[cfe-dev] Constexpr evaluation speed

[David Rector via cfe-dev]

> On Mar 2, 2021, at 10:03 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> 
> On Tue, 2 Mar 2021 at 16:46, David Rector via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
> I was reading http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1240r1.pdf <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1240r1.pdf>, which describes proposed reflection features currently under advanced consideration.  
> 
> On page 5, the authors give their rationale for defining `reflexpr(x)` to return an object of an opaque builtin type called `meta::info`, which is useful only when passed to other builtin functions able to access its various properties for different reflected entities.  This design is favored over the alternative of defining it to return an AST-class-like object specific to each reflected entity (ReflectedDeclaration, ReflectedStatement etc.).  
> 
> In other words given this design the user must write e.g. `meta::parameters_of(reflexpr(somefunc))` instead of e.g. `reflexpr(somefunc)->parameters()`.
> 
> One rationale the authors give for this choice is that they found that accessing subobjects of a constexpr class object is significantly slower than accessing values which are not subobjects, all else being equal.  The authors present an example on pp 5-6.  
> 
> I tried to reproduce this example and their results in CompilerExplorer, and was mostly just shocked at the apparent orders-of-magnitude-differences between GCC (by far the fastest), Clang, and MSVC (by far the slowest) at both constant-evaluation tasks.
> 
> Example A (NB `f()` deals only with complete objects):
> https://godbolt.org/z/TM5Wb6 <https://godbolt.org/z/TM5Wb6>
> 
> Example B (same as A, except `f()` now defined to dig through subobjects to get the data it needs):
> https://godbolt.org/z/5dPq3W <https://godbolt.org/z/5dPq3W>
> 
> Results of 5 trials:
>                     __Compilation_times__
> A
> GCC  (1793B):   337,   408,   325,   435,   242 ms
> Clang (218B):  9361,  5173,  4066,  5698,  4263 ms
> MSVC  (306B): 21850, 24616, 24957, 24925, 32323 ms
> 
> B
> GCC  (2295B):   471,   319,   403,   309,   323 ms
> Clang (218B): 17073, 15540, 17281, 13385, 18540 ms
> MSVC   (n/a): always >60k ms
> 
> Takeaways:
> 1. Clang performs constant evaluation 10-50 times slower than GCC.
> 2. While Clang performs B ~3 times slower than it performs A, it is not clear that GCC is likewise affected by having to dig through subobjects (if it is, the effect is slight).
> 
> Questions:
> 1. What am I missing?  Are there flags which might improve clang’s performance?  Is GCC somehow gaining an unfair advantage?  (Potential clue: note that the executable is the same small size, 218 bytes, for each of clang’s results, but it is larger and differs for GCC’s results…meaningful?)  
> 
> Yes, GCC is "cheating" (you're not testing what you think you are). GCC memoizes constant evaluations (at least when it's correct to do so). Here's a slightly modified version of your A that doesn't permit memoization: https://godbolt.org/z/3q3TYr <https://godbolt.org/z/3q3TYr>
> 
> 5 trials with that and GCC (1793B):  12340, 11106, 10204, 9983, 10771ms
> 
> (Times for Clang and MSVC seem similar to your measurements.) I don't know if compiler explorer uses the same machines for all compiles, or if all compilers are built in fully-optimized mode, but if so, that suggests that Clang is about 2x faster than GCC for this particular testcase.
> 
> 2. Given the "constexpr all the things" zeitgeist, and the constant evaluation speeds GCC has apparently realized, should the design of ExprConstant.cpp/APValue/etc. be reconsidered?
> 
> Ignoring the part about GCC, yes. We have a -fexperimental-new-constant-interpreter flag that enables a new interpreter, which was built to be substantially faster. Unfortunately it's not complete yet (the version in trunk doesn't support any looping constructs yet) but the early indications are very promising.
> 
> 3. If ExprConstant.cpp etc were overhauled for optimal speed, will it still ultimately be true that programs which dig through subobjects of compile time objects are necessarily slower than equivalent programs which deal only with complete compile-time objects?
> 
> I don't think that would necessarily be the case. I've also mentioned that in committee, but "one compiler can do X" doesn't necessarily translate into "everyone will do X", and folks representing other compilers have indicated they expect the more "bare-bones" approach will remain faster for their implementations.
> 

Thanks Richard, very thorough answer; that adjustment does indeed turn the tables on GCC.  Clang takes the gold, and that’s before even the fancy new interpreter is deployed.  Looking forward to it.

Assuming like machines are running the various compilers on CompilerExplorer, I think we can all hazard a pretty good guess at this point which particular compiler really benefits from keeping things as bare bones as possible ;)

Dave

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