[llvm-dev] Thumb frame pointer register

[Son Tuan VU via llvm-dev]

Thanks Tim & Peter,

I am referring to mpi_montmul function from library/bignum.c, and this
problem only occurs at clang -O3, so for the moment I am compiling the
library with clang -02.

As for GCC, if -fno-omit-frame-pointer is NOT used, it doesn't use r7 as
frame register, while clang uses r7 by default (even without
-fno-omit-frame-pointer).

So my question is how to turn the use of frame pointer off for clang/llc?

Son Tuan Vu


On Wed, Aug 21, 2019 at 11:18 AM Peter Smith <peter.smith at linaro.org> wrote:

> 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.
>
> Peter
>
> >
> > 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
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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