[llvm-dev] Compile with both arm and thumb mode

[Muhui Jiang via llvm-dev]

Hi Peter

Thanks for your reply. It seems that the mix of arm and thumb is determined
by the target attribute inside the source code. Is it right?

May I understand like this, I think the '-mthumb' is a compiling flag. If
no target information is provided. Then instruction mode will be determined
by the compiling flag. If I set -mthumb, all the instructions will be in
thumb mode. If I set nothing, all the instructions will be compiled into
arm mode by default. What if I set -mthumb, but there is a target
information specifying arm mode?

Besides, I looked at the link you provided, I don't know the valid value of
target if I need the function to be compiled to thumb mode. Maybe something
like  __attribute__((target("thumbv7a-linux-gnueabi")))? Thank you very
much.

Regards
Muhui

Peter Smith <peter.smith at linaro.org> 于2018年11月9日周五 上午12:17写道：

> On Thu, 8 Nov 2018 at 15:26, Muhui Jiang via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > Hi
> >
> > I would like to use clang to cross compile the ARM binary. I hope the
> target binary contains both arm and thumb instruction sets.
> >
> > I use the flag -mhwdiv=arm,thumb. I compiled several binaries. However,
> I found that the thumb mode instructions are few. Even for very big program
> like gcc. The number of thumb mode instruction is less than 100. I get the
> ground truth from the mapping table.
> >
> > Maybe the flag I use is not right? I think there should be many thumb
> mode instructions. How do clang select thumb or arm mode when compiling?
> Many Thanks
> >
> > Regards
> > Muhui
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> Hello Muhui,
>
> clang with a target like armv7a-linux-gnueabihf will default to
> compiling all the functions in the file in Arm state. If the option
> -mthumb or a target like thumbv7a-linux-gnueabi is used then all of
> the functions will be compiled in Thumb state. Depending on how the
> flags vary across the program will determine the mix of Arm and Thumb
> in the binary.
>
> Off the top of my head I think that you can use the target attribute
> to change the state of individual function
> https://clang.llvm.org/docs/AttributeReference.html#target
>
> Peter
>
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