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Re: [4.9 PATCH, alpha]: Switch alpha to LRA


2013/4/23 Steven Bosscher <stevenb.gcc@gmail.com>:
> On Mon, Apr 22, 2013 at 7:33 PM, Jeff Law wrote:
>> On 04/22/2013 11:17 AM, Uros Bizjak wrote:
>>>
>>> On Tue, Jan 29, 2013 at 12:34 AM, Richard Henderson <rth@redhat.com>
>>> wrote:
>>>>
>>>> On 01/28/2013 03:14 PM, Uros Bizjak wrote:
>>>>>
>>>>>
>>>>> 2013-01-28  Uros Bizjak<ubizjak@gmail.com>
>>>>>
>>>>>          * config/alpha/alpha.c (TARGET_LRA_P): New define.
>>>>>
>>>>> Bootstrapped and regression tested [1] on alphaev68-unknown-linux-gnu.
>>>>>
>>>>> OK for 4.9?
>>>>>
>>>>
>>>> Yep.
>>>
>>>
>>> Unfortunately, alphas are much more tied to reload than it was hoped.
>>> While latest alphas (with FIX and BWX ISAs) survived transition to LRA
>>> without problems, further testing on ev4 and ev5 triggered various
>>> problems, one of them is PR57032 [1] that exposed rather unique way of
>>> handling aligned/nonaligned memory operands.
>>>
>>> The patch was reverted.
>>>
>>> I suspect that fixing older alphas to live with LRA would be quite
>>> involved task, and I guess nobody (including me) wants to spend
>>> considerable amount of time on a dying architecture. Consequently,
>>> this also means that alphas will die together with reload as far as
>>> gcc is concerned.
>>>
>>> [1] http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57032
>>
>> Would it make sense to deprecate the older Alpha implementations without
>> killing the "modern" ones?
>
> Right. That would also eliminate the NOTE_INSN_EH_REGION notes bug (PR
> target/56858).
>
> I think it would be a shame to not enable LRA on alpha. It will only
> be another excuse to never let reload die, and it will hurt stability
> and reliability for Alpha EV6 in the long term as other targets switch
> over to LRA.
>
> Is it possible to add some EV4/EV5 splitters to work around this Alpha
> EV4/EV5 oddity? Even if it comes at a code quality cost, it's IMHO
> still better than tying the fate of apha to reload and vice versa..
>
> Ciao!
> Steven


How about using follow constraints?

Index: alpha.c
===================================================================
--- alpha.c     (revision 198216)
+++ alpha.c     (working copy)
@@ -9871,6 +9871,9 @@
 #undef TARGET_LEGITIMATE_ADDRESS_P
 #define TARGET_LEGITIMATE_ADDRESS_P alpha_legitimate_address_p

+#undef TARGET_LRA_P
+#define TARGET_LRA_P hook_bool_void_true
+
 #undef TARGET_CONDITIONAL_REGISTER_USAGE
 #define TARGET_CONDITIONAL_REGISTER_USAGE alpha_conditional_register_usage

Index: alpha.md
===================================================================
--- alpha.md    (revision 198216)
+++ alpha.md    (working copy)
@@ -4073,9 +4073,9 @@

 (define_insn "*movdi"
   [(set (match_operand:DI 0 "nonimmediate_operand"
-                               "=r,r,r,r,r,r,r,r, m, *f,*f, Q, r,*f")
+                               "=r,r,r,r,r,r,r,r, m, *f,*f, m, r,*f")
        (match_operand:DI 1 "input_operand"
-                               "rJ,K,L,T,s,n,s,m,rJ,*fJ, Q,*f,*f, r"))]
+                               "rJ,K,L,T,s,n,s,m,rJ,*fJ, m,*f,*f, r"))]
   "register_operand (operands[0], DImode)
    || reg_or_0_operand (operands[1], DImode)"
   "@


As Uros said, the 'Q' is ignored by LRA.
The reason is that the predicate function normal_memory_operand()
may change op to a memory location by using resolved_reload_operand().
However, if LRA is enabled, resolve_reload_operand() always returns
original reg op because reload_in_progress would never be true,
resulting reload loop in this case.

So I guess using 'm' constraint instead of 'Q' is able to avoid
such abnormal behavior, leaving all the reload jobs to LRA.
IMHO that is a simplest solution.  At least it passes the case in
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57032
and successfully build libgcc.


Best regards,
jasonwucj


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