Some of the meta-class proposals (and prototypes!) allow for printf style debugging at compile time.  I agree that it would be awful nice if I could debug the compile time constexpr things, or step through overload resolution somehow.


From: SG14 <> On Behalf Of Clément Grégoire
Sent: Thursday, February 14, 2019 2:13 AM
To: Low Latency:Game Dev/Financial/Trading/Simulation/Embedded Devices <>
Subject: [EXTERNAL] Re: [SG14] [EXT] Ideas on debug constexpr functions or other functions that have been 100% removed from the EXE



I do agree with what's being said, but I think you will get better feedback if you reach SG15 (tooling group) if not done already!





Le jeu. 14 févr. 2019 à 08:33, Scott Wardle <> a écrit :

Thanks for the feedback Paul, I fixed the image VPU0SINCOS.png forgot to upload to my web server.

You have the idea exactly. 

There are a lot of different versions of the idea I can think of.  I have not written a debugger my self but I have worked with people who have back on ps1.  I don’t think the debugger runtime is hard to write.  As you can see with the python mixed language debuggers it is not that original or do I think that hard.

The compiler side however I don’t know at all and don’t know how to start work on an idea like this. But as I have never seen any ISO papers on debugging maybe I should write a paper anyways :)

Maybe Paul, you are getting at this. I don’t really talk about parameters in the blog post.  Each return value has to be mapped to a load of a bunch of parameters and then a call to the code to debug.  I should write something on that. It is more than just the debug version of the code.

Would any of these parameters be non-const if the function is “constexper”? I don’t see how, but if they can be then you would need to load a value into this virtual co-processor memory space not so hard either way.


> On Feb 13, 2019, at 10:40 PM, Paul Hampson <> wrote:
> I had a 404'd image about 30% down:
> Thinking about the practicalities of this idea, what if the debug symbol blobs contains unoptimized versions of constexpr, inlined, and other (perhaps #pragma-marked?) functions, which were used for breakpointing? The debug blobs could know that a particular static value is the result of a constexpr call, since the code=>asm mapping must already see something like "call constExpr(X)" => "LOAD accumulator, some constant value". Same for inlining, a function call that maps to something other than a function-call preamble is presumably an inlined function.
> Then the debugger could step-through that side-lined code blob, but discard the results (and side-effects?) at the end, and execute the code in the function itself. *Or* it could actually execute the side-lined code, giving you the runtime effect of MSVC's "#pragma optimize(off)" triggered only by placing a breakpoint at a certain point.
> Constexpr might be an easier place to start, since we know the side-effects of such a function are limited, and it's really a code->value transform, and they're evaluated down to constants, while inlined and optimised functions are more-arbitrary code->code transforms.
> I don't know enough about the internals of debug tables and debuggers to know if this is practical, but it sounds useful for the use-case you've described.
> On a side-note, we use mixed C++/Python, and it's a delight when we (occasionally) use the MSVC mixed-mode debugging support for Python, and get mixed Python/C++ stack traces. It sounds like the VPU debugger gave the same thing, so there' definitely mixed-mode debugging traditions around we could leverage.
> Not sure if there's a C++ paper in this, probably more of a clang/lld/gdb hackathon opportunity and a nice presentation at CppCon. ^_^
> --
> Paul “Hampy” Hampson
>> -----Original Message-----
>> From: SG14 <> On Behalf Of Scott Wardle
>> Sent: Thursday, 14 February 2019 12:26 PM
>> To: Low Latency:Game Dev/Financial/Trading/Simulation/Embedded Devices
>> <>
>> Subject: [EXT] [SG14] Ideas on debug constexpr functions or other functions
>> that have been 100% removed from the EXE
>> Hi Everyone,
>> I was looking through C++ papers and since 2010 (and maybe before) not one
>> iso C++ paper has been submitted on debugging. So maybe now is a good
>> time to think about how we should debug C++ in a new way.
>> To that end I had an idea the other day on debugging and wrote a blog on it
>> and finally posted it.  I don’t see how this would effect the standard but it is
>> quality of implementation idea.
>> If we want to constexpr all of the things it would nice if we could printf debug
>> at least.  But my idea is maybe we can get the whole debugger to work.
>> Check it out on my blog.
>> Tell me if you have any feedback (spelling grammar or good counter ideas
>> that shows why the idea is not posable etc...)
>> Scott
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