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Subject: Re: [EXT] Ideas on debug constexpr functions or other functions that have been 100% removed from the EXE
From: Paul Hampson (p_hampson_at_[hidden])
Date: 2019-02-14 00:40:43

I had a 404'd image about 30% down: https://www.swardle.com/sweb/img/VPU0SINCOS.png

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 <sg14-bounces_at_[hidden]> On Behalf Of Scott Wardle
> Sent: Thursday, 14 February 2019 12:26 PM
> To: Low Latency:Game Dev/Financial/Trading/Simulation/Embedded Devices
> <sg14_at_[hidden]>
> 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. http://www.swardle.com/sweb/blog5.html
> 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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