesn't get a hard reg, reload replaces 
the pseudo with a mem as expected. Reload finishes with something like this:

(insn 100 98 101 15 (set (mem/f:SI (pre_dec:SI (reg/f:SI 7 sp)) [8  S4 A32])
         (mem/c:SI (plus:SI (reg/f:SI 7 sp)
                 (const_int 8 [0x8])) [9 %sfp+-24 S4 A32])) "j.c":62:11 
19 {*movsix}
      (expr_list:REG_ARGS_SIZE (const_int 4 [0x4])
         (nil)))

Which, isn't a valid instruction on the H8.  The insn's condition 
verifies that one of the two operands must be a REG.  But reload never 
bothered to re-recognize the insn after makng the substitution, then 
naturally it blows up in reload_cse with a constraint failure because 
the pre-inc destination constraints require a reg for the source 
operand.  But the real culprit here is reload making the substitution 
and not validing that the result is valid.

Arggh!

Which brings me back to pondering just removing the autoinc magic 
checking in the H8 port :-)  I've actually got that spinning in the 
tester just to see if there's any obvious fallout.  I've already spent 
more time on this than I can reasonably justify.


Jeff