[llvm-dev] [RFC] Usage of NDEBUG as a guard for non-assert debug code

Chris Tetreault via llvm-dev llvm-dev at lists.llvm.org
Thu Apr 9 09:59:18 PDT 2020


   In my opinion, NDEBUG is one of those gross old C things that everybody complains about. It's called "Not Debug", but really it means "Assert Disabled". I think one could be forgiven for actually using it as a heuristic of whether or not a build is a debug build, especially since no other options are provided. I appreciate your desire, but I think it'd be unfortunate if the build system grew yet another flag to control debugness.

   As far as I can tell, as it currently works, LLVM_ENABLE_ASSERTIONS just makes sure that NDEBUG is not defined, even in release builds. So if I do -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=TRUE, I'll get an optimized build with no debug symbols but with asserts enabled, which in my mind isn't a terribly useful thing to have. Furthermore, none of this works on Visual Studio because it has a UI menu to control the build type. I personally would be very disappointed to see Visual Studio's build type dropdown break.

   Since we C's assert, it is intrinsically tied to NDEBUG. What we need is proper custom asserts. In codebases I've seen in my travels that have this it usually looks like:

// If asserts are enabled, evaluate and assert that expr is truthy. If it is not, complain with msg.
LLVM_ASSERT(expr, msg)

// If asserts are enabled, evaluate and assert that expr is truthy. If it is not, complain with msg.
// If asserts are disabled, evaluate expr, do not assert.
// either way, return expr
LLVM_VERIFY(expr, msg)

   The first one is useful as a traditional assert. The second one is useful if you are calling a function, and want to assert that it succeeds, but still need it to be evaluated in release builds:

auto *Foo = LLVM_VERIFY(ReturnsAPointerThatShouldNeverActuallyBeNull(), "this should never return null");

   If we have custom asserts, then we can have custom assert guard macros:

// true if this is any sort of debug build

// true if asserts are turned on (Debug build on Windows,
// Debug build or -DLLVM_ASSERTIONS_ENABLED=TRUE on other platforms)

   These flags could be derived from just CMAKE_BUILD_TYPE, and LLVM_ENABLE_ASSERTIONS can go away (assuming we agree that an asserting build with optimizations and no debug info is worse than useless). Custom asserts also have the advantage of having a proper message parameter and not needing to rely on the truthiness of string literals. Obviously this is a much more invasive change than what you are proposing, but in my opinion it's the correct thing to do.

   Christopher Tetreault

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of David Truby via llvm-dev
Sent: Thursday, April 9, 2020 7:26 AM
To: llvm-dev at lists.llvm.org
Subject: [EXT] [llvm-dev] [RFC] Usage of NDEBUG as a guard for non-assert debug code

Hi all,

During discussions about assertions in the Flang project, we noticed that there are a lot of cases in LLVM that #ifndef NDEBUG is used as a guard for non-assert code that we want enabled in debug builds.
This works fine on its own, however it affects the behaviour of LLVM_ENABLE_ASSERTIONS;  since NDEBUG controls whether assertions are enabled or not, a lot of debug code gets enabled in addition to asserts if you specify this flag. This goes contrary to the name of the flag I believe also its intention. Specifically in Flang we have a case where someone wants to ship a build with assertions enabled, but doesn't want to drag in all the extra things that are controlled by NDEBUG in LLVM.

In my opinion we ideally want LLVM_ENABLE_ASSERTIONS to _only_ enable assertions and do nothing else. I don't think this is possible without changing the use of NDEBUG elsewhere as NDEBUG controls whether assert is enabled.
I propose we should be using another macro (something like LLVM_DEBUG_CHECKS ?) that is enabled in Debug builds, and possibly controlled by another cmake flag (LLVM_ENABLE_DEBUG_CHECKS ?) for code that we want enabled for debugging but not in releases. This would allow LLVM_ENABLE_ASSERTIONS to do what it says on the tin and actually enable assertions only.

Does anyone else have any thoughts on this?

David Truby
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