[PATCH] D126485: [SystemZ] Use SystemZVectorConstantInfo for more immediate stores.

Thu May 26 09:37:22 PDT 2022

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This is a follow-up patch that came up when working on the handling of stores of replicated values. This is the case that involves loading any integer constant into a register and immediately storing it. There are some cases where VGBM / VGM can be used for this.

This (at least currently) involves a new SystemZ method "isSupportedByScalarStore()" which is used to find cases where it would be worthwhile to use a vector constant instead. I have tried two variants:

1. Replace all immediate loads that would require one or two scalar instructions.
2. Only replace cases where two scalar instructions would be needed (-keep-imm-load passed).

(1) might make sense as there are more vector registers so register pressure might be improved. It seems that one time that gave less spills, and the other time it was more, and only with small differences, so that does not really seem to be a big deal.

(2) Is a bit more of a clear improvement as one immediate load is used instead of two scalar ones always.

  main <> patch  (1)
  vsteg          :                 6564                 9016    +2452
  stg            :               388272               385858    -2414
  vstef          :                 6615                 8761    +2146
  st             :               130605               128843    -1762
  vgmf           :                13125                14464    +1339
  vgmg           :                 8152                 9472    +1320
  llilh          :                 6956                 5825    -1131
  llihh          :                 2549                 1692     -857
  llihl          :                 7114                 6668     -446
  stfh           :                17089                16791     -298
  vgbm           :                11085                11288     +203
  oilh           :                 1539                 1351     -188
  llihf          :                11156                10978     -178
  oilf           :                 8959                 8819     -140
  iilf           :                 5084                 4965     -119
  iihf           :                 4918                 4814     -104
  lay            :                56028                56126      +98
  llilf          :                 2249                 2177      -72
  oill           :                18106                18045      -61
  ...
  OPCDIFFS: -212

  main <> patch with experimental option "-keep-imm-load" passed (2)
  stg            :               388272               387648     -624
  vsteg          :                 6564                 7177     +613
  vgmg           :                 8152                 8355     +203
  oilh           :                 1539                 1351     -188
  llihh          :                 2549                 2367     -182
  vgmf           :                13125                13304     +179
  oilf           :                 8959                 8819     -140
  llihl          :                 7114                 7003     -111
  llihf          :                11156                11065      -91
  oill           :                18106                18045      -61
  lg             :              1050050              1050003      -47
  vgbm           :                11085                11122      +37
  lay            :                56028                56058      +30
  mvc            :                59773                59755      -18
  lgr            :               842765               842758       -7
  cgije          :               142634               142629       -5
  iihf           :                 4918                 4913       -5
  je             :               337365               337370       +5
  ldr            :                51266                51270       +4
  OPCDIFFS: -408

With the latest build (2) seems better, but again I am not sure how stable that is as it seems to change over time.

The patch is not quite finalized and the next step would be to decide between (1) and (2)...

Nightly performance measurements do not show any changes above 1%. Given these measurements I would take (2) as there are a few 0.25% improvements with that, but I am not sure so small differences are trustworthy...

https://reviews.llvm.org/D126485

Files:
  llvm/lib/Target/SystemZ/SystemZISelDAGToDAG.cpp
  llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
  llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
  llvm/lib/Target/SystemZ/SystemZInstrInfo.h

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