[committed] libstdc++: Fix code size regressions in std::vector [PR110060]

[Jakub Jelinek]

On Thu, Jun 08, 2023 at 10:05:43AM +0100, Jonathan Wakely via Gcc-patches wrote:
> > Looking at assembly, one of the differences I see is that the "after"
> > version has calls to realloc_insert(), while "before" version seems to have
> > them inlined [2].
> >
> > [1]
> > https://git.linaro.org/toolchain/ci/interesting-commits.git/tree/gcc/sha1/b7b255e77a271974479c34d1db3daafc04b920bc/tcwg_bmk-code_size-cpu2017fast/status.txt
> >
> >
> I find it annoying that adding `if (n < sz) __builtin_unreachable()` seems
> to affect the size estimates for the function, and so perturbs inlining
> decisions. That code shouldn't add any actual instructions, so shouldn't
> affect size estimates.
> 
> I mentioned this in a meeting last week and Jason suggested checking
> whether using __builtin_assume has the same undesirable consequences, so I

We don't support __builtin_assume (intentionally), if you mean [[assume(n>=sz)]],
then because n >= sz doesn't have side-effects, it will be lowered to
exactly that if (n < sz) __builtin_unreachable(); - you can look at
-fdump-tree-all to confirm that.

I agree that the inliner should ignore if (comparison) __builtin_unreachable();
from costs estimation.  And inliner should ignore what we emit for [[assume()]]
if there are side-effects.  CCing Honza.

	Jakub