On Tue, May 21, 2019 at 6:15 PM Iain Sandoe <i...@sandoe.co.uk> wrote: > > Hi Uros, > > It seems to me that (even if it was working “properly”, which it isn't) > ‘-mfentry’ would break ABI on Darwin for both 32 and 64b - which require > 16byte stack alignment at call sites. > > For Darwin, the dynamic loader enforces the requirement when it can and will > abort a program that tries to make a DSO linkage with the stack in an > incorrect alignment. We previously had a bug against profiling caused by > exactly this issue (but when the mcount call was in the post-prologue > position). > > Actually, I’m not sure why it’s not an issue for other 64b platforms that use > the psABI (AFAIR, it’s only the 32b case that’s Darwin-specific).
The __fentry__ in glibc is written as a wrapper around the call to __mcount_internal, and is written in such a way that it compensates stack misalignment in a call to __mcount_internal. __fentry__ survives stack misalignment, since no xmm regs are saved to the stack in the function. > Anyway, my current plan is to disable mfentry (for Darwin) - the alternative > might be some kind of “almost at the start of the function, but needing some > stack alignment change”, > > I’m interested in if you know of any compelling use-cases that would make it > worth finding some work-around instead of disabling. Unfortunately, not from the top of my head... Uros.
