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[CodeGen][AArch64][SVE] Fold [rdffr, ptest] => rdffrs; bugfix for…

Authored by peterwaller-arm on Apr 27 2021, 5:29 AM.

Description

[CodeGen][AArch64][SVE] Fold [rdffr, ptest] => rdffrs; bugfix for optimizePTestInstr

When a ptest is used to set flags from the output of rdffr, the ptest
can be eliminated, using a flags-setting rdffrs instead.

Additionally, check that nothing consumes flags between rdffr and ptest;
this case appears to have been missed previously.

  • There is no unpredicated RDFFRS instruction.
  • If substituting RDFFR_PP, require that the mask argument of the PTEST matches that of the RDFFR_PP.
  • Move some precondition code up inside optimizePTestInstr, so that it covers the new code paths for RDFFR which return earlier.
    • Only consider RDFFR, PTEST in same basic block.
    • Check for other flag setting instructions between the two, abort if found.
    • Drop an old TODO comment about removing dead PTEST instructions.

RDFFR_P to follow in later patch.

Differential Revision: https://reviews.llvm.org/D101357