[llvm-dev] Thumb frame pointer register

Peter Smith via llvm-dev llvm-dev at lists.llvm.org
Wed Aug 21 02:18:12 PDT 2019

On Wed, 21 Aug 2019 at 06:05, Son Tuan VU via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Hello all,
> I noticed that for ARM Thumb target, llc uses r7 as frame pointer (gcc does not do this AFAIK), and this register should therefore not be used as a general-purpose register.
> However, when compiling mbedTLS, which contains some code that is highly optimized for ARM platform and uses r7 to efficiently perform an operation. This raises an exception since r7 is now the frame pointer.
> I tried passing -fomit-frame-pointer but this is just a hacky workaround and does not work in this case, since the function here does indeed need frame pointer.
> ```
> The -fomit-frame-pointer option instructs the compiler to not store stack frame pointers if the function does not need it. You can use this option to reduce the code image size.
> ```

Can you let us know which function in mbedTLS you are referring to. I
found https://tls.mbed.org/kb/development/arm-thumb-error-r7-cannot-be-used-in-asm-here
on the mbedTLS website which implies that you should use
-fomit-frame-pointer. I would be very surprised for code in mbedTLS to
simultaneously require a frame pointer and reserve r7 in Thumb state.
There may be some confusion about what GCC does as arm-none-eabi-gcc
uses -fomit-frame-pointer by default. As Tim points out, I think
you'll find that gcc will use r7 as the frame pointer if
-fno-omit-frame-pointer is used.


> My question is, how can I instruct the register allocator to use another register as frame pointer, or at least turn off the use of frame pointer? I tried changing the ABI with -mabi and -meabi but nothing works so far.
> Thanks a lot for your help,
> Cheers,
> Son Tuan Vu
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