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

[dexonsmith]

I like this idea.

Not sure if LLVM_UNREACHABLE_SHOULD_TRAP is quite right... would LLVM_UNREACHABLE_ALWAYS_TRAP be better?
(I'm honestly not sure. "should" seems a bit off to me, since it doesn't sound like a guarantee.)

Or: LLVM_UNREACHABLE_OPTIMIZE=OFF?
(Reasoning: it's disabling the optimization opportunity in no-asserts builds but leaving behind the debugging feature in asserts builds.)

llvm/include/llvm/Support/ErrorHandling.h

126–127

I think this part of the comment should be updated.

[mehdi_amini]

Address Duncan's comment: rename the flag to LLVM_UNREACHABLE_OPTIMIZE and update the comment

[mehdi_amini]

Or: LLVM_UNREACHABLE_OPTIMIZE=OFF?
(Reasoning: it's disabling the optimization opportunity in no-asserts builds but leaving behind the debugging feature in asserts builds.)

I like the naming! I had the same reasoning but didn't find a good name :)

llvm/include/llvm/Support/ErrorHandling.h

126–127

Done! PTAL

[dexonsmith]

It feels to me like this is worth documenting in llvm/docs/CMake.rst, maybe next to LLVM_ABI_BREAKING_CHECKS since they both seem assertion related, so that it's more easily discovered.

Otherwise, LGTM, with more wording suggestion inline.

llvm/CMakeLists.txt

468

I wonder if the wording should be tied to the word "optimize". Also, note that trapping is a legit implementation of undefined behaviour; so another interpretation is that llvm_unreachable() is being treated as UB either way.

E.g.:

A "reached" llvm_unreachable() is undefined behaviour; ON optimizes the unreachable control flow (default), while OFF triggers a guaranteed trap

WDYT?
(I'm okay with landing the wording as you had it if you prefer it; can always update it later)

[dexonsmith]

llvm/CMakeLists.txt

468

Or maybe this should be short and sweet:

Optimize control flow based on llvm_unreachable(); guaranteed trap when OFF

(and leave the longer explanation for llvm/docs/...)

[jdoerfert]

Cool :)

[mehdi_amini]

llvm/CMakeLists.txt

468

Trapping is a legit implementation of UB, but the opposite isn't and this is the important bit I was trying to express.
I see what you mean with "optimize" but it may not be obvious for everyone that we're turning a trap into UB here.

[mehdi_amini]

llvm/CMakeLists.txt

468

What about: "Optimize llvm_unreachable() as undefined behavior (default), guaranteed trap when OFF"?

[mehdi_amini]

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

[mehdi_amini]

Fix doc: s/STRING/BOOL/

[dexonsmith]

llvm/CMakeLists.txt

468

Sure, sounds fine to me!

llvm/include/llvm/Support/ErrorHandling.h

134–135

I'd prefer one of:

/// * When "OFF", a builtin_trap is emitted instead.
/// * When "OFF", a builtin_trap is emitted instead of enabling
//    optimizations or printing a reduced message.
/// * When "OFF", a builtin_trap is emitted instead of an
//    optimizer hint or printing a reduced message.

But your text here LGTM too; I don't think it needs to be perfect.

[dyung]

This seems to be breaking our internal Release Windows build with errors of the sort:

7>C:\src\upstream\llvm_clean_git\llvm\lib\Support\Compression.cpp(104): error C4716: 'llvm::zlib::uncompress': must return a value
7>C:\src\upstream\llvm_clean_git\llvm\lib\Support\Compression.cpp(107): error C4716: 'llvm::zlib::crc32': must return a value
7>C:\src\upstream\llvm_clean_git\llvm\lib\Support\Compression.cpp(99): error C4716: 'llvm::zlib::uncompress': must return a value
...
5>C:\src\upstream\llvm_clean_git\clang\utils\TableGen\MveEmitter.cpp(480): error C4716: '`anonymous namespace'::Result::getIntegerValue': must return a value
...
3>C:\src\upstream\llvm_clean_git\llvm\utils\TableGen\GlobalISelEmitter.cpp(1175): error C4716: '`anonymous namespace'::PredicateMatcher::getValue': must return a value
3>C:\src\upstream\llvm_clean_git\llvm\utils\TableGen\GlobalISelEmitter.cpp(803): error C4716: '`anonymous namespace'::SwitchMatcher::getFirstCondition': must return a value
3>C:\src\upstream\llvm_clean_git\llvm\utils\TableGen\GlobalISelEmitter.cpp(800): error C4716: '`anonymous namespace'::SwitchMatcher::popFirstCondition': must return a value

