Missed ssa-copyrename optimization?

[Bin.Cheng]

On Tue, Jan 8, 2013 at 6:42 PM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Tue, Jan 8, 2013 at 8:51 AM, Bin.Cheng <amker.cheng@gmail.com> wrote:
>> Hi,
>> For attached preprocessed file, dump file
>> lib_a-s_frexp.E.021t.copyrename1 contains gimple sequences like:
>>
>>   <bb 4>:
>>   x_41 = x_8(D);
>>   goto <bb 8>;
>>
>>   <bb 5>:
>>   if (ix_15 <= 1048575)
>>     goto <bb 6>;
>>   else
>>     goto <bb 7>;
>>
>>   <bb 6>:
>>   x_19 = x_8(D) * 1.8014398509481984e+16;
>>   gh_u.value = x_19;
>>   _21 = gh_u.parts.msw;
>>   hx_22 = (__int32_t) _21;
>>   gh_u ={v} {CLOBBER};
>>   ix_24 = hx_22 & 2147483647;
>>   *eptr_16(D) = -54;
>>
>>   <bb 7>:
>>   # x_1 = PHI <x_8(D)(5), x_19(6)>
>>   # hx_2 = PHI <hx_11(5), hx_22(6)>
>>   # ix_3 = PHI <ix_15(5), ix_24(6)>
>>   _26 = *eptr_16(D);
>>   _27 = ix_3 >> 20;
>>   _28 = _27 + -1022;
>>   _29 = _26 + _28;
>>   *eptr_16(D) = _29;
>>   hx.0_31 = (unsigned int) hx_2;
>>   _32 = hx.0_31 & 2148532223;
>>   _33 = _32 | 1071644672;
>>   hx_34 = (__int32_t) _33;
>>   sh_u.value = x_1;
>>   hx.1_36 = (unsigned int) hx_34;
>>   sh_u.parts.msw = hx.1_36;
>>   x_38 = sh_u.value;
>>   sh_u ={v} {CLOBBER};
>>   x_40 = x_38;
>>
>>   <bb 8>:
>>   # _4 = PHI <x_41(4), x_40(7)>
>>   return _4;
>>
>> I am not sure if phi node (_4, x_41, x_40) in basic block 8 should be
>> rename/partitioned into one partition.
>> Trunk does not do this rename/partition, which causing big regression
>> in code size on Thumb1.
>>
>> BTW, the command line is:
>> arm-none-eabi-gcc -mthumb -mcpu=cortex-m0 -Os/-O2 ...
>>
>> I noticed the behavior is intended by PR54200, but not sure how the
>> issue should be handled. Any suggestions?
>
> Missed coalescing should be handled by adjusting the out-of-SSA coalescing
> code, not by copyrename (which really should go - it should not be needed
> with VTA and var-tracking around).
>
> Richard.
>
Thanks for answering, I will learn that part of code.

--
Best Regards.