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Jeff Law

Jeff Law

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  1. Use bseti when profitable.  Many cases of poor constant synthesis are failure to use bseti.
  2. Use "uw" variants of shifts/arithmetic instructions when the constant has bit 0x80000000 set.
  3. Use shNadd for constants evenly divisible by 9, 5 or 3.
  4. Use blcri in conjunction with lui or addi to clear a small number of bits, particularly in the high 33 bits of a 64bit word
  5. Adjust constant so low 13 bits are 0x1800, recursively re-synthesize and restore low bits with trailing addi.
  6. Explore if we are using NOT effectively.  ie, synthesize the inverted constant, then NOT the result to get the desired constantSynthesize C' from C using bit inversion.  Synthesize C', then invert the result.  If that's better than synthesizing C directly, then use the inversion sequence.
  7. Use pack for repeating constants
    1. Depends on reassociation of constant in logical ops with shifts
    2. Depends on basic Zbkb support
  8. Use pack as synthesis of last resort

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Stakeholders/Partners

RISE:

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Page Properties


Development

Status
colourBlueGreen
titleIN PROGRESSCOMPLETE


Development Timeline1H2024
Upstreaming

Status
colourBlueGreen
titleIN PROGRESSCOMPLETE



Upstream Version

gcc-15 (target)

(Spring 2025)





Contacts

Jeff Law (Ventana)


DependenciesNone



Updates

  • Project done. 

  •  zbkb as synthesis of last support submitted upstream for CI testing
  • Additional ideas for improvements in constant synthesis pushed to 2H2024.

 

  • Pack for repeating constants is integrated.

 

  • Lyut's patterns to allow generation of pack, packw, packh automatically have been integrated.

 

  • Reassociation of constants in logical ops with shifts integrated upstream
  • Using pack to handle constants with equal high/low halves implemented
  • Synthesizing the inverted constant, then inverting the result integrated upstream
  • Adjusting the constant to have 0s in the low bits, synthesizing, then restoring low bits with addi integrated upstream.

  • bclr method for generating constants going through integrated upstream CI now
  • Adjustment of low 13 bits to make easier to synthesize constant development done submitted upstream

 

  • Updated list of various deficiencies in GCC's constant synthesis
  • Note that some of these are recently fixed (first week of May)
  • Hoping to have this done within the next week or so.

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