On Wed, Sep 14, 2022 at 04:55:27PM +0200, Peter Zijlstra wrote: > On Wed, Sep 14, 2022 at 02:28:26PM +0000, Michael Matz wrote: > > Don't mix DWARF debug info with DWARF-based unwinding info, the latter > > doesn't imply the former. Out of interest: how does ORC get around the > > need for CFI annotations (or equivalents to restore registers) and what > > Objtool 'interprets' the stackops. So it follows the call-graph and is > an interpreter for all instructions that modify the stack. Doing that it > konws what the stackframe is at 'most' places.
To get correct backtraces on e.g. PowerPC you need to emulate many of the integer insns. That is why GCC enables -fasynchronous-unwind-tables by default for us. The same is true for s390, aarch64, and x86 (unless 32-bit w/ frame pointer). The problem is that you do not know how to access anything on the stack, whether in the current frame or in a previous frame, from a random point in the program. GDB has many heuristics for this, and it still does not get it right in all cases. > > makes it fast? I want faster unwinding for DWARF as well, when there's > > feature parity :-) Maybe something can be learned for integration into > > dwarf-unwind. > > I think we have some details here: > > https://www.kernel.org/doc/html/latest/x86/orc-unwinder.html It is faster because it does a whole lot less. Is that still enough? It's not clear (to me) what exact information it wants to provide :-( Segher
