mfentry and Darwin.

[Iain Sandoe]

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).

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.

thanks
Iain