[fpc-devel] CFI

Jonas Maebe jonas at freepascal.org
Tue Jun 7 14:07:04 CEST 2022

On 2022-06-07 11:09, Joost van der Sluis via fpc-devel wrote:

> This is a snippet of the information that the compiler typically
> generates: (x86_64-linux)
> fde:
> <    0><0x00401090:0x004010c1><JoJo><cie offset 0x00000000::cie index
> 0><fde offset 0x00000014 length: 0x00000024><eh offset none>
>         0x00401090: <off cfa=08(r7) > <off r16=-8(cfa) >
>         0x00401091: <off cfa=16(r7) > <off r3=-16(cfa) > <off 
> r16=-8(cfa) >
>         0x004010c0: <off cfa=08(r7) > <off r16=-8(cfa) >
> This basically means that when the instruction-pointer is at
> 0x00401090, the CFA (frame) can be obtained by taking register 7 (rsp)
> and add 8 to it.
> The return address (called r16 here) if obtained by taking the CFA,
> substract 8 and then read the value at that memory location.
> So far so good. But then I need the next frame. So use the return
> address, substract one (To obtain the call address). And start over...
> but, the value of r7 is not available anymore.

The CFA (call frame address) is defined as "the value of %rsp at the 
call site in the previous frame" (*). So the value you calculated above 
in the first rule is the value of r7 in the previous frame and the call 
site address.


definition of _Unwind_GetCFA

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