On 16/12/2018 22:45, Uecker, Martin wrote: > Am Sonntag, den 16.12.2018, 09:13 -0700 schrieb Jeff Law: >> Ultimately using function descriptors is an ABI breaking choice and we >> might declare that function descriptors imply higher function >> alignments. > > Increasing the alignment is not an ABI breaking change.
increasing alignment _requirement_ is an abi breaking change. and it's not clear who would benefit from the new abi: - it affects everything that does indirect calls (if alignment requirement is increased then in addition everything that has functions whose address may be taken), so it can easily affect existing handwritten asm and it definitely requires the rebuild of the c runtime to support this abi (i think it even requires asm changes there if you allow a thread or makecontext start function to be a nested function). - it makes indirect calls more expensive everywhere, even if nested functions are not used. i think to fix the executable stack problem in practice, the new nested function mechanism should only require the rebuild of code that actually contains nested functions and thus have no abi or performance impact on code that never uses them. i believe this can be achieved by some restrictions on nested function usage in a way that covers most practical use-cases: e.g. only allowing one active parent function call frame per thread, no recursive calls to it, the nested function must be invoked in the same thread as the parent using the same stack, etc. (then the new mechanism can be used safely if nested functions are known to follow the restrictions, the compiler may even emit code to check the constraints at runtime.)
