Make _LIBUNWIND_SUPPORT_FRAME_APIS not conditional on target architecure.

[saugustine]

Summary

The functions this macro protects are stubs for libgcc-compatibility.
Today, libunwind as a drop-in replacement for libgcc_eh links on x86,
x86_64, and powerpc, but not aarch64, which doesn't really make
sense. As there is nothing architecture specific about these, they
should be provided everywhere or nowhere.

The target-specific protection goes all the way back to the original
code contribution in 312fcd0e1cf14482b2ae8eee8234541dcc3bc2c4 from
2013, so the original reason is lost to history, and probably not
relevant today.

It may make sense to make this a top-level configure option, or
possibly to get it on defined(linux) but either default has some
compatibility issues--they would either be available in unexpected
places, or not available in expected places. Just like with this
patch. Please advise.

[compnerd]

I think that this needs to be more surgical. These shouldn't be enabled on Darwin I believe if this is for libgcc compatibility.

[saugustine]

What exactly do you recommend here? The situation is pretty messy.

Today the macro only guards them for target architecture, so they are enabled for Darwin on x86 today, but not Darwin on aarch64. That is true for every single OS that runs on (one of x86, x86_64, and powerpc) and anything else.

So the same problems that exist for Darwin also exist for Fuschia and FreeBSD and many others.

Perhaps a top-level CMake option would be best.

[philnik]

In D158011#4589757, @saugustine wrote:

What exactly do you recommend here? The situation is pretty messy.

Today the macro only guards them for target architecture, so they are enabled for Darwin on x86 today, but not Darwin on aarch64. That is true for every single OS that runs on (one of x86, x86_64, and powerpc) and anything else.

So the same problems that exist for Darwin also exist for Fuschia and FreeBSD and many others.

Perhaps a top-level CMake option would be best.

I think this is actually required on Darwin, since Apple used to ship libstdc++ on x86 at some point in the past. A CMake option seems to me like the sensible solution. Adding libc++ vendors, since they might have some insight here.

[compnerd]

Yeah, having control at build time seems entirely reasonable to me.

[saugustine]

Switch to _LIBUNWIND_SUPPORT_FRAME_APIS a build-time option

[MaskRay]

In D158011#4592705, @philnik wrote:

In D158011#4589757, @saugustine wrote:

What exactly do you recommend here? The situation is pretty messy.

Today the macro only guards them for target architecture, so they are enabled for Darwin on x86 today, but not Darwin on aarch64. That is true for every single OS that runs on (one of x86, x86_64, and powerpc) and anything else.

So the same problems that exist for Darwin also exist for Fuschia and FreeBSD and many others.

Perhaps a top-level CMake option would be best.

I think this is actually required on Darwin, since Apple used to ship libstdc++ on x86 at some point in the past. A CMake option seems to me like the sensible solution. Adding libc++ vendors, since they might have some insight here.

These symbols are unneeded by ELF systems. See D158241 for an analysis of GCC and Clang.

It's fairly likely that Mac OS doesn't need them as well.
It's possible that Power Mac (llvm-project has dropped support) needed the symbols at one point as a workaround, or these empty symbols were just added for no good reason. (https://en.wikipedia.org/wiki/Mac_transition_to_Intel_processors https://en.wikipedia.org/wiki/Mac_transition_to_Apple_silicon)
It's likely 10+ years no new __register_frame_info references has been generated.

I think defaulting LIBUNWIND_ENABLE_FRAME_APIS to OFF is fine.

[saugustine]

Any last comments on this?

[MaskRay]

LGTM if LIBUNWIND_ENABLE_FRAME_APIS defaults to OFF

libunwind/CMakeLists.txt

54

LGTM if this is changed to OFF

55

delete this blank line

[MaskRay]

We shall need D158241 first for the llvm-libgcc subproject to suppress ld.lld --no-undefined-version errors.

[xingxue]

GCC uses __register_frame_info and friends on platforms where the eh_frame_hdr/eh_frame sections are not supported such as AIX. But we can turn the macro on if GCC compat is needed on those OSs.

[saugustine]

Adjust based on feedback

[glandium]

It's really confusing that the commit that landed says:

Default this to on, so as not to remove the apis from environments that already have them.

FWIW, this is used by rust on i686 mingw: https://github.com/rust-lang/rust/blob/36b8e4aa7588c70947419a5b435b6faa66669cbd/library/rtstartup/rsbegin.rs#L82

[MaskRay]

In D158011#4643901, @glandium wrote:

It's really confusing that the commit that landed says:

Default this to on, so as not to remove the apis from environments that already have them.

