[LLVMbugs] [Bug 177] NEW: [JIT] Functions pointed to by global variables should be lazily compiled!

[bugzilla-daemon at zion.cs.uiuc.edu]

http://llvm.cs.uiuc.edu/bugs/show_bug.cgi?id=177

           Summary: [JIT] Functions pointed to by global variables should be
                    lazily compiled!
           Product: libraries
           Version: 1.0
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Generic Execution Engine Support
        AssignedTo: sabre at nondot.org
        ReportedBy: sabre at nondot.org


The LLVM JIT currently compiles a functions pointed to by global variables
immediately, though they probably won't ever be referenced, especially in C++
programs.  Here's a simple C program that demonstrates what is going on. 
Multiply it a thousand fold for a medium-sized C++ program that has vtables
pointing to LOTS of functions.

---
void bar() {}
void baz() {}
void (*P1)(void) = bar;   // P1 is never used, should not codegen bar
void (*P2)(void) = baz;

void qux() {
  P2();
}

int main(int argc, char**argv) {
  if (argc > 1)
    qux();
}
---

Use like so:

$ llvmgcc test.c -c -Wa,-disable-inlining
$ lli -debug-only=jit test.o <optional arg>

Currently I get this on the testcase:

$ lli -debug-only=jit test.o
Global 'P1' -> 0x84e3808
Global 'P2' -> 0x84e3818
Finished CodeGen of [0x4018e000] Function: bar: 7 bytes of text
Finished CodeGen of [0x4018e008] Function: baz: 7 bytes of text
Finished CodeGen of [0x4018e010] Function: main: 57 bytes of text
WARNING: Cannot resolve fn '__main' using a dummy noop function instead!

$ lli -debug-only=jit test.o x
Global 'P1' -> 0x84e3808
Global 'P2' -> 0x84e3818
Finished CodeGen of [0x4018e000] Function: bar: 7 bytes of text
Finished CodeGen of [0x4018e008] Function: baz: 7 bytes of text
Finished CodeGen of [0x4018e010] Function: main: 57 bytes of text
WARNING: Cannot resolve fn '__main' using a dummy noop function instead!
Finished CodeGen of [0x4018e04c] Function: qux: 16 bytes of text



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