llvm dev - Jan 2019 - (in x86, ) Use ebp or esp to access local variable? what's different?

Hi

I use LLVM/Clang to compile some program.

I found that if we use -O0 flag, the program will use ebp to access local
variables.

For example :  mov ecx,DWORD PTR [ebp-0x8]

If we use -O2 flag, the program will use esp to access local variables.

For example : mov eax,DWORD PTR [esp+0x8]

Is there any different between them?

Can user decide esp or ebp to access the local variable?

Or can I modify LLVM backend code to do it?

Is there any calling convention related to it?
I  found it is the same in gcc.

Thank you!
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When EBP is used, it's known as the frame pointer. It points to a fixed
location on the stack throughout the body of the function. It makes local
variables easy to find because they are always at the same place relative
to EBP. ESP's value can change through the function as other functions are
called. So the location of a local variable can require a different offset
depending on how much ESP has been changed throughout the function. Having
a frame pointer makes a debugging tool like gdb's job easier. But using a
frame pointer ties up another register. Leaving only 6 registers available
for computation in 32-bit mode. So by default for optimized code we try not
to use it. There are still situations where it is required. You can force
the frame pointer to always be used by passing -fno-omit-frame-pointer to
clang.

Some more information can be found here
https://stackoverflow.com/questions/579262/what-is-the-purpose-of-the-ebp-frame-pointer-register

~Craig


On Tue, Jan 22, 2019 at 9:39 PM PenYiWang via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi
>
> I use LLVM/Clang to compile some program.
>
> I found that if we use -O0 flag, the program will use ebp to access local
> variables.
>
> For example :  mov ecx,DWORD PTR [ebp-0x8]
>
> If we use -O2 flag, the program will use esp to access local variables.
>
> For example : mov eax,DWORD PTR [esp+0x8]
>
> Is there any different between them?
>
> Can user decide esp or ebp to access the local variable?
>
> Or can I modify LLVM backend code to do it?
>
> Is there any calling convention related to it?
> I  found it is the same in gcc.
>
> Thank you!
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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