The cmake command I ran for this build was:

cmake.exe -G "Visual Studio 16 2019" -DCLANG_ENABLE_ARCMT=OFF -DCMAKE_BUILD_TYPES=Release -DLLVM_BUILD_RUNTIME=OFF -DLLVM_DEFAULT_TARGET_TRIPLE=x86_64-ps4-scei -DLLVM_ENABLE_TIMESTAMPS=OFF -DLLVM_INCLUDE_EXAMPLES=OFF -DLLVM_TARGETS_TO_BUILD=X86 -DLLVM_TOOL_LLD_BUILD=OFF -DLLVM_VERSION_SUFFIX= -DLLVM_LIT_ARGS="--verbose -j24" -Thost=x64 -DLLVM_ENABLE_PROJECTS=compiler-rt;clang;clang-tools-extra C:\src\upstream\llvm_clean_git\llvm

Can you take a look?

[dexonsmith]

llvm/include/llvm/Support/ErrorHandling.h

143

Based on:

7>C:\src\upstream\llvm_clean_git\llvm\lib\Support\Compression.cpp(104): error C4716: 'llvm::zlib::uncompress': must return a value

it looks like LLVM_BUILTIN_TRAP isn't guaranteed to be something the compiler understands as noreturn. Maybe this logic needs to fallback to llvm_unreachable_internal if LLVM_BUILTIN_TRAP is not a function.

[mehdi_amini]

llvm/include/llvm/Support/ErrorHandling.h

143

The bug is that when you suggested a renaming the macro I didn't inverse the condition: the previous name was something like SHOULD_TRAP but with the new name the logic is reversed! I need #elif !LLVM_UNREACHABLE_OPTIMIZE I think.

[mehdi_amini]

Fix the condition

llvm/include/llvm/Support/ErrorHandling.h

143

I fixed the condition but I'm not sure how to address the windows situation. I think I'd try to add an attribute or something to make it "noreturn" there as well, but rather leave it to someone who could actually test it.

[mehdi_amini]

By the way: it seems that there is no bot upstream building on Windows without assertions, I didn't get an email about the initial breakage!

[dexonsmith]

llvm/include/llvm/Support/ErrorHandling.h

143

Hah, that's what I thought at first, and then I convinced myself that it wasn't wrong and jumped to the other conclusion :).

For the windows thing, maybe don't rely on LLVM_BUILTIN_TRAP for now, if it's not guaranteed to be noreturn. Could change the builtin trap logic to export another variable:

#if __has_builtin(__builtin_trap) || defined(__GNUC__)
# define LLVM_BUILTIN_TRAP __builtin_trap()
# define LLVM_BUILTIN_TRAP_NO_RETURN 1
#elif defined(_MSC_VER)
// The __debugbreak intrinsic is supported by MSVC, does not require forward
// declarations involving platform-specific typedefs (unlike RaiseException),
// results in a call to vectored exception handlers, and encodes to a short
// instruction that still causes the trapping behavior we want.
# define LLVM_BUILTIN_TRAP __debugbreak()
# define LLVM_BUILTIN_TRAP_NO_RETURN 0
#else
# define LLVM_BUILTIN_TRAP *(volatile int*)0x11 = 0
# define LLVM_BUILTIN_TRAP_NO_RETURN 0
#endif

Assuming something similar creates a variable LLVM_BUILTIN_UNREACHABLE_NON_EMPTY (to fix the bug mentioned in the other inline comment):

#ifndef NDEBUG
#define llvm_unreachable(msg) \
  ::llvm::llvm_unreachable_internal(msg, __FILE__, __LINE__)
#elif LLVM_UNREACHABLE_OPTIMIZE && LLVM_BUILTIN_UNREACHABLE_NON_EMPTY
#define llvm_unreachable(msg) LLVM_BUILTIN_UNREACHABLE
#elif LLVM_BUILTIN_TRAP_NO_RETURN
#define llvm_unreachable(msg) LLVM_BUILTIN_TRAP
#else
#define llvm_unreachable(msg) ::llvm::llvm_unreachable_internal()
#endif

144

Just noticed that this pre-existing logic was wrong.

LLVM_BUILTIN_UNREACHABLE is defined whenever llvm/Support/Compiler.h is included:

#if __has_builtin(__builtin_unreachable) || defined(__GNUC__)
# define LLVM_BUILTIN_UNREACHABLE __builtin_unreachable()
#elif defined(_MSC_VER)
# define LLVM_BUILTIN_UNREACHABLE __assume(false)
#else
# define LLVM_BUILTIN_UNREACHABLE
#endif

That probably didn't used to be true.

