[MachO] Fix dead-stripping __eh_frame

This section is marked S_ATTR_LIVE_SUPPORT in input files, which meant
that on arm64, we were unnecessarily preserving FDEs if we e.g. had
multiple weak definitions for a function. Worse, we would actually
produce an invalid `__eh_frame` section in that case, because the CIE
associated with the unnecessary FDE would still get dead-stripped and
we'd end up with a dangling FDE. We set up associations from functions
to their FDEs, so dead-stripping will just work naturally, and we can
clear S_ATTR_LIVE_SUPPORT from our input `__eh_frame` sections to fix
dead-stripping.

Reviewed By: #lld-macho, int3

Differential Revision: https://reviews.llvm.org/D132489

lld/MachO/InputFiles.cpp

@@ -1593,6 +1593,15 @@ void ObjFile::registerEhFrames(Section &ehFrameSection) {
     funcSym->unwindEntry = isec;
     ehRelocator.commit();
   }
+
+  // __eh_frame is marked as S_ATTR_LIVE_SUPPORT in input files, because FDEs
+  // are normally required to be kept alive if they reference a live symbol.
+  // However, we've explicitly created a dependency from a symbol to its FDE, so
+  // dead-stripping will just work as usual, and S_ATTR_LIVE_SUPPORT will only
+  // serve to incorrectly prevent us from dead-stripping duplicate FDEs for a
+  // live symbol (e.g. if there were multiple weak copies). Remove this flag to
+  // let dead-stripping proceed correctly.
+  ehFrameSection.flags &= ~S_ATTR_LIVE_SUPPORT;
 }
 
 std::string ObjFile::sourceFile() const {

lld/test/MachO/eh-frame-dead-strip.s

@@ -0,0 +1,46 @@
+# REQUIRES: x86, aarch64
+
+# RUN: rm -rf %t; split-file %s %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos11.0 %t/strong.s -o %t/strong_x86_64.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos11.0 %t/weak.s -o %t/weak_x86_64.o
+# RUN: %lld -dylib -dead_strip %t/strong_x86_64.o %t/weak_x86_64.o -o %t/libstrongweak_x86_64.dylib
+# RUN: llvm-dwarfdump --eh-frame %t/libstrongweak_x86_64.dylib | FileCheck --check-prefixes CHECK,X86_64 %s
+# RUN: %lld -dylib -dead_strip %t/weak_x86_64.o %t/strong_x86_64.o -o %t/libweakstrong_x86_64.dylib
+# RUN: llvm-dwarfdump --eh-frame %t/libweakstrong_x86_64.dylib | FileCheck --check-prefixes CHECK,X86_64 %s
+
+# RUN: llvm-mc -filetype=obj -triple=arm64-apple-macos11.0 %t/strong.s -o %t/strong_arm64.o
+# RUN: llvm-mc -filetype=obj -triple=arm64-apple-macos11.0 %t/weak.s -o %t/weak_arm64.o
+# RUN: %lld -arch arm64 -dylib -dead_strip %t/strong_arm64.o %t/weak_arm64.o -o %t/libstrongweak_arm64.dylib
+# RUN: llvm-dwarfdump --eh-frame %t/libstrongweak_arm64.dylib | FileCheck --check-prefixes CHECK,ARM64 %s
+# RUN: %lld -arch arm64 -dylib -dead_strip %t/weak_arm64.o %t/strong_arm64.o -o %t/libweakstrong_arm64.dylib
+# RUN: llvm-dwarfdump --eh-frame %t/libweakstrong_arm64.dylib | FileCheck --check-prefixes CHECK,ARM64 %s
+
+## Verify that unneeded FDEs (and their CIEs) are dead-stripped even if they
+## point to a live symbol (e.g. because we had multiple weak definitions).
+
+# CHECK: .eh_frame contents:
+# X86_64: 00000000 00000014 00000000 CIE
+# X86_64: 00000018 0000001c 0000001c FDE cie=00000000
+# ARM64: 00000000 00000010 00000000 CIE
+# ARM64: 00000014 00000018 00000018 FDE cie=00000000
+# CHECK-NOT: CIE
+# CHECK-NOT: FDE
+
+#--- strong.s
+.globl _fun
+_fun:
+  .cfi_startproc
+  ## cfi_escape cannot be encoded in compact unwind
+  .cfi_escape 0
+  ret
+  .cfi_endproc
+
+#--- weak.s
+.globl _fun
+.weak_definition _fun
+_fun:
+  .cfi_startproc
+  ## cfi_escape cannot be encoded in compact unwind
+  .cfi_escape 0
+  ret
+  .cfi_endproc