[PATCH] D121750: Add a cmake flag to turn `llvm_unreachable()` into builtin_trap() when assertions are disabled

[Mehdi AMINI via Phabricator via llvm-commits]

mehdi_amini updated this revision to Diff 416059.
mehdi_amini added a comment.

Update CMake.rst and the description of the option in CMake


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D121750/new/

https://reviews.llvm.org/D121750

Files:
  llvm/CMakeLists.txt
  llvm/docs/CMake.rst
  llvm/include/llvm/Config/llvm-config.h.cmake
  llvm/include/llvm/Support/ErrorHandling.h


Index: llvm/include/llvm/Support/ErrorHandling.h
===================================================================
--- llvm/include/llvm/Support/ErrorHandling.h
+++ llvm/include/llvm/Support/ErrorHandling.h
@@ -124,15 +124,23 @@
 
 /// Marks that the current location is not supposed to be reachable.
 /// In !NDEBUG builds, prints the message and location info to stderr.
-/// In NDEBUG builds, becomes an optimizer hint that the current location
-/// is not supposed to be reachable.  On compilers that don't support
-/// such hints, prints a reduced message instead and aborts the program.
+/// In NDEBUG builds, the behavior is controlled by the CMake flag
+///   -DLLVM_UNREACHABLE_OPTIMIZE
+/// * When "ON" (default) llvm_unreachable() becomes an optimizer hint
+///   that the current location is not supposed to be reachable: the hint
+///   turns such code path into undefined behavior.  On compilers that don't
+///   support such hints, prints a reduced message instead and aborts the
+///   program.
+/// * When "OFF", a builtin_trap is emitted instead of involving undefined
+///   behavior.
 ///
 /// Use this instead of assert(0).  It conveys intent more clearly and
 /// allows compilers to omit some unnecessary code.
 #ifndef NDEBUG
 #define llvm_unreachable(msg) \
   ::llvm::llvm_unreachable_internal(msg, __FILE__, __LINE__)
+#elif LLVM_UNREACHABLE_OPTIMIZE
+#define llvm_unreachable(msg) LLVM_BUILTIN_TRAP
 #elif defined(LLVM_BUILTIN_UNREACHABLE)
 #define llvm_unreachable(msg) LLVM_BUILTIN_UNREACHABLE
 #else
Index: llvm/include/llvm/Config/llvm-config.h.cmake
===================================================================
--- llvm/include/llvm/Config/llvm-config.h.cmake
+++ llvm/include/llvm/Config/llvm-config.h.cmake
@@ -112,4 +112,8 @@
 /* Define if building LLVM with LLVM_FORCE_USE_OLD_TOOLCHAIN_LIBS */
 #cmakedefine LLVM_FORCE_USE_OLD_TOOLCHAIN ${LLVM_FORCE_USE_OLD_TOOLCHAIN}
 
+/* Define if llvm_unreachable should be optimized with undefined behavior
+ * in non assert builds */
+#cmakedefine01 LLVM_UNREACHABLE_OPTIMIZE
+
 #endif
Index: llvm/docs/CMake.rst
===================================================================
--- llvm/docs/CMake.rst
+++ llvm/docs/CMake.rst
@@ -300,6 +300,12 @@
   enabled or not.  A version of LLVM built with ABI breaking checks
   is not ABI compatible with a version built without it.
 
+**LLVM_UNREACHABLE_OPTIMIZE**:STRING
+  This flag controls the behavior of `llvm_unreachable()` in release build
+  (when assertions are disabled in general). When ON (default) then
+  `llvm_unreachable()` is considered "undefined behavior" and optimized as
+  such. When OFF it is instead replaced with a guaranteed "trap".
+
 **LLVM_APPEND_VC_REV**:BOOL
   Embed version control revision info (Git revision id).
   The version info is provided by the ``LLVM_REVISION`` macro in
Index: llvm/CMakeLists.txt
===================================================================
--- llvm/CMakeLists.txt
+++ llvm/CMakeLists.txt
@@ -465,6 +465,7 @@
 option(LLVM_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
 
 option(LLVM_ENABLE_DUMP "Enable dump functions even when assertions are disabled" OFF)
+option(LLVM_UNREACHABLE_OPTIMIZE "Optimize llvm_unreachable() as undefined behavior (default), guaranteed trap when OFF" ON)
 
 if( NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG" )
   option(LLVM_ENABLE_ASSERTIONS "Enable assertions" OFF)


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