3feb724496
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` like059e7cbb66, 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](8f36a13743)) will add a clang option to enable the new code path. --------- Co-authored-by: Ellis Hoag <ellis.sparky.hoag@gmail.com>
16 KiB
16 KiB