[lld] [lld][elf] add safe-thunks in ELF (PR #126695)

[via llvm-commits]

================
@@ -459,6 +467,58 @@ static void combineRelocHashes(unsigned cnt, InputSection *isec,
   isec->eqClass[(cnt + 1) % 2] = hash | (1U << 31);
 }
 
+// Given a range of identical icfInputs, replace address significant functions
+// with a thunk that is just a direct branch to the first function in the
+// series. This way we keep only one main body of the function but we still
+// retain the address uniqueness of relevant functions by having them be a
+// direct branch thunk rather than containing a full copy of the actual function
+// body.
+template <class ELFT>
+void ICF<ELFT>::applySafeThunksToRange(size_t begin, size_t end) {
+  InputSection *masterIsec = sections[begin];
+
+  uint32_t thunkSize = ctx.target->getICFSafeThunkSize();
+  // If the functions we're dealing with are smaller than the thunk size, then
+  // just leave them all as-is - creating thunks would be a net loss.
+  if (masterIsec->getSize() <= thunkSize)
+    return;
+
+  // Find the symbol to create the thunk for.
+  Symbol *masterSym = nullptr;
+  for (Symbol *sym : masterIsec->file->getSymbols()) {
+    if (auto *d = dyn_cast<Defined>(sym)) {
+      if (d->section == masterIsec) {
----------------
alx32 wrote:

I don't think this is enough, we also want to check that the offset is 0 within the section - we don't want the thunk to be pointing to the middle of the function. Can we just use `getEnclosingSymbol` ?

https://github.com/llvm/llvm-project/pull/126695