[lld] [lld][MachO] Warn on underaligned arm64 functions (PR #221636)

[Chak-Kuen Lam via llvm-commits]

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@@ -896,6 +896,23 @@ void ObjFile::parseSymbols(ArrayRef<typename LP::section> sectionHeaders,
       return StringRef(strtab + sym.n_strx);
     };
 
+    // arm64 instructions must be 4-byte aligned, so a symbol in a code section
+    // whose guaranteed alignment is lower than that names a function that can
+    // never be executed. ld64 diagnoses this, so we do too. Note that we check
+    // each symbol rather than each subsection: an alt-entry symbol does not
+    // induce a new subsection, but it is still an entry point that has to be
+    // aligned.
+    const bool checkCodeAlign = target->cpuType == CPU_TYPE_ARM64 &&
+                                (sections[i]->flags & S_ATTR_PURE_INSTRUCTIONS);
+
+    // Mach-O reserves the 'l' and 'L' prefixes for labels that the assembler
+    // generates for its own use, such as the ltmp0 anchor it emits for each
+    // section. Those do not name functions, so ld64 does not warn about them.
+    auto isAssemblerTemp = [](const NList &sym, StringRef name) {
+      return !(sym.n_type & N_EXT) &&
+             (name.starts_with("l") || name.starts_with("L"));
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dukecat0 wrote:

`isPrivateLabel` treats all `l`/`L`-prefixed names as private, which doesn't work for the example you provided here.

https://github.com/llvm/llvm-project/pull/221636