[llvm-branch-commits] [lld] ELF: CFI jump table relaxation. (PR #147424)

[Peter Collingbourne via llvm-branch-commits]

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@@ -312,6 +313,196 @@ bool X86_64::deleteFallThruJmpInsn(InputSection &is,
   return true;
 }
 
+void X86_64::relaxCFIJumpTables() const {
+  // Relax CFI jump tables.
+  // - Split jump table into pieces and place target functions inside the jump
+  //   table if small enough.
+  // - Move jump table before last called function and delete last branch
+  //   instruction.
+  DenseMap<InputSection *, SmallVector<InputSection *, 0>> sectionReplacements;
+  SmallVector<InputSection *, 0> storage;
+  for (OutputSection *osec : ctx.outputSections) {
+    if (!(osec->flags & SHF_EXECINSTR))
+      continue;
+    for (InputSection *sec : getInputSections(*osec, storage)) {
+      if (sec->type != SHT_LLVM_CFI_JUMP_TABLE || sec->entsize == 0 ||
+          sec->size % sec->entsize != 0)
+        continue;
+
+      // We're going to replace the jump table with this list of sections. This
+      // list will be made up of slices of the original section and function
+      // bodies that were moved into the jump table.
+      SmallVector<InputSection *, 0> replacements;
+
+      // r is the only relocation in a jump table entry. Figure out whether it
+      // is a branch pointing to the start of a statically known section that
+      // hasn't already been moved while processing a different jump table
+      // section, and if so return it.
+      auto getMovableSection = [&](Relocation &r) -> InputSection * {
+        if (r.type != R_X86_64_PC32 && r.type != R_X86_64_PLT32)
+          return nullptr;
+        auto *sym = dyn_cast_or_null<Defined>(r.sym);
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pcc wrote:

I think you're right. We can clean up `applyBranchToBranchOptImpl` as that does the same thing.

https://github.com/llvm/llvm-project/pull/147424