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

[Connor Davis via cfe-dev]

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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