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Passing a stack frame by value
https://forum.osdev.org/viewtopic.php?f=13&t=34660		 Page 1 of 1
Author:  unablegrid [ Fri Sep 20, 2019 1:57 am ]
Post subject:  Passing a stack frame by value
I spent about three hours yesterday debugging an issue triggered by a gcc optimisation. I'm posting it in the hope that it may save someone some time. Basically, my ISR look like this:
Code:
...
pusha
call handler
popa
...
iret

where handler is the following C function:
Code:
void handler(isr_stack_frame_t stack)
{
   /* possibly do something to the stack */
}

Obviously (in retrospect), this is wrong, because the compiler is free to elide any modifications to stack that the function makes (and indeed it did so when compiling with -O2).

I wonder if there is a way to beat the compiler into submission by explaining that the stack is going to be valid after the call. For example, adding a call to an empty function (defined in another compilation unit!) that takes a pointer to the stack as argument does the trick, but I don't think it's guaranteed to. I guess the calling convention makes no guarantees about the validity of the arguments on the stack after the call, so I should probably give up and pass the stack frame by reference, shouldn't I?
Author:  Octocontrabass [ Fri Sep 20, 2019 2:08 am ]
Post subject:  Re: Passing a stack frame by value
You should.

Were you perhaps following a tutorial known to have this bug?
Author:  unablegrid [ Fri Sep 20, 2019 2:11 am ]
Post subject:  Re: Passing a stack frame by value
Quote:
Were you perhaps following a tutorial known to have this bug?


Ahah, no, I take all the blame. :)
Author:  unablegrid [ Fri Sep 20, 2019 2:44 am ]
Post subject:  Re: Passing a stack frame by value
Note that the Little Book about OS Development makes the same mistake. Here's the github issue about it.
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