[LLVMdev] ToT ARM Code generator causing - Error: invalid constant (xxx) after fixup in assembly output

[Pawel Wodnicki]

On 8/18/2010 1:47 PM, Dale Johannesen wrote:
>
> On Aug 18, 2010, at 11:37 AMPDT, Pawel Wodnicki wrote:
>
>> On 8/18/2010 12:39 PM, Dale Johannesen wrote:
>>> I can look at this, but you'll need to send the .bc file.  Please 
>>> open a PR?
>>
>> I would do it but I am in  a bit of a pickle as the .bc is from 
>> propriety code and
>> I can  not post it.
>
> Can't help much then.  You might look into obfuscating the code, some 
> companies are OK with distributing obfuscated IR.
> -debug-only=arm-cp-islands will show you what ARMConstantIslands is 
> thinking pretty well.
Oh -debug-only=arm-cp-islands works very very well! It just that now 
after running it I have
a 1.5GB log file problem in addition to the original issue.
>
>> Anyway, I have been trying to re-create the problem in a simpler test 
>> case.
>> Since, I do not have access to the source for the .bc I am trying to 
>> guess
>> the kind of code that created the problem. So far I was able to 
>> replicate
>> the one large basic block but I not sure how to produce the constant 
>> pool
>> with the jumptable like in the description below.
>>
>>>
>>> There have been lots of bugs in ARMConstantIslands where it's off by 
>>> 1, but I haven't seen one where it's off by a lot like this.
>> The big offset is due to this one large basic block and is not a 
>> problem with alignment or any off by 1  issue.
>>>
>>> On Aug 17, 2010, at 7:53 PMPDT, Pawel Wodnicki wrote:
>>>
>>>> Hello,
>>>>
>>>> This problem happens in ToT under specific conditions - namely 
>>>> there is
>>>> a big BB#671 basic block of code
>>>> the just copies data from memory location to another. At the beginning
>>>> of BBB#671 r0 is loaded
>>>> from the jumptable in the constant pool immediately after it.
>>>> Displacement from the pc
>>>> in this case is #1476 which is way above magic #1020 hence the error
>>>> after fixup.
>>>> Both ARMCodeEmitter::emitLEApcrelJTInstruction() and 
>>>> emitJumpTableAddress()
>>>> are responsible for the offending instruction adr    r0, 
>>>> #.LJTI8485_1_1
>>>> But besides the fact that they do not complain about the invalid 
>>>> offset
>>>> from the pc I do not see
>>>> anything wrong here.
>>>>
>>>> The problem seems to be in the ARMConstantIslands which is not 
>>>> splitting
>>>> BB#671 into
>>>> smaller pieces and thus producing over the limit 'add' opcode.
>>>>
>>>> This code fragment below is produced from a rather large bitcode file
>>>> that resists being reduced to something
>>>> more manageable. As a result keystrokes in gdb are very "expensive 
>>>> " and
>>>> taxing my  machine rather heavily.
>>>>
>>>> Any thoughts and hints on debugging this issue would be much 
>>>> appreciated!
>>>>
>>>> I have used  llc with these options to produce the output:
>>>>
>>>> llc -O0 -regalloc=fast  -relocation-model=pic  l.bc -o ll-2.8.s
>>>>
>>>>    bhi    .LBB8485_455
>>>> @ BB#671:                               @ %bb154
>>>>                                        @   in Loop: Header=BB8485_1112
>>>> Depth=5
>>>>    adr    r0, #.LJTI8485_1_1
>>>>    ldr    r1, [sp, #3300]
>>>>    ldr    r2, [r0, r1, lsl #2]
>>>>    ...
>>>>    lots of boring code omitted
>>>>    ...
>>>>    add    pc, r2, r0
>>>> .LJTI8485_1_1:
>>>>    .set    .L8485_1_1_set_70,.LBB8485_70-.LJTI8485_1_1
>>>>    .long     .L8485_1_1_set_70
>>>>    .set    .L8485_1_1_set_49,.LBB8485_49-.LJTI8485_1_1
>>>>    .long     .L8485_1_1_set_49
>>>>    .set    .L8485_1_1_set_419,.LBB8485_419-.LJTI8485_1_1
>>>>    .long     .L8485_1_1_set_419
>>>>    .set    .L8485_1_1_set_1085,.LBB8485_1085-.LJTI8485_1_1
>>>>    .long     .L8485_1_1_set_1085
>>>>    .set    .L8485_1_1_set_439,.LBB8485_439-.LJTI8485_1_1
>>>>
>>>> When assembler runs on the output we get:
>>>>
>>>> ll-2.8.s:1916640: Error: invalid constant (5c4) after fixup
>>>>
>>>> Standard disclaimer:
>>>> All of the tools used were proper ARM cross compiler,assembler, etc
>>>> running on x86_64
>>>> and no bitcode was ever harmed during debugging.
>>>>
>>>> Cheers,
>>>> Pawel
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>>
>>>
>>
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