[LLVMdev] switching ARM modes and integrated-as

[Greg Fitzgerald]

Looks like the bug here is that the InlineAsm parser and the ARM code
emitter point to two different TargetSubtargetInfo instances.  The attached
patch corrects the issue, but is incomplete.  For starters, how can I remove
that const_cast?  Also, the patch does not restore the TargetSubtargetInfo
to its original state.  So if you changed from Thumb to ARM mode, but not
back to Thumb, the code emitter will generate broken machine code.
Unfortunately, the TargetSubtargetInfo class does not expose a copy
constructor or a setFeatureBits() method.  Should it?  Or should we expect
the inline assembly to be responsible for restoring the mode itself?

Thanks,
Greg

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Greg Fitzgerald
Sent: Tuesday, November 19, 2013 11:07 AM
To: Jim Grosbach
Cc: llvmdev at cs.uiuc.edu List
Subject: Re: [LLVMdev] switching ARM modes and integrated-as

> 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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