From: Russell King > Sent: 21 March 2024 11:24 > > On Thu, Mar 21, 2024 at 10:22:30AM +0000, David Laight wrote: > > How aggressively does the compiler optimise 'noreturn' functions? > > I've seen cases where the compiler emits a BL instruction as the very > last thing in the function, and nothing after it. I've also seen the compiler defer generating a stack frame until after an initial conditional. That might mean you can get the BL in the middle of a function but where the following instruction is for the 'no stack frame' side of the branch. That is very likely to break any stack offset calculations. > This is where the problem lies - because the link register value > created by the BL instruction will point to the instruction after the > BL which will _not_ part of the function that invoked the BL. That > will probably cause issues for the ELF unwinder, which means this > issue probably goes beyond _just_ printing the function name. Isn't this already in the unwinder? A BL itself isn't going to fault with PC = next-instruction. For pretty much all code isn't *(LR-4) going to be BL? On arm that is probably testable. (It is pretty much impossible to detect a ACLL on x86.) If it is a direct BL then you'd normally expect to the be a call the function containing the current 'PC'. The obvious exception is if there was a tail call, and printing the called address would then be useful. (It might help with leaf functions that don't generate a stack frame.) I remember issues with the solaris sparc backtrace that used to get confused by leaf functions... > I have vague memories that Ard has been involved in the unwinder, > maybe he could comment on this problem? Maybe we need the unwinder > itself to do the correction? I also wonder whether we should only > do the correction if we detect that we're pointing at the first > instruction of a function, and the previous instruction in the > text segment was a BL. It might be enough to depend on whether the address is that of a fault (where the instruction could be retried) or from a call/trap instruction where it will be the following instruction. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
