Liu Xin
2013-Sep-20 02:50 UTC
[LLVMdev] Does Mips resolve hazard in pre-ra-sched or post-ra-sched?
Akira, Thanks you for response. I understand Post-RA schedule make uses of scoreboardHazardRecognizer. But I found mips codes are good enough by default. basically, I can not easily eyeball any bubbles. I don't understand how they can do that without post-RA-sched. pre-ra-scheduler eg. (SelectionDAG/ScheduleDAGRRList.cpp) has little information and they can only schedule node in topology order. It assumes any SU is one cycle delay. I don't think pre-ra-sched consider any pipeline details. thanks, --lx On Fri, Sep 20, 2013 at 5:39 AM, Akira Hatanaka <ahatanak at gmail.com> wrote:> Mips invokes the post-RA scheduler only when OptLevel > Aggressive, so you > will have to compile with -O3. > > You can also invoke the MI (pre-RA) scheduler with llc option > "-enable-misched". As you have pointed out, the post-isel scheduler is > mandatory, and therefore you don't have to give any command line options. > > Currently, mips has only one generic scheduling itinerary model in > MipsSchedule.td that is not tailored to any specific core, so you might > have to tweak it to have the scheduler generate efficient code for your > target. > > > On Wed, Sep 18, 2013 at 7:41 PM, Liu Xin <navy.xliu at gmail.com> wrote: > >> Hi, LLVM, >> >> I found LLVM codegen has 3 passes for instruction scheduling: >> >> 1) pre-ra sched >> 2) post-ra sched >> 3) mi sched. >> >> for RISC machines, there are data hazard cases appear only after Register >> Allocation(RA). for example, $t0 is used immediately after writing(RAW): >> >> ld $t0, MEM >> add $t2, $t0, $0 >> >> There may be one or more stall in pipeline. Instruction scheduler can >> detect this kinds of conflict and insert other instructions to avoid >> pipeline bubble. I think this work only can be done after RA. If so, >> what's the purpose for 1). I found 1) is mandatory and 2/3) are optional. >> Further, at least one target enable pre-RA-sched with harzardRecognizer. >> Does it really work out? you can resolve data hazard using pre-RA-sched >> only? >> >> thanks, >> --lx >> >> >> >> >> >> >> >> _______________________________________________ >> LLVM Developers mailing list >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >> >> >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20130920/c7e865e4/attachment.html>
Liu Xin
2013-Sep-20 10:30 UTC
[LLVMdev] Does Mips resolve hazard in pre-ra-sched or post-ra-sched?
Hi, Akira,
I found you maintain mips MipsSchedule.td. does it correct? in
MipsSchedule.td, every InstrItinData only uses one InstrStage. there's no
ByPass info out there.
are you sure this reflects the real R4xxx/R5xxx processors.
why IILoad uses funcition unit ALU?
InstrItinData<IILoad , [InstrStage<3, [ALU]>]>
for my previous question, I have new input after reading the code.
pre-RA-sched is derived from ScheduleDAGSNodes, but post-RA-sched and
mi-sched are both derived from ScheduleDAGInstrs.that means pre-RA-sched
schedules SDNodes. post-RA-sched schedules MIs.
from -debug-pass=Structure, we can see that the order is
"mi-sched"==>
RegisterAllocation==>post-RA TD.
Simple Register Coalescing
Machine Instruction Scheduler
Machine Block Frequency Analysis
...
Greedy Register Allocator
Virtual Register Rewriter
...
Post RA top-down list latency scheduler
Analyze Machine Code For Garbage Collection
...
for my testcase, I found -enable-misched is helpful for ARM, they reduce
stall numbers from 205 to 160. however, mips is adverse impact. the stall
number increases from 554 to 560. this doesn't make any sense.
thanks
--lx
On Fri, Sep 20, 2013 at 10:50 AM, Liu Xin <navy.xliu at gmail.com> wrote:
> Akira,
>
> Thanks you for response.
>
> I understand Post-RA schedule make uses of scoreboardHazardRecognizer. But
> I found mips codes are good enough by default. basically, I can not easily
> eyeball any bubbles.
> I don't understand how they can do that without post-RA-sched.
> pre-ra-scheduler eg. (SelectionDAG/ScheduleDAGRRList.cpp) has little
> information and they can only schedule node in topology order. It assumes
> any SU is one cycle delay. I don't think pre-ra-sched consider any
> pipeline details.
>
> thanks,
> --lx
>
>
>
> On Fri, Sep 20, 2013 at 5:39 AM, Akira Hatanaka <ahatanak at
gmail.com>wrote:
>
>> Mips invokes the post-RA scheduler only when OptLevel > Aggressive,
so
>> you will have to compile with -O3.
>>
>> You can also invoke the MI (pre-RA) scheduler with llc option
>> "-enable-misched". As you have pointed out, the post-isel
scheduler is
>> mandatory, and therefore you don't have to give any command line
options.
>>
>> Currently, mips has only one generic scheduling itinerary model in
>> MipsSchedule.td that is not tailored to any specific core, so you might
>> have to tweak it to have the scheduler generate efficient code for your
>> target.
>>
>>
>> On Wed, Sep 18, 2013 at 7:41 PM, Liu Xin <navy.xliu at gmail.com>
wrote:
>>
>>> Hi, LLVM,
>>>
>>> I found LLVM codegen has 3 passes for instruction scheduling:
>>>
>>> 1) pre-ra sched
>>> 2) post-ra sched
>>> 3) mi sched.
