[cfe-dev] Smart Pointer Lifetime Optimizations

[Arthur O'Dwyer via cfe-dev]

On Wed, Jun 10, 2020 at 11:14 AM Johannes Doerfert via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On 6/8/20 8:13 PM, Richard Smith via cfe-dev wrote
> >> You wouldn’t be the first person to be surprised by the result of this
> sort
> >> of analysis, but I’m afraid it’s what we’re working with.
> >>
> >> Unfortunately, there’s really no way to eliminate this one without
> either
> >> interprocedural information or language changes. trivial_abi eliminates
> >> the other one because it changes the convention for passing by value,
> but
> >> to pass an “immutably borrowed” value in C++ we have to pass by
> reference,
> >> which allows the reference to be escaped and accessed (and even
> mutated, if the
> >> original object wasn’t declared const) as long as those accesses happen
> >> before destruction.
> >>
> > Perhaps we should expose LLVM's nocapture attribute to the source level?
>
> Yes please ;)
>

As someone (John?) has said in this thread, the nocapture attribute is
already exposed to C/C++ via __attribute__((noescape)).

Here is an example of __attribute__((noescape)) doing its thing:
https://godbolt.org/z/rJbEmZ
In function `one`, the call to `noargs` is assumed to trash the value of
`i`.
In function `two`, the call to `noargs` is *not* assumed to trash the value
of `i`.

However, notice that the store corresponding to the assignment `i = 42;`
cannot be removed in either case!  We initialize `i` to 42, then pass it by
const reference to `dont_escape`, and yet *cannot assume that its value is
unchanged by that call*. This is required by C++'s loose rules around
`const`. But it doesn't match up with human intuition very well.

I mentioned to Zoe offline that I'd like to see Clang add an opt-in
non-conforming optimization mode, on the lines of `-ffast-math`, which
would *assume* things like "things passed by const reference don't change"
and "things passed by reference don't escape," and then measure to what
degree codegen is improved on real codebases by that non-conforming
optimization mode.

–Arthur
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