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

Pawel Wodnicki
Wed, 18 Aug 2010 02:53:39 -0000

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