Alternatives to self assignment for 'breakpoint code'

Question

I recently tried to build a project with clang and clang++. Up to now it has only been built with GCC.

One of the problems that I saw was with code that is of the following form

if (expression)
{
   var = var;
}

I assume that this was done to give a place in the code to put breakpoints in debug builds. In optimized builds the code should all be optimized away.

We build with -Werror and this fails to build with

error: explicitly assigning value of variable of type 'int' to itself [-Werror,-Wself-assign]

I can get rid of the warning/error with -Wno-self-assign or -Wno-error=self-assign or locally with

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wself-assign"
var = var;
#pragma clang diagnostic pop

(possibly in a MACRO).

Does anyone have any other suggestions for code without side effects that could be used for breakpoints?

EvilTeach · Accepted Answer · 2024-10-22 17:10:47Z

0

This is what I use. It also issues an unused warning that make them easy to find.

int x = 3;

answered Oct 22, 2024 at 17:10

EvilTeach

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2 Comments

Barmar Over a year ago

But -Werror will turn the unused warning into an error, which is part of the OP's problem.

EvilTeach Over a year ago

add a (void) x; to supress that.

phuclv · Accepted Answer · 2024-12-17 17:12:34Z

0

The standard way in C++ is to use std::breakpoint or std::breakpoint_if_debugging() which is a new C++26 feature, the latter is probably more suitable

This function standardizes many similar existing facilities: __builtin_debugtrap from LLVM, __debugbreak from Win32 API, debugger_break from boost.test, assert(false), _asm { int 3 } (MSVC) and asm("int3") (GCC/clang) for x86 targets, etc.

#if DEBUG
if (std::is_debugger_present())
    std::breakpoint();
std::breakpoint_if_debugging();
#endif

As can be seen from the quote, in LLVM you can use __builtin_debugtrap, or if you have Boost you can use debugger_break

There are other methods that you can see in Is there a portable equivalent to DebugBreak()/__debugbreak?

edited Dec 17, 2024 at 17:12

answered Dec 15, 2024 at 16:34

phuclv

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4 Comments

Nate Eldredge Dec 15, 2024 at 17:08

I think the goal is to have a statement where you can set a breakpoint, but are not forced to. With std::breakpoint the program will always stop when it is reached, and it doesn't go away on optimized builds.

phuclv Dec 16, 2024 at 1:16

@NateEldredge that's why I put an#if there. if (debug) {...} also works

Nate Eldredge Dec 16, 2024 at 1:19

But I think we want to be able to decide within the debugger whether to have a breakpoint there.

phuclv Dec 16, 2024 at 7:52

@NateEldredge yeah that's a valid concern. It's better to not use these at all though, because when we want to break at any line we should put a breakpoint manually at that line

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