[X86] shouldReduceLoadWidth - don't split loads if we can freely reuse full width legal binop (#129695)

Currently shouldReduceLoadWidth is very relaxed about when loads can be
split to avoid extractions from the original full width load - resulting
in many cases where the number of memory operations notably increases,
replacing the cost of a extract_subvector for additional loads.

This patch adjusts the 256/512-bit vector load splitting metric to
detect cases where ANY use of the full width load is be used directly -
in which case we will now reuse that load for smaller types, unless we'd
need to extract an upper subvector / integer element - i.e. we now
correctly treat (extract_subvector cst, 0) as free.

We retain the existing logic of never splitting loads if all uses are
extract+stores but we improve this by peeking through bitcasts while
looking for extract_subvector/store chains.

This required a number of fixes - shouldReduceLoadWidth now needs to
peek through bitcasts UP the use-chain to find final users (limited to
hasOneUse cases to reduce complexity). It also exposed an issue in
isTargetCanonicalConstantNode which assumed that a load of vector
constant data would always extract, which is no longer the case.
12 files changed
tree: 9624bc31910070bb6012a6cdd0c9a60da40b89ca
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. flang-rt/
  11. libc/
  12. libclc/
  13. libcxx/
  14. libcxxabi/
  15. libunwind/
  16. lld/
  17. lldb/
  18. llvm/
  19. llvm-libgcc/
  20. mlir/
  21. offload/
  22. openmp/
  23. polly/
  24. pstl/
  25. runtimes/
  26. third-party/
  27. utils/
  28. .clang-format
  29. .clang-format-ignore
  30. .clang-tidy
  31. .git-blame-ignore-revs
  32. .gitattributes
  33. .gitignore
  34. .mailmap
  35. CODE_OF_CONDUCT.md
  36. CONTRIBUTING.md
  37. LICENSE.TXT
  38. pyproject.toml
  39. README.md
  40. SECURITY.md
README.md

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