[LLVMdev] Resolving a function symbol using JIT.

[sumeeth kc]

Hey,

I have a situation where in I need to intercept a call to a particular
function and return a pointer to a separate implementation of that function
using JIT. The scenario is like this:

1. A test code or client code calls a function A() for which a dummy
implementation is provided in a library which the test code/client links
with during compilation.
2. I create the .bc using -emit-llvm and run it with lli.
3. Now, in the JIT, I have provided the concrete implementation of the
function A(). I have to intercept this call to function A() and return a
pointer to the concrete implementation in JIT.

I tried the way external library symbols are being resolved in
JITMemoryManager.cpp but the error I get is as follows:

pseudo instructions should be removed before code emission
UNREACHABLE executed at /llvm/lib/Target/X86/X86CodeEmitter.cpp:1175!
0  lli             0x00000000011fa92e llvm::sys::PrintStackTrace(_IO_FILE*)
+ 46
1  lli             0x00000000011fabeb
2  lli             0x00000000011fae4a
3  libpthread.so.0 0x00007f08378cecb0
4  libc.so.6       0x00007f0836915425 gsignal + 53
5  libc.so.6       0x00007f0836918b8b abort + 379
6  lli             0x00000000011e1726
7  lli             0x0000000000688f83
8  lli             0x0000000000688700
9  lli             0x0000000000b5789e
llvm::MachineFunctionPass::runOnFunction(llvm::Function&) + 110
10 lli             0x0000000001168e7b
llvm::FPPassManager::runOnFunction(llvm::Function&) + 427
11 lli             0x0000000001168777
llvm::FunctionPassManagerImpl::run(llvm::Function&) + 119
12 lli             0x00000000011686e7
llvm::FunctionPassManager::run(llvm::Function&) + 167
13 lli             0x0000000000ac478f
llvm::JIT::jitTheFunction(llvm::Function*, llvm::MutexGuard const&) + 63
14 lli             0x0000000000ac467e
llvm::JIT::runJITOnFunctionUnlocked(llvm::Function*, llvm::MutexGuard
const&) + 94
15 lli             0x0000000000ac4a9e
llvm::JIT::getPointerToFunction(llvm::Function*) + 734
16 lli             0x0000000000ad7e18
17 lli             0x000000000077eb85
18 lli             0x000000000077e9ca X86CompilationCallback + 74
Stack dump:
0.    Program arguments: lli -jit-emit-debug test
1.    Running pass 'X86 Machine Code Emitter' on function
'@_ZN4llvm5APInt15clearUnusedBitsEv'
Aborted

If this approach is not proper, where else can I intercept the call to that
symbol and return a pointer to the concrete implementation?

Thanks in advance.

-Sumeeth
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