[all-commits] [llvm/llvm-project] 652bcf: CodeGenPrepare: Add support for llvm.threadlocal.a...

[Matthias Braun via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 652bcf685c72447f3cc46d93d6c9c1948e8499f3
      https://github.com/llvm/llvm-project/commit/652bcf685c72447f3cc46d93d6c9c1948e8499f3
  Author: Matthias Braun <matze at braunis.de>
  Date:   2024-04-17 (Wed, 17 Apr 2024)

  Changed paths:
    M llvm/include/llvm/CodeGen/TargetLowering.h
    M llvm/lib/CodeGen/CodeGenPrepare.cpp
    M llvm/lib/Target/X86/X86ISelLowering.cpp
    M llvm/lib/Target/X86/X86ISelLowering.h
    A llvm/test/CodeGen/X86/codegen-prepare-addrmode-tls.ll
    A llvm/test/Transforms/CodeGenPrepare/X86/sink-addrmode-tls.ll

  Log Message:
  -----------
  CodeGenPrepare: Add support for llvm.threadlocal.address address-mode sinking (#87844)

Depending on the TLSMode many thread-local accesses on x86 can be
expressed by adding a %fs: segment register to an addressing mode. Even
if there are mutliple users of a `llvm.threadlocal.address` intrinsic it
is generally not worth sharing the value in a register but instead fold
the %fs access into multiple addressing modes.

Hence this changes CodeGenPrepare to duplicate the
`llvm.threadlocal.address` intrinsic as necessary.

Introduces a new `TargetLowering::addressingModeSupportsTLS` callback
that allows targets to indicate whether TLS accesses can be part of an
addressing mode.

This is fixing a performance problem, as this folding of TLS-accesses
into multiple addressing modes happened naturally before the
introduction of the `llvm.threadlocal.address` intrinsic, but regressed
due to `SelectionDAG` keeping things in registers when accessed across
basic blocks, so CodeGenPrepare needs to duplicate to mitigate this. We
see a ~0.5% recovery in a codebase with heavy TLS usage (HHVM).

This fixes most of #87437



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