>>>
>>> for RISC machines, there are data hazard cases appear only after
>>> Register Allocation(RA). for example, $t0 is used immediately
after
>>> writing(RAW):
>>>
>>> ld $t0, MEM
>>> add $t2, $t0, $0
>>>
>>> There may be one or more stall in pipeline. Instruction scheduler
can
>>> detect this kinds of conflict and insert other instructions to
avoid
>>> pipeline bubble. I think this work only can be done after RA. If
so,
>>> what's the purpose for 1). I found 1) is mandatory and 2/3)
are optional.
>>> Further, at least one target enable pre-RA-sched with
harzardRecognizer.
>>> Does it really work out? you can resolve data hazard using
pre-RA-sched
>>> only?
>>>
>>> thanks,
>>> --lx
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>>
>>
>
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Akira Hatanaka
2013-Sep-25 03:48 UTC
[LLVMdev] Does Mips resolve hazard in pre-ra-sched or post-ra-sched?
On Fri, Sep 20, 2013 at 3:30 AM, Liu Xin <navy.xliu at gmail.com> wrote:> Hi, Akira, > > I found you maintain mips MipsSchedule.td. does it correct? in > MipsSchedule.td, every InstrItinData only uses one InstrStage. there's no > ByPass info out there. > are you sure this reflects the real R4xxx/R5xxx processors. > > why IILoad uses funcition unit ALU? > InstrItinData<IILoad , [InstrStage<3, [ALU]>]> > >This means IILoad instructions use resource ALU for three cycles. I don't remember why only two functional units (ALU and IMULDIV) are defined and used in this .td file, but this would be incorrect if load instructions did not have any resource conflicts with other ALU instructions on your target.> > for my previous question, I have new input after reading the code. > pre-RA-sched is derived from ScheduleDAGSNodes, but post-RA-sched and > mi-sched are both derived from ScheduleDAGInstrs.that means pre-RA-sched > schedules SDNodes. post-RA-sched schedules MIs. > > from -debug-pass=Structure, we can see that the order is "mi-sched"==> > RegisterAllocation==>post-RA TD. > > Simple Register Coalescing > Machine Instruction Scheduler > Machine Block Frequency Analysis > ... > Greedy Register Allocator > Virtual Register Rewriter > ... > Post RA top-down list latency scheduler > Analyze Machine Code For Garbage Collection > ... > > > for my testcase, I found -enable-misched is helpful for ARM, they reduce > stall numbers from 205 to 160. however, mips is adverse impact. the stall > number increases from 554 to 560. this doesn't make any sense. > >I don't know for sure why stalls increase. In my experience, both pre-RA and post-RA improve performance when I run the compiled executables on a mips board. But as I said in my previous email, the model in MipsSchedule.td is just a generic one, so it's possible that it isn't generating efficient code for your target.> thanks > --lx > > > > On Fri, Sep 20, 2013 at 10:50 AM, Liu Xin <navy.xliu at gmail.com> wrote: > >> Akira, >> >> Thanks you for response. >> >> I understand Post-RA schedule make uses of scoreboardHazardRecognizer. >> But I found mips codes are good enough by default. basically, I can not >> easily eyeball any bubbles. >> I don't understand how they can do that without post-RA-sched. >> pre-ra-scheduler eg. (SelectionDAG/ScheduleDAGRRList.cpp) has little >> information and they can only schedule node in topology order. It assumes >> any SU is one cycle delay. I don't think pre-ra-sched consider any >> pipeline details. >> >> thanks, >> --lx >> >> >> >> On Fri, Sep 20, 2013 at 5:39 AM, Akira Hatanaka <ahatanak at gmail.com>wrote: >> >>> Mips invokes the post-RA scheduler only when OptLevel > Aggressive, so >>> you will have to compile with -O3. >>> >>> You can also invoke the MI (pre-RA) scheduler with llc option >>> "-enable-misched". As you have pointed out, the post-isel scheduler is >>> mandatory, and therefore you don't have to give any command line options. >>> >>> Currently, mips has only one generic scheduling itinerary model in >>> MipsSchedule.td that is not tailored to any specific core, so you might >>> have to tweak it to have the scheduler generate efficient code for your >>> target. >>> >>> >>> On Wed, Sep 18, 2013 at 7:41 PM, Liu Xin <navy.xliu at gmail.com> wrote: >>> >>>> Hi, LLVM, >>>> >>>> I found LLVM codegen has 3 passes for instruction scheduling: >>>> >>>> 1) pre-ra sched >>>> 2) post-ra sched >>>> 3) mi sched. >>>> >>>> for RISC machines, there are data hazard cases appear only after >>>> Register Allocation(RA). for example, $t0 is used immediately after >>>> writing(RAW): >>>> >>>> ld $t0, MEM >>>> add $t2, $t0, $0 >>>> >>>> There may be one or more stall in pipeline. Instruction scheduler can >>>> detect this kinds of conflict and insert other instructions to avoid >>>> pipeline bubble. I think this work only can be done after RA. If so, >>>> what's the purpose for 1). I found 1) is mandatory and 2/3) are optional. >>>> Further, at least one target enable pre-RA-sched with harzardRecognizer. >>>> Does it really work out? you can resolve data hazard using pre-RA-sched >>>> only? >>>> >>>> thanks, >>>> --lx >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> _______________________________________________ >>>> LLVM Developers mailing list >>>> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >>>> >>>> >>> >> >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20130924/81556adb/attachment.html>
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