llvm dev - May 2013 - [LLVMdev] Global variables on LLVM JIT on ARM (Android)

Hi all,

I'm trying to extend the Dalvik VM to load some LLVM assembly from the SD
card, JIT it, and execute it. I'm using AOSP 4.0.4, a Galaxy Nexus, and the
version of LLVM that comes in the external project of AOSP (LLVM 2.8, I
believe?)

I have the following LLVM assembly:

target datalayout
"e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:32:64-v128:32:128-a0:0:32-n32-S32"
target triple = "armv7--"

@runtime_val = global i32 7, align 4

define i32 @method() {
entry:
%ret = load i32* @runtime_val, align 4
ret i32 %ret
}

When I use getPointerToFunction() after loading this assembly with
ParseAssemblyFile(), I get the following stderr output, and then a segfault:

UNREACHABLE executed!
Stack dump:
0. Running pass 'ARM Machine Code Emitter' on function '@method'

And here's the GDB backtrace:

#0  __libc_android_abort () at bionic/libc/unistd/abort.c:82
#1  0x40e35600 in llvm::llvm_unreachable_internal (msg=0x0, file=<optimized
out>, line=<optimized out>) at
external/llvm/lib/Support/ErrorHandling.cpp:98
#2  0x409d34ac in (anonymous namespace)::ARMCodeEmitter::getMovi32Value
(this=<optimized out>, MI=<optimized out>, MO=<optimized out>,
Reloc=8)
    at external/llvm/lib/Target/ARM/ARMCodeEmitter.cpp:439
#3  0x409e237c in (anonymous
namespace)::ARMCodeEmitter::emitDataProcessingInstruction (this=0x1a7a338,
MI=..., ImplicitRd=0, ImplicitRn=0)
    at external/llvm/lib/Target/ARM/ARMCodeEmitter.cpp:1036
#4  0x409e2ba0 in (anonymous namespace)::ARMCodeEmitter::emitInstruction
(this=0x1a7a338, MI=...) at
external/llvm/lib/Target/ARM/ARMCodeEmitter.cpp:555
#5  0x409e2e68 in (anonymous
namespace)::ARMCodeEmitter::runOnMachineFunction (this=0x1a7a338, MF=...)
    at external/llvm/lib/Target/ARM/ARMCodeEmitter.cpp:399
#6  0x40bb6f10 in llvm::MachineFunctionPass::runOnFunction (this=0x1a7a338,
F=<optimized out>) at external/llvm/lib/CodeGen/MachineFunctionPass.cpp:33
#7  0x40dfc234 in llvm::FPPassManager::runOnFunction (this=0x1a65100,
F=...) at external/llvm/lib/VMCore/PassManager.cpp:1498
#8  0x40dfc34c in llvm::FunctionPassManagerImpl::run (this=0x1a2ea50,
F=...) at external/llvm/lib/VMCore/PassManager.cpp:1449
#9  0x40dfc5a4 in llvm::FunctionPassManager::run (this=0x1a6c608, F=...) at
external/llvm/lib/VMCore/PassManager.cpp:1379
#10 0x409aa988 in llvm::JIT::jitTheFunction (this=0x1a6c550, F=0x1,
locked=...) at external/llvm/lib/ExecutionEngine/JIT/JIT.cpp:645
#11 0x409aa9ac in llvm::JIT::runJITOnFunctionUnlocked (this=0xdeadbaad,
F=0x1, locked=...) at external/llvm/lib/ExecutionEngine/JIT/JIT.cpp:624
#12 0x409aac50 in llvm::JIT::getPointerToFunction (this=0x1a6c550,
F=0x1a6aef8) at external/llvm/lib/ExecutionEngine/JIT/JIT.cpp:681
<..snip..>

Is there something obviously wrong with the way I'm declaring this global
variable and then reading its value, that would lead to this segfault?

Cheers,
Stephen
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Hi Stephen,
> UNREACHABLE executed!
> Stack dump:
> 0.	Running pass 'ARM Machine Code Emitter' on function
'@method'
LLVM currently has two completely independent JIT implementations, and this
fault indicates you're trying to use the legacy one which can just about be
relied on not to fail horribly for x86 (usually).

The newer system is called the MCJIT, and *should* work on ARM. If you look at
how lli is implemented (tools/lli/lli.cpp) you should be able to see calls to
"setUseMCJIT" which you'll want to duplicate in your code.

Your .ll code looks fine.

Cheers.

Tim

> The newer system is called the MCJIT, and *should* work on ARM. If you look
at how lli is implemented (tools/lli/lli.cpp) you should be able to see calls to
"setUseMCJIT" which you'll want to duplicate in your code.
Side note: this won't work in 2.8 :) In any case, asking for proper
ARM codegen in 2.8 is a big challenge.

--
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University