I think the intent of this code is to check whether LLVM_BUILTIN_UNREACHABLE is empty. If the Compiler.h logic exports another variable that can be used for detecting that, this could become:

#ifndef NDEBUG
#define llvm_unreachable(msg) \
  ::llvm::llvm_unreachable_internal(msg, __FILE__, __LINE__)
#elif LLVM_BUILTIN_UNREACHABLE_NON_EMPTY
#define llvm_unreachable(msg) LLVM_BUILTIN_UNREACHABLE
#else
#define llvm_unreachable(msg) ::llvm::llvm_unreachable_internal()
#endif

(before your patch)

[mehdi_amini]

llvm/include/llvm/Support/ErrorHandling.h

143

Right, that would be a valid path. But what about another path of making LLVM_BUILTIN_TRAP correctly "noreturn" on Windows as well? I'm not a Windows user but someone more versed into Windows may be able to help on this?

144

I wonder if just:

#ifndef NDEBUG
#define llvm_unreachable(msg) \
  ::llvm::llvm_unreachable_internal(msg, __FILE__, __LINE__)
#elif!LLVM_UNREACHABLE_OPTIMIZE
#define llvm_unreachable(msg) LLVM_BUILTIN_UNREACHABLE
#else
#define llvm_unreachable(msg) LLVM_BUILTIN_TRAP
#endif

Wouldn't be enough? Yes it would accounts to "nothing" when LLVM_BUILTIN_UNREACHABLE is empty, but that seems just correct and maybe even desirable: when LLVM_UNREACHABLE_OPTIMIZE is enabled "do nothing"
seems closer to the behavior we're looking for rather than calling llvm_unreachable_internal() right?

[dexonsmith]

llvm/include/llvm/Support/ErrorHandling.h

143

Maybe the windows case could be fixed with another intrinsic, other than __debugbreak. Note that __debugbreak() can return. From https://docs.microsoft.com/en-us/cpp/intrinsics/debugbreak?view=msvc-170 :

Causes a breakpoint in your code, where the user will be prompted to run the debugger.

Maybe @rnk has an idea for something else.

Even with that fixed, is there a way to mark the fall-through case as "no-return"?

# define LLVM_BUILTIN_TRAP *(volatile int*)0x11 = 0

That said, I'm not sure if making builtin_trap() no-return needs to be a blocker for this patch.

144

Did you mean:

#ifndef NDEBUG
#define llvm_unreachable(msg) \
  ::llvm::llvm_unreachable_internal(msg, __FILE__, __LINE__)
#elif LLVM_UNREACHABLE_OPTIMIZE
#define llvm_unreachable(msg) LLVM_BUILTIN_UNREACHABLE
#else
#define llvm_unreachable(msg) LLVM_BUILTIN_TRAP
#endif

? (Or maybe I'm reading it backwards)

Regardless, one of the important pieces of llvm_unreachable() is that it suppresses diagnostics about the control path it's on. If it expands to "empty"/nothing then it won't suppress diagnostics.

I think the behaviour here changed accidentally. @aeubanks updated LLVM_BUILTIN_UNREACHABLE as a drive-by in 26827337dff26ba3450721f880d4c6caaf2a8219 / https://reviews.llvm.org/D111581:

-#if __has_builtin(__builtin_unreachable) || LLVM_GNUC_PREREQ(4, 5, 0)
+#if __has_builtin(__builtin_unreachable) || defined(__GNUC__)
 # define LLVM_BUILTIN_UNREACHABLE __builtin_unreachable()
 #elif defined(_MSC_VER)
 # define LLVM_BUILTIN_UNREACHABLE __assume(false)
+#else
+# define LLVM_BUILTIN_UNREACHABLE
 #endif

Add a missing #endif for LLVM_ATTRIBUTE_UNREACHABLE.

Before that, the fallback behaviour was to hit the #else case here so that llvm_unreachable() always expanded to something that was no-return.

Maybe we should revert that part of 26827337dff26ba3450721f880d4c6caaf2a8219 patch before landing this.

[dexonsmith]

llvm/include/llvm/Support/ErrorHandling.h

144

Done in https://reviews.llvm.org/D122167 / 892c104fb71b86dc6399e36a82ae920e00ccae17.

[dexonsmith]

I suggest:

#ifndef NDEBUG
#define llvm_unreachable(msg) \
  ::llvm::llvm_unreachable_internal(msg, __FILE__, __LINE__)
#elif !defined(LLVM_BUILTIN_UNREACHABLE)
#define llvm_unreachable(msg) ::llvm::llvm_unreachable_internal()
#elif LLVM_UNREACHABLE_OPTIMIZE
#define llvm_unreachable(msg) LLVM_BUILTIN_UNREACHABLE
#else
#define llvm_unreachable(msg) LLVM_BUILTIN_TRAP, LLVM_BUILTIN_UNREACHABLE
#endif

I *think* this will have all the intended effects:

• If asserts are off, use the function (like a fancy assertion).
• Else, if there is no LLVM_BUILTIN_UNREACHABLE, call the function without args (works again as of https://reviews.llvm.org/D122167).
• Else, always call LLVM_BUILTIN_UNREACHABLE to suppress warnings about this control flow path...
  • ... but if !LLVM_UNREACHABLE_OPTIMIZE, first call LLVM_BUILTIN_TRAP to block optimizations.
  • ... and if LLVM_BUILTIN_TRAP happens to be marked no-return, the extra LLVM_BUILTIN_UNREACHABLE should get cleaned up by the optimizer.

