[LLVMdev] switching ARM modes and integrated-as

[Greg Fitzgerald]

> I just tried the above code in a .s file and it worked fine for me.
> Can you show exactly what you’re seeing that looks wrong?

Thanks Jim.  I can only reproduce the issue when that code is inline
assembly in a bitcode file.  Attached is a bitcode file and Makefile
to demonstrate.  Calling 'make' will dump two object files 'c.o' and
'asm.o'.  The former is created with llc directly, whereas the latter
first generates an assembly file and then assembles it via llvm-mc.
The issue is only present when directly generating the object file.
You can see that the first ARM instruction, push {r7}, is encoded as
7004e52d and not e52d7004.

Thanks,
Greg

On Mon, Nov 18, 2013 at 6:33 PM, Jim Grosbach <grosbach at apple.com> wrote:
>
> On Nov 18, 2013, at 6:18 PM, Greg Fitzgerald <garious at gmail.com> wrote:
>
>> Does the integrated assembler in the ARM backend support switching
>> between ARM and Thumb modes in the same file?
>
> Yes.
>
>>  I'm having trouble with
>> the following assembly:
>>
>>  .thumb_func
>>  @   Enter ARM mode
>>  adr r3, 1f
>>  bx  r3
>>  .align 4
>
> That .align is probably not what you want. ARM .align is a power of two, so you’re aligning at 16 bytes here, not 32-bits.
>
>>  .code 32
>> 1:  push {r7}
>>  mov r7, r12
>>  svc 0x0
>>  pop {r7}
>>  @   Enter thumb mode
>>  adr r3, 2f+1
>>  bx  r3
>>  .code 16
>> 2:
>>  bx  lr
>>
>>
>> As a standalone .s file, GNU-as can compile it but with "clang
>> -integrated-as", I get errors.  If using inline assembly in a C file,
>> no error is reported (when compiled targeting thumb 2), but the 32-bit
>> ARM instructions are encoded wrongly.  The two 16-bit halves need to
>> be swapped.  A bug, unsupported, or perhaps just user error?
>
> I just tried the above code in a .s file and it worked fine for me. Can you show exactly what you’re seeing that looks wrong? Note that Thumb2 wide instructions have the two half-words swapped from what one would expect. Possibly the disassembly isn’t identifying arm vs. thumb code and that’s what’s making things look wrong?
>
>>
>> Thanks,
>> Greg
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>
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