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

[via llvm-commits]

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@@ -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))
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alx32 wrote:

Normally sorting is only done by hash. For `safe_thunks` we also sort by `keepUnique` - see [here](https://github.com/llvm/llvm-project/blob/d57732358ab1a7ba59c892fb805135354a3bc2e5/lld/MachO/ICF.cpp#L413-L416). But we never sort by section type. 

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