On Wed, Oct 25, 2023 at 07:03:43PM +0000, Qing Zhao wrote:
> For the code generation impact:
>
> turning the original x.buf
> to a builtin function call
> __builtin_with_access_and_size(x,buf, x.L,-1)
>
> might inhibit some optimizations from happening before the builtin is
> evaluated into object size info (phase .objsz1). I guess there might be
> some performance impact.
>
> However, if we mark this builtin as PURE, NOTRROW, etc, then the negative
> performance impact will be reduced to minimum?
You can't drop it during objsz1 pass though, otherwise __bdos wouldn't
be able to figure out the dynamic sizes in case of normal (non-early)
inlining - caller takes address of a counted_by array, passes it down
to callee which is only inlined late and uses __bdos, or callee takes address
and returns it and caller uses __bdos, etc. - so it would need to be objsz2.
And while the builtin (or if it is an internal detail rather than user
accessible builtin an internal function) could be even const/nothrow/leaf if
the arguments contain the loads from the structure 2 fields, I'm afraid it
will still have huge code generation impact, prevent tons of pre-IPA
optimizations. And it will need some work to handle it properly during
inlining heuristics, because in GIMPLE the COMPONENT_REF loads aren't gimple
values, so it wouldn't be just the builtin/internal-fn call to be ignored,
but also the count load from memory.
Jakub
