Summary

Implement Step 3 of VPTOSoftPostUpdate for issue #941 : convert fixed-stride memory accesses within a single block into chained post-update operations.

Key changes

• Add a second analysis phase after loop-path rewriting.

  • Recollect blocks after all scf.for rewrites.
  • Scan normal blocks, scf.for bodies, and nested regions such as scf.if.
  • Skip operations already converted by the loop path.

• Normalize addresses as (rootBase, baseOffset) through pto.addptr chains.

• Reuse the loop-path infrastructure for:

  • element/block unit conversion;
  • effective stride calculation;
  • initial pointer construction;
  • type and range validation;
  • pure definition-chain cloning;
  • generic post-update op creation.

• Support constant and symbolic affine strides, including:

  • varying base and offset whose combined address step is fixed;
  • non-constant affine expressions;
  • exactly divisible block strides such as 8 * k for f32 vsstb.

• Group candidates by (op type, rootBase) and detect deterministic,
  non-overlapping maximal arithmetic runs.

  • Different buckets may be physically interleaved.
  • A stride break ends the current run and starts analysis from the breaking candidate.
  • A run must contain at least three operations before it is rewritten.

• Analyze all accepted runs before materialization, then rewrite operations in
  original program order while maintaining an independent pointer chain per run.

• Update the pass description and Step 3 design documentation.

Test coverage

Added 8 sequential-path lit test files containing 22 focused scenarios, including:

• fixed-base constant sequences;
• minimum run length and zero-stride rejection;
• run restart after invalid or too-short prefixes;
• multiple maximal runs;
• varying base/offset cancellation;
• multilevel pto.addptr normalization;
• constant and symbolic affine strides;
• pure symbolic leaf cloning and conservative rejection;
• scf.for body fallback and nested scf.if blocks;
• partial loop-path conversion followed by sequential fallback;
• interleaved buckets and same-op/different-root isolation;
• already-post-update operations interleaved with a run;
• vlds, vsts, and vsstb pointer chaining;
• exact and inexact block-unit conversion;
• nonzero initial vsstb repeat stride;
• narrow stride type overflow rejection;
• operation attribute preservation.

Validation

• Sequential-path tests: 8/8 passed
• All soft-post-update tests: 39/39 passed

Commands:

llvm-lit -sv -j 8 build/test/lit \
  --filter 'soft_postupdate_sequential'

llvm-lit -sv -j 8 build/test/lit \
  --filter 'soft_postupdate'