Re: [PATCH] PR 62173, re-shuffle insns for RTL loop invariant hoisting

[Jiong Wang <wong dot kwongyuan dot tools at gmail dot com>]

2014-12-19 15:21 GMT+00:00 Kenneth Zadeck <zadeck@naturalbridge.com>:
>
> however, since i am a nice person ....
>
> loop-invariant solves the DF_UD_CHAIN which builds a data structure that
> connects each use with all of the defs that reach it.   I believe that this
> is the opposite of what you want here.
>
> if you really need this, you need to also turn on the DF_DU_CHAIN which
> builds the opposite structure.    Both structures can be space hogs, but
> they are both turned on in other places in the compiler so it is unlikely to
> be an issue.

Exactly, Thanks, Kenneth.

This approach works from my experiment and look much better than
previous REG_NOTE approach.
while it do have one problem. We need the UD/DU chain built before we
do insn re-shuffling.
While after re-shuffling, UD chain needs update, otherwise, the later
"check_dependecies" in find_invariant_insn may fail.

although we have re-shuffle instruction 1 into 2, the later check
still using old UD info, thus
decide instruction 2 is not iv.

1: regA <- vfp + regB
2: regA <- vfp + const

my current fix is to insert those re-shuffled insn into a table named
"vfp_const_iv", then skip those
dependencies check  for them as they don't have any dependencies.

>
>
>
>>
>>>>> LOG_LINKs have nothing to do with single use; they point from the
>>>>> _first_
>>>>> use to its corresponding def.
>>>>>
>>>>> You might want to look at what fwprop does instead.
>>>>
>>>> Pass rtl fwprop uses df information in single-definition way, it
>>>> doesn't really take into consideration if register is a single use.
>>>> This often corrupts other optimizations like post-increment and
>>>> load/store pair.  For example:
>>>>
>>>>    add r2, r1, r0
>>>>    ldr rx, [r2]
>>>>    add r2, r2, #4
>>>> is transformed into below form:
>>>>    add r2, r1, r0
>>>>    ldr rx, [r1, r0]
>>>>    add r2, r2, #4
>>>>
>>>> As a result, post-increment opportunity is corrupted, also definition
>>>> of r2 can't be deleted because it's not single use.
>>>>
>>>> Thanks,
>>>> bin
>>>
>>> thanks for all these suggestion.
>>>
>>> Have look at the LOG_LINK build function, a simple reverse scan, while
>>> needs to allocate big map array for all pseudo regs. Haven't looked at
>>> similar code in fwprop,
>>>
>>> actually, when found the first matched insn pattern, I just want to
>>> scan several insns next, then abort quickly if nothing meet
>>> requirement. there is no need to build full single-use information.
>>>
>>> still can anyone confirm that it is safe to re-use REG_DEAD info there
>>> without calling df_note_add_problem and df_analysis first? or I am
>>> using those info passed down from the previous pass which calculated
>>> these info and maybe broken?
>>
>> It's not safe to use REG_DEAD info without re-computing it.
>
> not sure that reg_dead is the right answer even if you did re-compute it.
> I believe you can have two parallel uses (on both sides of an if-then-else)
> for a single def (above the if then else) and have two REG_DEAD notes.
>
>> Richard.
>
>