[llvm-dev] Errononous scheduling of COPY instruction.

[Dominique Torette via llvm-dev]

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
System Architect
Rue des Chasseurs Ardennais - Liège Science Park - B-4031 Angleur
Tel: +32 (0) 4 361 81 11 - Fax: +32 (0) 4 361 81 20
www.spacebel.be<http://www.spacebel.be/>



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