[cfe-dev] [clang-tidy] Determining constexpr evaluation time

[David Rector via cfe-dev]

I’m stumped — I am kind of surprised that `f(3)` is not always evaluated away before CodeGen, but even disregarding that, I am not sure how to determine which CallExprs are ultimately emitted as calls given the possibility of additional optimizations.

Does anyone else have insight?

Dave

> On Sep 1, 2020, at 12:48 AM, Connor Davis <connojd at pm.me> wrote:
> 
> Hi Dave,
> 
> That makes sense, however I'm looking for a way to determine if an actual
> call instruction will be emitted. Using your example in godbolt (using x86-64
> trunk with -std=c++17) there are four call instructions in main. But using
> ```constexpr int l = f(3);``` results in only three calls in main. And passing
> -O2 removes all of them. Is there some way to statically determine that the
> call instruction will be omitted?
> 
> For context, I have a patch for misc-no-recursion that does not warn when an
> SCC is found in the call graph where each node is marked consteval, and I thought
> that the "every node is constexpr but evaluated at compile time" is another case
> where recursion would be OK. Although now that I type it out it's really "every
> node is inlined".
> 
> Thanks,
> Connor
> 
> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> On Monday, August 31, 2020 5:17 PM, David Rector <davrecthreads at gmail.com> wrote:
> 
>> There is `Expr::isCXX11ConstantExpr(const ASTContext &C, …)`, which returns e.g.:
>> 
>> ```
>> constexpr int f(int i) {
>>   return i + 42;
>> }
>> int g(int i) {
>>   return
>>       f(i); // isCXX11ConstantExpr = false
>> }
>> int main() {
>>   int j =
>>       g(3); // isCXX11ConstantExpr = false
>>   int k =
>>       f(g(3)); // isCXX11ConstantExpr = false (both CallExprs)
>>   int l =
>>       f(3); // isCXX11ConstantExpr = true
>> }```
>> 
>> It is defined in terms of `EvaluateAsRValue` FWIW.  Note that all the `EvaluateAs*` methods refer to evaluating at compile time, regardless of the `InConstantContext` argument, and (I think) all will return false whenever the expression cannot fully "fold" (i.e. if there will necessarily be run-time dependencies remaining in the expression, or there was an error during evaluation).
>> 
>> Hope that helps,
>> 
>> Dave
>> 
>>> On Aug 30, 2020, at 7:15 PM, Connor Davis via cfe-dev <cfe-dev at lists.llvm.org> wrote:
>>> 
>>> Hello all,
>>> 
>>> Is it possible to determine if a CallExpr will be evaluated
>>> at compile time from clang-tidy? I notice some references to
>>> InConstantContext in the EvaluateAs* functions in ExprConstant.cpp,
>>> but I'm not sure if that is applicable/usable from clang-tidy.
>>> 
>>> Thanks,
>>> Connor Davis
>>> 
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> 
>