[clang] [clang] Avoid evaluating the BitWidth expression over and over again (PR #66203)

[Timm Baeder via cfe-commits]

tbaederr wrote:

> > Well, this is not going to make a noticeable difference in runtime. https://reviews.llvm.org/D155548 didn't land because there are no measurements to make where this makes a measurable difference.
> 
> Those changes didn't land because no measurements were attempted. Putting up a branch at https://llvm-compile-time-tracker.com/ would help get those measurements to at least start to see if there's benefit or harm from the changes.

I did measure it, but only by myself, I can ask Nikita do use the compile-time tracker as well.

> 
> > As for my earlier comment, it would also make sense to rename that function to `computeBitWidth()` or just cache the computed value (we compute it when parsing anyway to diagnose 0 size, etc. right?).
> 
> Caching the computed value would make sense, but that's sort of the goal of D155548, right? That's a generalized caching mechanism that should mean when we compute it to diagnose 0 size, we never need to re-compute it again.

Well yes, we would never compute it again, but we still go through quite a few function calls to figure out that we've already computed it before. For a function called `getBitWidthValue()` I basically assume it has exactly one statement: `return BitWidthValue`.

I thought the bitwidth expression will always be a `ConstantExpr`, unless it's value-dependent (in which case it will be a `ConstantExpr` when we instantiate it, right?), but I might be wrong.



https://github.com/llvm/llvm-project/pull/66203