[mehdi_amini]

In D121750#3397382, @dexonsmith wrote:

I suggest:

#ifndef NDEBUG
#define llvm_unreachable(msg) \
  ::llvm::llvm_unreachable_internal(msg, __FILE__, __LINE__)
#elif !defined(LLVM_BUILTIN_UNREACHABLE)
#define llvm_unreachable(msg) ::llvm::llvm_unreachable_internal()
#elif LLVM_UNREACHABLE_OPTIMIZE
#define llvm_unreachable(msg) LLVM_BUILTIN_UNREACHABLE
#else
#define llvm_unreachable(msg) LLVM_BUILTIN_TRAP, LLVM_BUILTIN_UNREACHABLE
#endif

I *think* this will have all the intended effects:

• If asserts are off, use the function (like a fancy assertion).
• Else, if there is no LLVM_BUILTIN_UNREACHABLE, call the function without args (works again as of https://reviews.llvm.org/D122167).
• Else, always call LLVM_BUILTIN_UNREACHABLE to suppress warnings about this control flow path...
  • ... but if !LLVM_UNREACHABLE_OPTIMIZE, first call LLVM_BUILTIN_TRAP to block optimizations.
  • ... and if LLVM_BUILTIN_TRAP happens to be marked no-return, the extra LLVM_BUILTIN_UNREACHABLE should get cleaned up by the optimizer.

LG: https://reviews.llvm.org/D122170

llvm/include/llvm/Support/ErrorHandling.h

144

Regardless, one of the important pieces of llvm_unreachable() is that it suppresses diagnostics about the control path it's on. If it expands to "empty"/nothing then it won't suppress diagnostics.

Ah I forgot about this. That said: when you encounter a toolchain that does not support a proper "unreachable", that would just mean they don't have a way to suppress the diagnostics and have to live with warnings. For the *(volatile int*)0x11 = 0, you can wrap it in a macro with a noreturn attribute: []() __attribute__((__noreturn__)) { *(volatile int*)0x11 = 0; }(); (the same may apply to Windows debugbreak() builtin by the way).

Anyway I think your solution will work just fine!

llvm/CMakeLists.txt

 option(LLVM_ENABLE_LOCAL_SUBMODULE_VISIBILITY "Compile with -fmodules-local-submodule-visibility." ON)
 option(LLVM_ENABLE_LIBCXX "Use libc++ if available." OFF)
 option(LLVM_STATIC_LINK_CXX_STDLIB "Statically link the standard library." OFF)
 option(LLVM_ENABLE_LLD "Use lld as C and C++ linker." OFF)
 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)
 else()
   option(LLVM_ENABLE_ASSERTIONS "Enable assertions" ON)
 endif()
 
 option(LLVM_ENABLE_EXPENSIVE_CHECKS "Enable expensive checks" OFF)

llvm/docs/CMake.rst

**LLVM_ABI_BREAKING_CHECKS**:STRING
   Used to decide if LLVM should be built with ABI breaking checks or
   not.  Allowed values are `WITH_ASSERTS` (default), `FORCE_ON` and
   `FORCE_OFF`.  `WITH_ASSERTS` turns on ABI breaking checks in an
   assertion enabled build.  `FORCE_ON` (`FORCE_OFF`) turns them on
   (off) irrespective of whether normal (`NDEBUG`-based) assertions are
   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
   ``llvm/include/llvm/Support/VCSRevision.h``. Developers using git who don't
   need revision info can disable this option to avoid re-linking most binaries
   after a branch switch. Defaults to ON.
 
 **LLVM_BUILD_32_BITS**:BOOL

llvm/include/llvm/Config/llvm-config.h.cmake

 #cmakedefine LLVM_BUILD_LLVM_DYLIB
 
 /* Define if building LLVM with BUILD_SHARED_LIBS */
 #cmakedefine LLVM_BUILD_SHARED_LIBS
 
 /* 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

llvm/include/llvm/Support/ErrorHandling.h

 /// Use the llvm_unreachable macro (that adds location info), instead of
 /// calling this function directly.
 [[noreturn]] void
 llvm_unreachable_internal(const char *msg = nullptr, const char *file = nullptr,
                           unsigned line = 0);
 }
 
 /// 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
 #define llvm_unreachable(msg) ::llvm::llvm_unreachable_internal()
 #endif
 
 #endif