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

[Peter Smith via llvm-dev]

Hello Muhui,

Yes the -mthumb (and the corresponding -marm) flag work at compile
time, and assembly time if you are using the GNU assembler. In clang
the -mthumb and -marm flag are currently implemented by changing the
target triple. For example --target=armv7a-linux-gnueabihf -mthumb
changes the the target to thumbv7a-linux-gnueabihf.

If you use a target starting with arm like armv7a-linux-gnueabihf then
by default you will get Arm instructions. If you use a target
thumbv7a-linux-gnueabihf then by default you will get Thumb
instructions.

For an example of using attributes see
clang/test/CodeGen/arm-target-attr.c . The available attributes are:
__attribute__((target("arm"))) void test_target_arm() {
...
}

__attribute__((target("thumb"))) void test_target_thumb() {
...
}
The effect of these attributes is to add -thumb-mode or +thumb-mode to
the target-features attribute.

Peter
On Fri, 9 Nov 2018 at 02:49, Muhui Jiang <jiangmuhui at gmail.com> wrote:
>
> 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