[llvm] 7b98391 - Add a cmake flag to turn `llvm_unreachable()` into builtin_trap() when assertions are disabled

[Mehdi Amini via llvm-commits]

Author: Mehdi Amini
Date: 2022-03-18T19:24:14Z
New Revision: 7b983917d455500f6b861f289f252718290f36ac

URL: https://github.com/llvm/llvm-project/commit/7b983917d455500f6b861f289f252718290f36ac
DIFF: https://github.com/llvm/llvm-project/commit/7b983917d455500f6b861f289f252718290f36ac.diff

LOG: Add a cmake flag to turn `llvm_unreachable()` into builtin_trap() when assertions are disabled

This re-lands 6316129e066e after fixing the condition logic.

The new flag seems to not be working yet on Windows, where the builtin
trap isn't "no return".

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D121750

Added: 
    

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

Removed: 
    


################################################################################
diff  --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index 9892c6cd2c515..988c77b68c937 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -465,6 +465,7 @@ option(LLVM_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)
 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)

diff  --git a/llvm/docs/CMake.rst b/llvm/docs/CMake.rst
index ac635fc7df885..4f5a0bb3617c1 100644
--- a/llvm/docs/CMake.rst
+++ b/llvm/docs/CMake.rst
@@ -300,6 +300,12 @@ enabled sub-projects. Nearly all of these variable names begin with
   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**:BOOL
+  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

diff  --git a/llvm/include/llvm/Config/llvm-config.h.cmake b/llvm/include/llvm/Config/llvm-config.h.cmake
index d8edf33ba0ce1..f8281d8297929 100644
--- a/llvm/include/llvm/Config/llvm-config.h.cmake
+++ b/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

diff  --git a/llvm/include/llvm/Support/ErrorHandling.h b/llvm/include/llvm/Support/ErrorHandling.h
index f980510d37f06..42dcf4b4f99df 100644
--- a/llvm/include/llvm/Support/ErrorHandling.h
+++ b/llvm/include/llvm/Support/ErrorHandling.h
@@ -124,15 +124,23 @@ llvm_unreachable_internal(const char *msg = nullptr, const char *file = nullptr,
 
 /// 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 an
+//    optimizer hint or printing a reduced message.
 ///
 /// 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