[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@gmail.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/Target/TargetLoweringObjectFile.cpp b/llvm/lib/Target/TargetLoweringObjectFile.cpp
index 4fe9d13..02c1010 100644
--- a/llvm/lib/Target/TargetLoweringObjectFile.cpp
+++ b/llvm/lib/Target/TargetLoweringObjectFile.cpp
@@ -348,6 +348,12 @@ TargetLoweringObjectFile::SectionForGlobal(const GlobalObject *GO,
 
 MCSection *TargetLoweringObjectFile::getSectionForJumpTable(
     const Function &F, const TargetMachine &TM) const {
+  return getSectionForJumpTable(F, TM, /*JTE=*/nullptr);
+}
+
+MCSection *TargetLoweringObjectFile::getSectionForJumpTable(
+    const Function &F, const TargetMachine &TM,
+    const MachineJumpTableEntry *JTE) const {
   Align Alignment(1);
   return getSectionForConstant(F.getDataLayout(),
                                SectionKind::getReadOnly(), /*C=*/nullptr,