Re: [fpc-pascal] get_caller_frame

[Sven Barth via fpc-pascal]

Ryan Joseph via fpc-pascal  schrieb am
Fr., 22. Nov. 2019, 01:51:

>
>
> > On Nov 21, 2019, at 1:41 AM, Sven Barth via fpc-pascal <
> fpc-pascal@lists.freepascal.org> wrote:
> >
> > No. On x86 it's essentially the content of the EBP/RBP register which is
> (assuming no optimizations are done) essentially the ESP/RSP register of
> the calling function. This is only used by the exception handling to have
> the exception appear to be raised somewhere else.
>
> So what's missing then to be able make saving a stack frame to the heap
> and then restoring it?
>

That the compiler doesn't throw a wrench in your plans and decides to use
EBP/RBP for its own purposes due to optimization? Not to mention that on
other platforms this will likely be completely different?

Regards,
Sven

>
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Re: [fpc-pascal] get_caller_frame

[Ryan Joseph via fpc-pascal]

> On Nov 21, 2019, at 1:41 AM, Sven Barth via fpc-pascal 
>  wrote:
> 
> No. On x86 it's essentially the content of the EBP/RBP register which is 
> (assuming no optimizations are done) essentially the ESP/RSP register of the 
> calling function. This is only used by the exception handling to have the 
> exception appear to be raised somewhere else.

So what's missing then to be able make saving a stack frame to the heap and 
then restoring it?

Regards,
Ryan Joseph

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Re: [fpc-pascal] get_caller_frame

[Sven Barth via fpc-pascal]

Am 20.11.2019 um 23:52 schrieb Ryan Joseph via fpc-pascal:



On Nov 20, 2019, at 1:56 AM, Sven Barth via fpc-pascal 
 wrote:

It returns the address of the caller's frame pointer. See also 
https://www.freepascal.org/docs-html/rtl/system/get_caller_frame.html

It's mainly used in context of raising exceptions with the help of a second 
function. See here: 
https://freepascal.org/docs-html/current/ref/refse112.html#x227-24900017.1


I guess I don't know what a frame pointer is. I thought it meant a pointer to 
the current stack frame and so I was curious if the RTL could include a way to 
copy the stack with the pointer and restore it later. Is that not how it works?
No. On x86 it's essentially the content of the EBP/RBP register which is 
(assuming no optimizations are done) essentially the ESP/RSP register of 
the calling function. This is only used by the exception handling to 
have the exception appear to be raised somewhere else.


The exception handling itself does not need it. The generic mechanism 
for example is a stack of exception frames (both except- and 
finally-blocks) and the exception handling code calls the handlers from 
top to bottom (thus executing explicit destructors (bla.Free) and 
implicit reference count decreases) before restoring the state using 
LongJmp (which also restores the original stack pointer register). The 
SEH and PSABIEH mechanisms that FPC supports work in a similar way (e.g. 
the SEH on Win32 is a single linked list).


Regards,
Sven
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Re: [fpc-pascal] get_caller_frame

[Ryan Joseph via fpc-pascal]

> On Nov 20, 2019, at 1:56 AM, Sven Barth via fpc-pascal 
>  wrote:
> 
> It returns the address of the caller's frame pointer. See also 
> https://www.freepascal.org/docs-html/rtl/system/get_caller_frame.html
> 
> It's mainly used in context of raising exceptions with the help of a second 
> function. See here: 
> https://freepascal.org/docs-html/current/ref/refse112.html#x227-24900017.1
> 

I guess I don't know what a frame pointer is. I thought it meant a pointer to 
the current stack frame and so I was curious if the RTL could include a way to 
copy the stack with the pointer and restore it later. Is that not how it works?

Regards,
Ryan Joseph

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Re: [fpc-pascal] get_caller_frame

[Florian Klämpfl]

Am 20.11.19 um 07:56 schrieb Sven Barth via fpc-pascal:
Ryan Joseph via fpc-pascal > schrieb am Mi., 20. Nov. 2019, 
04:36:


I came across get_caller_frame  in some unrelated code and I was
just curious about this so I wanted to ask.

What does get_caller_frame return exactly? Is this a pointer to a
stack frame that could be copied to the heap? I'm still interested
in how we could add some form of coroutine like behaviors to pascal
and so I was wondering if we could copy/restore the current stack
pointer so that SetJmp and LongJmp would not blow things up.


It returns the address of the caller's frame pointer. See also 
https://www.freepascal.org/docs-html/rtl/system/get_caller_frame.html


It's mainly used in context of raising exceptions with the help of a 
second function. See here: 
https://freepascal.org/docs-html/current/ref/refse112.html#x227-24900017.1


For the record: the reliability of this function is limited, in 
particular in combination with optimization.


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Re: [fpc-pascal] get_caller_frame

[Sven Barth via fpc-pascal]

Ryan Joseph via fpc-pascal  schrieb am
Mi., 20. Nov. 2019, 04:36:

> I came across get_caller_frame  in some unrelated code and I was just
> curious about this so I wanted to ask.
>
> What does get_caller_frame return exactly? Is this a pointer to a stack
> frame that could be copied to the heap? I'm still interested in how we
> could add some form of coroutine like behaviors to pascal and so I was
> wondering if we could copy/restore the current stack pointer so that SetJmp
> and LongJmp would not blow things up.
>

It returns the address of the caller's frame pointer. See also
https://www.freepascal.org/docs-html/rtl/system/get_caller_frame.html

It's mainly used in context of raising exceptions with the help of a second
function. See here:
https://freepascal.org/docs-html/current/ref/refse112.html#x227-24900017.1

Regards,
Sven

>
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