[all-commits] [llvm/llvm-project] 3feb72: [AsmPrinter][ELF] Support profile-guided section p...

[Mingming Liu via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 3feb724496238ce10d32e8c2bd84b4ea50f9977e
      https://github.com/llvm/llvm-project/commit/3feb724496238ce10d32e8c2bd84b4ea50f9977e
  Author: Mingming Liu <mingmingl at google.com>
  Date:   2025-01-28 (Tue, 28 Jan 2025)

  Changed paths:
    M llvm/include/llvm/CodeGen/CommandFlags.h
    M llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h
    M llvm/include/llvm/Target/TargetLoweringObjectFile.h
    M llvm/include/llvm/Target/TargetMachine.h
    M llvm/include/llvm/Target/TargetOptions.h
    M llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
    M llvm/lib/CodeGen/CommandFlags.cpp
    M llvm/lib/CodeGen/StaticDataSplitter.cpp
    M llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
    M llvm/lib/CodeGen/TargetPassConfig.cpp
    M llvm/lib/Target/TargetLoweringObjectFile.cpp
    M llvm/test/CodeGen/X86/jump-table-partition.ll

  Log Message:
  -----------
  [AsmPrinter][ELF] Support profile-guided section prefix for jump tables' (read-only) data sections (#122215)

https://github.com/llvm/llvm-project/pull/122183 adds a codegen pass to
infer machine jump table entry's hotness from the MBB hotness. This is a
follow-up PR to produce `.hot` and or `.unlikely` section prefix for
jump table's (read-only) data sections in the relocatable `.o` files.

When this patch is enabled, linker will see {`.rodata`, `.rodata.hot`,
`.rodata.unlikely`} in input sections. It can map `.rodata.hot` and
`.rodata` in the input sections to `.rodata.hot` in the executable, and
map `.rodata.unlikely` into `.rodata` with a pending extension to
`--keep-text-section-prefix` like
https://github.com/llvm/llvm-project/commit/059e7cbb66a30ce35f3ee43197eed1a106b50c5b,
or with a linker script.

1. To partition hot and jump tables, the AsmPrinter pass slices a function's jump table indices into two groups, one for hot and the other for cold jump tables. It then emits hot jump tables into a `.hot`-prefixed data section and cold ones into a `.unlikely`-prefixed data section, retaining the relative order of `LJT<N>` labels within each group.

2. [ELF only] To have data sections with _dynamic_ names (e.g., `.rodata.hot[.func]`), we implement
`TargetLoweringObjectFile::getSectionForJumpTable` method that accepts a `MachineJumpTableEntry` parameter, and update `selectELFSectionForGlobal` to generate `.hot` or `.unlikely` based on
MJTE's hotness.
    - The dynamic JT section name doesn't depend on `-ffunction-section=true` or `-funique-section-names=true`, even though it leverages the similar underlying mechanism to have a MCSection with on-demand name as `-ffunction-section` does.

3. The new code path is off by default.
    - Typically, `TargetOptions` conveys clang or LLVM tools' options to code generation passes. To follow the pattern, add option `EnableStaticDataPartitioning` bit in `TargetOptions` and make it
readable through `TargetMachine`.
    - To enable the new code path in tools like `llc`, `partition-static-data-sections` option is introduced in
`CodeGen/CommandFlags.h/cpp`.
    -  A subsequent patch
([draft](https://github.com/llvm/llvm-project/commit/8f36a1374365862b3ca9be5615dd38f02a318c45)) will add a clang option to enable the new code path.

---------

Co-authored-by: Ellis Hoag <ellis.sparky.hoag at gmail.com>



To unsubscribe from these emails, change your notification settings at https://github.com/llvm/llvm-project/settings/notifications