[lld-macho] Improve ICF thunk folding logic

[alx32]

Refactor to add some early return logic to applySafeThunksToRange so that we completely skip irrelevant ranges.

Also add a check for isCodeSection to ensure we only apply branch thunks to code section (they don't make sense for anything else). Currently this isn't an issue since there are no keepUnique non-code sections - but this is not a hard restriction and may be implemented in the future, so we should be able to handle (i.e. avoid) this scenario.

[llvmbot]

@llvm/pr-subscribers-lld-macho

@llvm/pr-subscribers-lld

Author: None (alx32)

Changes

Refactor to add some early return logic to applySafeThunksToRange so that we completely skip irrelevant ranges.

Also add a check for isCodeSection to ensure we only apply branch thunks to code section (they don't make sense for anything else). Currently this isn't an issue since there are no keepUnique non-code sections - but this is not a hard restriction and may be implemented in the future, so we should be able to handle (i.e. avoid) this scenario.

---

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

1 Files Affected:

• (modified) lld/MachO/ICF.cpp (+14-5)

diff --git a/lld/MachO/ICF.cpp b/lld/MachO/ICF.cpp
index acba0a2a824b4..9b8aa1837a4aa 100644
--- a/lld/MachO/ICF.cpp
+++ b/lld/MachO/ICF.cpp
@@ -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))
+    return;
+
   // 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.
   uint32_t thunkSize = target->getICFSafeThunkSize();
-  if (icfInputs[begin]->data.size() <= thunkSize)
+  if (masterIsec->data.size() <= thunkSize)
     return;
 
-  // When creating a unique ICF thunk, use the first section as the section that
-  // all thunks will branch to.
-  ConcatInputSection *masterIsec = icfInputs[begin];
-
   // Get the symbol that all thunks will branch to.
   Symbol *masterSym = getThunkTargetSymbol(masterIsec);
 

[DataCorrupted]

In that case all LGTM