FWIW, this is used by rust on i686 mingw: https://github.com/rust-lang/rust/blob/36b8e4aa7588c70947419a5b435b6faa66669cbd/library/rtstartup/rsbegin.rs#L82

Note that these __register_frame_* symbols are empty in llvm-project/libunwind and do not really work.
rust i686 mingw should use weak references like GCC libgcc/crtstuff.c does. mingw supports .refptr. to make an unresolved symbol value 0.
(I probably should add more detail to https://maskray.me/blog/2021-04-25-weak-symbol when I understand mingw better...)

[mati865]

Rust doesn't use LLVM's libunwind for mingw-w64, it relies on libgcc.

[glandium]

In D158011#4644678, @mati865 wrote:

Rust doesn't use LLVM's libunwind for mingw-w64, it relies on libgcc.

Only if one uses rust-mingw. That's not the only way rustc can be used.

[mati865]

If you use windows-gnu triples Rust will link libgcc_s or libgcc_eh directly unless you modify it's source not to do it: https://github.com/rust-lang/rust/blob/e5fedceabf4e0564231db592b6d1f35e1ca27908/compiler/rustc_target/src/spec/windows_gnu_base.rs#L64

libunwind/CMakeLists.txt

 option(LIBUNWIND_WEAK_PTHREAD_LIB "Use weak references to refer to pthread functions." OFF)
 option(LIBUNWIND_USE_COMPILER_RT "Use compiler-rt instead of libgcc" OFF)
 option(LIBUNWIND_INCLUDE_DOCS "Build the libunwind documentation." ${LLVM_INCLUDE_DOCS})
 option(LIBUNWIND_INCLUDE_TESTS "Build the libunwind tests." ${LLVM_INCLUDE_TESTS})
 option(LIBUNWIND_IS_BAREMETAL "Build libunwind for baremetal targets." OFF)
 option(LIBUNWIND_USE_FRAME_HEADER_CACHE "Cache frame headers for unwinding. Requires locking dl_iterate_phdr." OFF)
 option(LIBUNWIND_REMEMBER_HEAP_ALLOC "Use heap instead of the stack for .cfi_remember_state." OFF)
 option(LIBUNWIND_INSTALL_HEADERS "Install the libunwind headers." ON)
+option(LIBUNWIND_ENABLE_FRAME_APIS "Include libgcc-compatible frame apis." OFF)
 
 set(LIBUNWIND_LIBDIR_SUFFIX "${LLVM_LIBDIR_SUFFIX}" CACHE STRING
     "Define suffix of library directory name (32/64)")
 option(LIBUNWIND_INSTALL_LIBRARY "Install the libunwind library." ON)
 cmake_dependent_option(LIBUNWIND_INSTALL_STATIC_LIBRARY
   "Install the static libunwind library." ON
   "LIBUNWIND_ENABLE_STATIC;LIBUNWIND_INSTALL_LIBRARY" OFF)
 cmake_dependent_option(LIBUNWIND_INSTALL_SHARED_LIBRARY
   "Install the shared libunwind library." ON
else()
   endif()
 endif()
 
 # Cross-unwinding
 if (NOT LIBUNWIND_ENABLE_CROSS_UNWINDING)
   add_compile_flags(-D_LIBUNWIND_IS_NATIVE_ONLY)
 endif()
 
+# Include stubs for __register_frame_info_bases and related
+if (LIBUNWIND_ENABLE_FRAME_APIS)
+  add_compile_flags(-D_LIBUNWIND_SUPPORT_FRAME_APIS)
+endif()
+
 # Threading-support
 if (NOT LIBUNWIND_ENABLE_THREADS)
   add_compile_flags(-D_LIBUNWIND_HAS_NO_THREADS)
 endif()
 
 # ARM WMMX register support
 if (LIBUNWIND_ENABLE_ARM_WMMX)
   # __ARM_WMMX is a compiler pre-define (as per the ACLE 2.0). Clang does not

libunwind/src/config.h

 #endif
 
 // Apple/armv7k defaults to DWARF/Compact unwinding, but its libunwind also
 // needs to include the SJLJ APIs.
 #if (defined(__APPLE__) && defined(__arm__)) || defined(__USING_SJLJ_EXCEPTIONS__)
 #define _LIBUNWIND_BUILD_SJLJ_APIS
 #endif
 
-#if defined(__i386__) || defined(__x86_64__) || defined(__powerpc__)
-#define _LIBUNWIND_SUPPORT_FRAME_APIS
-#endif
-
 #if defined(__i386__) || defined(__x86_64__) || defined(__powerpc__) ||        \
     (!defined(__APPLE__) && defined(__arm__)) || defined(__aarch64__) ||       \
     defined(__mips__) || defined(__riscv) || defined(__hexagon__) ||           \
     defined(__sparc__) || defined(__s390x__) || defined(__loongarch__)
 #if !defined(_LIBUNWIND_BUILD_SJLJ_APIS)
 #define _LIBUNWIND_BUILD_ZERO_COST_APIS
 #endif
 #endif