llvm dev - Aug 2012 - [LLVMdev] question about ExectuionEngine::Create

I found the following problem when I try to debug "target does not support
mc emission" in linux (the same code executes OK in windows):

Below is a snippet extracted from this method,

if (UseMCJIT && ExecutionEngine::MCJITCtor) {
        ExecutionEngine *EE           ExecutionEngine::MCJITCtor(M, ErrorStr,
JMM, OptLevel,
                                     AllocateGVsWithCode, TM);
        if (EE) return EE;
      } else if (ExecutionEngine::JITCtor) {
        ExecutionEngine *EE           ExecutionEngine::JITCtor(M, ErrorStr, JMM,
OptLevel,
                                   AllocateGVsWithCode, TM);
        if (EE) return EE;
      }

Both ExecutionEngine::MCJITCtor and ExecutionEngine::JITCtor are function
pointers (static members in class ExecutionEngine) and are initialized at
the very beginning of ExecutionEngine.cpp to be 0 (NULL):

ExecutionEngine *(*ExecutionEngine::JITCtor)(
  Module *M,
  std::string *ErrorStr,
  JITMemoryManager *JMM,
  CodeGenOpt::Level OptLevel,
  bool GVsWithCode,
  TargetMachine *TM) = 0;
ExecutionEngine *(*ExecutionEngine::MCJITCtor)(
  Module *M,
  std::string *ErrorStr,
  JITMemoryManager *JMM,
  CodeGenOpt::Level OptLevel,
  bool GVsWithCode,
  TargetMachine *TM) = 0;


When I create the engine, I set UseMCJIT to "true" using
engineBuilder.setUseMCJIT(true);

Then I call
engineBuilder.crate()
to create the Execution engine.

In windows and Linux (Ubuntu), I got a different execution path when it
reaches the if/else structure mentioned in the very beginning of this email:
Windows: it goes into the "else if" branch, which means
that  ExecutionEngine::MCJITCtor is evaluated as 0/NULL. It then
uses ExecutionEngine::JITCtor to create the engine and successful. However,
this is not what I meant to do.
Linux: it goes into the "if" branch, which means
ExecutionEngine::MCJITCtor
is not 0/NULL, and it must have been assigned a value somewhere. Then it
uses ExecutionEngine::MCJITCtor (which is what I meant to do) but fails,
giving me an error "target does not support mc emission"

In Windows, I am linking all the libraries (.lib) statically; In Linux, I
am dynamic linking libLLVM3.0.so

Question:
1. Where and when do  ExecutionEngine::MCJITCtor
and ExecutionEngine::JITCtor get assigned a different value other than
0/NULL?
2. Why the value of ExecutionEngine::MCJITCtor are different in Windows and
Linux?

Thanks.

-- 
Xinglin Zhang
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The MCJITCtor and JITCtor are initialized from static constructors defined in
MCJIT.cpp and JIT.cpp respectively.

The problem you are seeing is probably caused by the linker optimizing out these
static constructors on Windows.  In order to force these constructors to be
linked in, you need to include "llvm/ExecutionEngine/MCJIT.h" from
some module that you know contains non-static code.

-Andy

From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Xinglin Zhang
Sent: Thursday, August 09, 2012 10:04 AM
To: llvmdev at cs.uiuc.edu
Subject: [LLVMdev] question about ExectuionEngine::Create

I found the following problem when I try to debug "target does not support
mc emission" in linux (the same code executes OK in windows):

Below is a snippet extracted from this method,

if (UseMCJIT && ExecutionEngine::MCJITCtor) {
        ExecutionEngine *EE           ExecutionEngine::MCJITCtor(M, ErrorStr,
JMM, OptLevel,
                                     AllocateGVsWithCode, TM);
        if (EE) return EE;
      } else if (ExecutionEngine::JITCtor) {
        ExecutionEngine *EE           ExecutionEngine::JITCtor(M, ErrorStr, JMM,
OptLevel,
                                   AllocateGVsWithCode, TM);
        if (EE) return EE;
      }

Both ExecutionEngine::MCJITCtor and ExecutionEngine::JITCtor are function
pointers (static members in class ExecutionEngine) and are initialized at the
very beginning of ExecutionEngine.cpp to be 0 (NULL):

ExecutionEngine *(*ExecutionEngine::JITCtor)(
  Module *M,
  std::string *ErrorStr,
  JITMemoryManager *JMM,
  CodeGenOpt::Level OptLevel,
  bool GVsWithCode,
  TargetMachine *TM) = 0;
ExecutionEngine *(*ExecutionEngine::MCJITCtor)(
  Module *M,
  std::string *ErrorStr,
  JITMemoryManager *JMM,
  CodeGenOpt::Level OptLevel,
  bool GVsWithCode,
  TargetMachine *TM) = 0;


When I create the engine, I set UseMCJIT to "true" using
engineBuilder.setUseMCJIT(true);

Then I call
engineBuilder.crate()
to create the Execution engine.

In windows and Linux (Ubuntu), I got a different execution path when it reaches
the if/else structure mentioned in the very beginning of this email:
Windows: it goes into the "else if" branch, which means that 
ExecutionEngine::MCJITCtor is evaluated as 0/NULL. It then uses
ExecutionEngine::JITCtor to create the engine and successful. However, this is
not what I meant to do.
Linux: it goes into the "if" branch, which means
ExecutionEngine::MCJITCtor is not 0/NULL, and it must have been assigned a value
somewhere. Then it uses ExecutionEngine::MCJITCtor (which is what I meant to do)
but fails, giving me an error "target does not support mc emission"

In Windows, I am linking all the libraries (.lib) statically; In Linux, I am
dynamic linking libLLVM3.0.so<http://libLLVM3.0.so>

Question:
1. Where and when do  ExecutionEngine::MCJITCtor and ExecutionEngine::JITCtor
get assigned a different value other than 0/NULL?
2. Why the value of ExecutionEngine::MCJITCtor are different in Windows and
Linux?

Thanks.

--
Xinglin Zhang
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