[libc] -Wframe-address in libc/src/__support/threads/linux/thread.cpp

[nickdesaulniers]

doing an aarch64-linux-gnu runtimes cross build results in the following warning (blocking enabling -Werror):

/android0/llvm-project/libc/src/__support/threads/linux/thread.cpp:171:38: error: calling '__builtin_frame_address' with a nonzero argument is unsafe [-Werror,-Wframe-address]
  171 |   return reinterpret_cast<uintptr_t>(__builtin_frame_address(1));
      |                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

doing an aarch64-linux-gnu runtimes cross build results in the following warning (blocking enabling -Werror): ``` /android0/llvm-project/libc/src/__support/threads/linux/thread.cpp:171:38: error: calling '__builtin_frame_address' with a nonzero argument is unsafe [-Werror,-Wframe-address] 171 | return reinterpret_cast<uintptr_t>(__builtin_frame_address(1)); | ^~~~~~~~~~~~~~~~~~~~~~~~~~ ```

[nickdesaulniers]

annoying, adding -Wno-frame-address isn't enough, because we add the per TU compiler flags AFTER the global ones.

diff --git a/libc/src/__support/threads/linux/CMakeLists.txt b/libc/src/__support/threads/linux/CMakeLists.txt
index 642eead72772..148a0ba061c5 100644
--- a/libc/src/__support/threads/linux/CMakeLists.txt
+++ b/libc/src/__support/threads/linux/CMakeLists.txt
@@ -39,6 +39,8 @@ add_object_library(
     -O3
     -fno-omit-frame-pointer # This allows us to sniff out the thread args from
                             # the new thread's stack reliably.
+    -Wno-frame-address      # Yes, calling __builtin_return_address with a
+                            # value other than 0 is dangerous. We know.
 )
 
 add_object_library(

we still get an error:

FAILED: libc/src/__support/threads/linux/CMakeFiles/libc.src.__support.threads.linux.thread.dir/thread.cpp.o 
/android0/llvm-project/llvm/build/bin/clang++ -DLIBC_NAMESPACE=__llvm_libc____ -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/android0/llvm-project/libc -isystem /android0/llvm-project/build_runtimes/libc/include -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=gnu++17 --target=aarch64-linux-gnu -O3 -fno-omit-frame-pointer -Wno-frame-address -fpie -ffreestanding -fno-builtin -fno-exceptions -fno-lax-vector-conversions -fno-unwind-tables -fno-asynchronous-unwind-tables -fno-rtti -Wall -Wextra -Werror -Wconversion -Wno-sign-conversion -Wimplicit-fallthrough -Wwrite-strings -Wextra-semi -Wnewline-eof -Wnonportable-system-include-path -Wstrict-prototypes -Wthread-safety -Wglobal-constructors -MD -MT libc/src/__support/threads/linux/CMakeFiles/libc.src.__support.threads.linux.thread.dir/thread.cpp.o -MF libc/src/__support/threads/linux/CMakeFiles/libc.src.__support.threads.linux.thread.dir/thread.cpp.o.d -o libc/src/__support/threads/linux/CMakeFiles/libc.src.__support.threads.linux.thread.dir/thread.cpp.o -c /android0/llvm-project/libc/src/__support/threads/linux/thread.cpp

note: -Wno-frame-address is in the list, BEFORE -Wall -Wextra. I can't imagine that design was intentional...