[all-commits] [llvm/llvm-project] cbd89f: [AArch64] Form CCMP for CBB and CBH

[David Tellenbach via All-commits]

  Branch: refs/heads/users/dtellenbach/cmpbr_ccmp2/ccmp_for_cbb_cbh
  Home:   https://github.com/llvm/llvm-project
  Commit: cbd89f8405a400ee5f892832f72f2d8a08fcd75a
      https://github.com/llvm/llvm-project/commit/cbd89f8405a400ee5f892832f72f2d8a08fcd75a
  Author: David Tellenbach <dtellenbach at apple.com>
  Date:   2026-09-10 (Thu, 10 Sep 2026)

  Changed paths:
    M llvm/lib/Target/AArch64/AArch64ConditionalCompares.cpp
    M llvm/test/CodeGen/AArch64/cmpbr-ccmp.ll
    M llvm/test/CodeGen/AArch64/cmpbr-ccmp.mir

  Log Message:
  -----------
  [AArch64] Form CCMP for CBB and CBH

AArch64ConditionalCompares forms CMP/CCMP chains to transform patterns
such as

                 Head                        Head
                 / |                         CmpBB
                /  |                         / |
               |  CmpBB        =>           /  |
               |  / |                    Tail  |
               | /  |                      |   |
              Tail  |                      |   |
                |   |                      |   |
               ... ...                    ... ...

where Head is terminated by a conditional branch and CmpBB contains
a cmp + conditional branch.

We usually try to split any fused conditional branches to be able to
form CCMP. However, in the case of CBB and CBH, splitting the fused
conditional branch in CmpBB could require the insertion of explicit
zero- or sign-extensions for both operands. In the Head block we can
cheaply fuse at least one of the extensions into cmp.

We therefore convert CBB and CBH unconditionally when we find them in
the Head block. In the CmpBB block, we only do the transformation if
neither operands needs an extension.



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