Dominique Torette via llvm-dev
2018-Sep-20 15:27 UTC
[llvm-dev] Errononous scheduling of COPY instruction.
Hi,
I've instruction scheduling problem that I cannot further investigate by
myself... Could someone give me some clues?
After Instruction selection, here is part of the generated instruction.
NOP
MOV_AB_ro @s1, %fab_roff0
%6:fpuaoffsetclass = COPY %fab_roff0; FPUaOffsetClass:%6
MOV_A_oo %6, def %5; FPUaOffsetClass:%6,%5
MOVSUTO_A_iSLo 24575, def %7; FPUaOffsetClass:%7
The order of instruction is very important: the COPY shall take place after the
MOV_AB_ro!
But during Instruction scheduling, these two instructions have been permutated!
SU(18): NOP
SU(20): %6:fpuaoffsetclass = COPY %fab_roff0; FPUaOffsetClass:%6
SU(19): MOV_AB_ro @s1, %fab_roff0
SU(21): MOV_A_oo %6, def %5; FPUaOffsetClass:%6,%5
SU(22): MOVSUTO_A_iSLo 24575, def %7; FPUaOffsetClass:%7
I'm trying to understand why the Instruction scheduler has swapped these two
instructions...
>From the trace, it seems that the Successors of SU(19) is SU(21) and not
SU(20) as expected...
What kind of information between the MOV_AB_ro and COPY could be missing?
How to define that the pred of SU(20) should be SU(19)?
SU(19): MOV_AB_ro @s1, %fab_roff0
# preds left : 1
# succs left : 1
# rdefs left : 0
Latency : 0
Depth : 21
Height : 25
Predecessors:
SU(18): Ord Latency=1 Barrier
Successors:
SU(21): Ord Latency=0 Barrier
Pressure Diff :
Single Issue : false;
SU(20): %6:fpuaoffsetclass = COPY %fab_roff0; FPUaOffsetClass:%6
# preds left : 0
# succs left : 1
# rdefs left : 0
Latency : 0
Depth : 0
Height : 25
Successors:
SU(21): Data Latency=0 Reg=%6
Pressure Diff : FPUaOffsetClass -1 FPUabOffsetClass -1
Single Issue : false;
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Dominique Torette
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Tim Northover via llvm-dev
2018-Sep-20 15:42 UTC
[llvm-dev] Errononous scheduling of COPY instruction.
Hi Dominique, On Thu, 20 Sep 2018 at 16:27, Dominique Torette via llvm-dev <llvm-dev at lists.llvm.org> wrote:> MOV_AB_ro @s1, %fab_roff0 > %6:fpuaoffsetclass = COPY %fab_roff0; FPUaOffsetClass:%6The MOV_AB_ro instruction there is only reading %fab_roff0 so there's no reason for LLVM to think it can't be swapped with the following copy. Should it maybe write that register instead? If not, what's the architectural constraint that *does* prevent movement? we might be able to help with how you need to describe that to LLVM. Cheers. Tim.
Dominique Torette via llvm-dev
2018-Sep-20 17:10 UTC
[llvm-dev] Errononous scheduling of COPY instruction.
Tim,
You are right, here is the definition of the MOV_AB_ro. No 'outs' !
def MOV_AB_ro : CLPFPU_AB_ro<0b1000010011,
(outs ),
(ins FPUabRegisterOperand:$RegA, FPUabOffsetOperand:$OffsetB),
[],[],
"mov_ab\t$RegA,$OffsetB","",
[],NoItinerary> {
let mayLoad = 1;
let TSFlags{12} = 1;
let DecoderMethod = "DecodeFPURO_moveInstruction";
}
My problem is that my processor has _not_ load/store instructions.
It does _not_ have external memory, only a bank of 512 internal registers...
So data memory, stack frames and temps are allocated from these 512 registers.
It only has some MOV instructions, supporting two modes of addressing: register
number addressing (r) or offset register related addressing (o).
These MOV instructions are sometime used to lower LOAD and sometime used to
lower STORE.
So MOV instructions are a little ambiguous. Feel free to comment this
architecture ;-)
To solve this ambiguity, I've introduced some LOAD PseudoInstroductions
(with customInserters to MOV instructions).
These PseudoInstructions are used during Instruction Selection and Register
Allocation. That works quite well.
But customInserters are called before the final scheduling. So the final
scheduling is performed using the MOV instructions.
If well understood, I've to define the (outs ) for the destination of all my
MOV definitions.
This will have many impact on my code (cusstomInserters, decoderMethods,...).
Could I use ImplictOuts to define dependency (less impact on the my code) ?
Thanks so much, Dominique T.
-----Original Message-----
From: Tim Northover [mailto:t.p.northover at gmail.com]
Sent: jeudi 20 septembre 2018 17:43
To: Dominique Torette
Cc: LLVM Developers Mailing List
Subject: Re: [llvm-dev] Errononous scheduling of COPY instruction.
Hi Dominique,
On Thu, 20 Sep 2018 at 16:27, Dominique Torette via llvm-dev
<llvm-dev at lists.llvm.org> wrote:> MOV_AB_ro @s1, %fab_roff0
> %6:fpuaoffsetclass = COPY %fab_roff0; FPUaOffsetClass:%6
The MOV_AB_ro instruction there is only reading %fab_roff0 so there's
no reason for LLVM to think it can't be swapped with the following
copy. Should it maybe write that register instead?
If not, what's the architectural constraint that *does* prevent
movement? we might be able to help with how you need to describe that
to LLVM.
Cheers.
Tim.
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