[cfe-dev] Folding constexpr function calls with constant arguments in non constant expression contexts

[David Blaikie via cfe-dev]

I'm going to guess it's mostly (3). Checking and trying to constant
evaluate every subexpression would be expensive - the systems to do so in
Clang have to do a lot of work to properly report diagnostics, to act on
the full AST form, etc. It's easier to leave it to the LLVM optimization
passes to do the work, when possible.

On Wed, Feb 22, 2017 at 1:58 AM Roger Ferrer Ibanez via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi all,
>
> maybe this is just a silly question. But I wondered why clang is not
> attempting to fold calls to constexpr functions in non-constant expression
> contexts.
>
> Example:  https://godbolt.org/g/KBaMjp just compare "one" vs "two" in the
> IR
>
> I understand that clang does not have to do this and I'm not saying it
> should.
>
> But I'd like to know what is the rationale here.
> - Is it a C++ Standard issue?, i.e. doing that would trigger some
> side-effects that would be non-compliant.
> - Is it a "separation of concerns" issue?, i.e. let LLVM optimize these
> cases and improve if it can't.
> - Is it a speed issue?, i.e. even if the original AST is preserved for
> fidelity, computing an associated constant value to it takes time which is
> precious for some clients.
>
> Or maybe it is something else that didn't occur to me.
>
> Thank you,
> Roger
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