[MC] Don't treat altentry symbols as atoms

[MaskRay]

The current setAtom is inaccurate: a .alt_entry label can also be
recognized as an atom. This is mostly benign, but might cause two
locations only separated by an .alt_entry to have different atoms.

https://reviews.llvm.org/D153167 changed a evaluateKnownAbsolute to
evaluateAsAbsolute and would not fold A-B even if they are only
separated by a .alt_entry label, leading to a spurious error
invalid CFI advance_loc expression.

The fix is similar to #82268: add a special case for .alt_entry.

Fix #97116

[llvmbot]

@llvm/pr-subscribers-mc

Author: Fangrui Song (MaskRay)

Changes

The current setAtom is inaccurate: a .alt_entry label can also be
recognized as an atom. This is mostly benign, but might cause two
locations only separated by an .alt_entry to have different atoms.

The evaluateAsAbsolute code added by https://reviews.llvm.org/D153167
would not fold A-B when they are only separated by a .alt_entry
label, leading to a spurious error.

The fix is similar to #82268: add a special case for .alt_entry.

Fix #97116

---

Full diff: https://github.com/llvm/llvm-project/pull/97479.diff

2 Files Affected:

• (modified) llvm/lib/MC/MCMachOStreamer.cpp (+1-1)
• (modified) llvm/test/MC/MachO/cfi-advance-loc-err.s (+5)

diff --git a/llvm/lib/MC/MCMachOStreamer.cpp b/llvm/lib/MC/MCMachOStreamer.cpp
index d67b95820a8a9..0b34d87033b7b 100644
--- a/llvm/lib/MC/MCMachOStreamer.cpp
+++ b/llvm/lib/MC/MCMachOStreamer.cpp
@@ -508,7 +508,7 @@ void MCMachOStreamer::finishImpl() {
   DenseMap<const MCFragment *, const MCSymbol *> DefiningSymbolMap;
   for (const MCSymbol &Symbol : getAssembler().symbols()) {
     if (getAssembler().isSymbolLinkerVisible(Symbol) && Symbol.isInSection() &&
-        !Symbol.isVariable()) {
+        !Symbol.isVariable() && !cast<MCSymbolMachO>(Symbol).isAltEntry()) {
       // An atom defining symbol should never be internal to a fragment.
       assert(Symbol.getOffset() == 0 &&
              "Invalid offset in atom defining symbol!");
diff --git a/llvm/test/MC/MachO/cfi-advance-loc-err.s b/llvm/test/MC/MachO/cfi-advance-loc-err.s
index 3143dd84efc63..77b6544cb12d8 100644
--- a/llvm/test/MC/MachO/cfi-advance-loc-err.s
+++ b/llvm/test/MC/MachO/cfi-advance-loc-err.s
@@ -9,6 +9,11 @@ _foo:
   subq $8, %rsp
   .cfi_adjust_cfa_offset 8
 
+  .alt_entry _bar
+_bar: # alt_entry label can appear here as it is not an atom
+  addq $8, %rsp
+  .cfi_adjust_cfa_offset -8
+
 tmp0: # non-private label cannot appear here
   addq $8, %rsp
 # CHECK: :[[#@LINE+1]]:3: error: invalid CFI advance_loc expression

[filipnavara]

JFYI The original issue is somehow present in Xcode 16 beta 2/3 which is based on LLVM 17 AFAICT. I filed Apple feedback to get appropriate backport of the fix (FB14191360).

[MaskRay]

JFYI The original issue is somehow present in Xcode 16 beta 2/3 which is based on LLVM 17 AFAICT. I filed Apple feedback to get appropriate backport of the fix (FB14191360).

Thank you. I am glad that this resolved an issue for you:)