[lld] [lld-macho] Improve ICF thunk folding logic (PR #131186)

[via llvm-commits]

================
@@ -297,16 +297,25 @@ static Symbol *getThunkTargetSymbol(ConcatInputSection *isec) {
 // direct branch thunk rather than containing a full copy of the actual function
 // body.
 void ICF::applySafeThunksToRange(size_t begin, size_t end) {
+  // When creating a unique ICF thunk, use the first section as the section that
+  // all thunks will branch to.
+  ConcatInputSection *masterIsec = icfInputs[begin];
+
+  // If the first section is not address significant, sorting guarantees that
+  // there are no address significant functions. So we can skip this range.
+  if (!masterIsec->keepUnique)
+    return;
+
+  // Skip anything that is not a code section.
+  if (!isCodeSection(masterIsec))
----------------
alx32 wrote:

So `ICF::applySafeThunksToRange` is called with `begin` `end` - which forms a range of identical input sections. The `ICF::applySafeThunksToRange` is only operating on this range, so we can ignore the rest.

This range is basically guaranteed to be composed of identical sections. 
The "identical-ity" is determined by the hash (sorted as discussed before) and also by segregating the sections [like this](https://github.com/llvm/llvm-project/blob/d57732358ab1a7ba59c892fb805135354a3bc2e5/lld/MachO/ICF.cpp#L419). The segregation checks that [the parents are equal](https://github.com/llvm/llvm-project/blob/d57732358ab1a7ba59c892fb805135354a3bc2e5/lld/MachO/ICF.cpp#L103).

So basically in this call we are guaranteed that all sections in `[begin, end]` range are of the same type. So we only need to check the first one to check weather all of them are code section or not. 


https://github.com/llvm/llvm-project/pull/131186