[lld-macho] Fix Defined size increase with -mms-bitfields

[BertalanD]

Under the Microsoft ABI, only those bit fields can be merged whose underlying types have the same size.

d175616 ([lld-macho][arm64] Enhance safe ICF with thunk-based deduplication) added an enum field (identicalCodeFoldingKind) next to booleans in the Defined class, which increased the size under the MS ABI. On MinGW targets, this triggered the static_assert which checks the size of Defined (for MSVC targets, the check is disabled due to another problem). Let's store it as a uint8_t to allow merging to take place.

Fixes #107511

[llvmbot]

@llvm/pr-subscribers-lld

@llvm/pr-subscribers-lld-macho

Author: Daniel Bertalan (BertalanD)

Changes

Under the Microsoft ABI, only those bit fields can be merged whose underlying types have the same size.

d175616 ([lld-macho][arm64] Enhance safe ICF with thunk-based deduplication) added an enum field (identicalCodeFoldingKind) next to booleans in the Defined class, which increased the size under the MS ABI. This triggered the static_assert which checks the size of Defined. Let's store it as a uint8_t to allow merging to take place.

Fixes #107511

---

Full diff: https://github.com/llvm/llvm-project/pull/107545.diff

1 Files Affected:

• (modified) lld/MachO/Symbols.h (+3-1)

diff --git a/lld/MachO/Symbols.h b/lld/MachO/Symbols.h
index 70fd195f25c929..beb97b35bf8817 100644
--- a/lld/MachO/Symbols.h
+++ b/lld/MachO/Symbols.h
@@ -14,6 +14,7 @@
 #include "Target.h"
 
 #include "llvm/Object/Archive.h"
+#include "llvm/Support/Compiler.h"
 #include "llvm/Support/MathExtras.h"
 
 namespace lld {
@@ -152,7 +153,8 @@ class Defined : public Symbol {
   // Whether this symbol should appear in the output symbol table.
   bool includeInSymtab : 1;
   // The ICF folding kind of this symbol: None / Body / Thunk.
-  ICFFoldKind identicalCodeFoldingKind : 2;
+  LLVM_PREFERRED_TYPE(ICFFoldKind)
+  uint8_t identicalCodeFoldingKind : 2;
   // Symbols marked referencedDynamically won't be removed from the output's
   // symbol table by tools like strip. In theory, this could be set on arbitrary
   // symbols in input object files. In practice, it's used solely for the

[mstorsjo]

LGTM, thanks!

The PR message is great, but it could also be good to further clarify that the failing static assert already is ifdeffed out for MSVC targets, but MinGW targets also use the same MS bitfield packing logic (for binary compatibility of C structs).