lib/Target/ARM/ARMJITInfo.cpp

//===-- ARMJITInfo.cpp - Implement the JIT interfaces for the ARM target --===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the JIT interfaces for the ARM target.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "jit"
#include "ARMJITInfo.h"
#include "ARMRelocations.h"
#include "ARMSubtarget.h"
#include "llvm/Function.h"
#include "llvm/CodeGen/MachineCodeEmitter.h"
#include "llvm/Config/alloca.h"
#include "llvm/Support/Streams.h"
#include "llvm/System/Memory.h"
#include <cstdlib>
using namespace llvm;

void ARMJITInfo::replaceMachineCodeForFunction(void *Old, void *New) {
  abort();
}

/// JITCompilerFunction - This contains the address of the JIT function used to
/// compile a function lazily.
static TargetJITInfo::JITCompilerFn JITCompilerFunction;

// Get the ASMPREFIX for the current host.  This is often '_'.
#ifndef __USER_LABEL_PREFIX__
#define __USER_LABEL_PREFIX__
#endif
#define GETASMPREFIX2(X) #X
#define GETASMPREFIX(X) GETASMPREFIX2(X)
#define ASMPREFIX GETASMPREFIX(__USER_LABEL_PREFIX__)

// CompilationCallback stub - We can't use a C function with inline assembly in
// it, because we the prolog/epilog inserted by GCC won't work for us (we need
// to preserve more context and manipulate the stack directly).  Instead,
// write our own wrapper, which does things our way, so we have complete 
// control over register saving and restoring.
extern "C" {
#if defined(__arm__)
  void ARMCompilationCallback(void);
  asm(
    ".text\n"
    ".align 2\n"
    ".globl " ASMPREFIX "ARMCompilationCallback\n"
    ASMPREFIX "ARMCompilationCallback:\n"
    // Save caller saved registers since they may contain stuff
    // for the real target function right now. We have to act as if this
    // whole compilation callback doesn't exist as far as the caller is
    // concerned, so we can't just preserve the callee saved regs.
    "push {r0, r1, r2, r3, lr}\n"
    // The LR contains the address of the stub function on entry.
    // pass it as the argument to the C part of the callback
    "mov  r0, lr\n"
    "sub  sp, sp, #4\n"
    // Call the C portion of the callback
    "bl   " ASMPREFIX "ARMCompilationCallbackC\n"
    "add  sp, sp, #4\n"
    // Restoring the LR to the return address of the function that invoked
    // the stub and de-allocating the stack space for it requires us to
    // swap the two saved LR values on the stack, as they're backwards
    // for what we need since the pop instruction has a pre-determined
    // order for the registers.
    //      +--------+
    //   0  | LR     | Original return address
    //      +--------+    
    //   1  | LR     | Stub address (start of stub)
    // 2-5  | R3..R0 | Saved registers (we need to preserve all regs)
    //      +--------+    
    //
    //      We need to exchange the values in slots 0 and 1 so we can
    //      return to the address in slot 1 with the address in slot 0
    //      restored to the LR.
    "ldr  r0, [sp,#20]\n"
    "ldr  r1, [sp,#16]\n"
    "str  r1, [sp,#20]\n"
    "str  r0, [sp,#16]\n"
    // Return to the (newly modified) stub to invoke the real function.
    // The above twiddling of the saved return addresses allows us to
    // deallocate everything, including the LR the stub saved, all in one
    // pop instruction.
    "pop  {r0, r1, r2, r3, lr, pc}\n"
      );
#else  // Not an ARM host
  void ARMCompilationCallback() {
    assert(0 && "Cannot call ARMCompilationCallback() on a non-ARM arch!\n");
    abort();
  }
#endif
}

/// ARMCompilationCallbackC - This is the target-specific function invoked 
/// by the function stub when we did not know the real target of a call.  
/// This function must locate the start of the stub or call site and pass 
/// it into the JIT compiler function.
extern "C" void ARMCompilationCallbackC(intptr_t StubAddr) {
  // Get the address of the compiled code for this function.
  intptr_t NewVal = (intptr_t)JITCompilerFunction((void*)StubAddr);

  // Rewrite the call target... so that we don't end up here every time we
  // execute the call. We're replacing the first two instructions of the
  // stub with:
  //   ldr pc, [pc,#-4]
  //   <addr>
#if defined(__APPLE__)
  bool ok = sys::Memory::setRangeWritable ((void*)StubAddr, 8);
  if (!ok)
    {
      cerr << "ERROR: Unable to mark stub writable\n";
      abort();
    }
#endif
  *(intptr_t *)StubAddr = 0xe51ff004;
  *(intptr_t *)(StubAddr+4) = NewVal;
#if defined(__APPLE__)
  ok = sys::Memory::setRangeExecutable ((void*)StubAddr, 8);
  if (!ok)
    {
      cerr << "ERROR: Unable to mark stub executable\n";
      abort();
    }
#endif
}

TargetJITInfo::LazyResolverFn
ARMJITInfo::getLazyResolverFunction(JITCompilerFn F) {
  JITCompilerFunction = F;
  return ARMCompilationCallback;
}

void *ARMJITInfo::emitFunctionStub(const Function* F, void *Fn,
                                   MachineCodeEmitter &MCE) {
  unsigned addr = (intptr_t)Fn;
  // If this is just a call to an external function, emit a branch instead of a
  // call.  The code is the same except for one bit of the last instruction.
  if (Fn != (void*)(intptr_t)ARMCompilationCallback) {
    // branch to the corresponding function addr
    // the stub is 8-byte size and 4-aligned
    MCE.startFunctionStub(F, 8, 4);
    MCE.emitWordLE(0xe51ff004); // LDR PC, [PC,#-4]
    MCE.emitWordLE(addr);       // addr of function
  } else {
    // The compilation callback will overwrite the first two words of this
    // stub with indirect branch instructions targeting the compiled code. 
    // This stub sets the return address to restart the stub, so that
    // the new branch will be invoked when we come back.
    //
    // branch and link to the compilation callback.
    // the stub is 16-byte size and 4-byte aligned.
    MCE.startFunctionStub(F, 16, 4);
    // Save LR so the callback can determine which stub called it.
    // The compilation callback is responsible for popping this prior
    // to returning.
    MCE.emitWordLE(0xe92d4000); // PUSH {lr}
    // Set the return address to go back to the start of this stub
    MCE.emitWordLE(0xe24fe00c); // SUB LR, PC, #12
    // Invoke the compilation callback
    MCE.emitWordLE(0xe51ff004); // LDR PC, [PC,#-4]
    // The address of the compilation callback
    MCE.emitWordLE((intptr_t)ARMCompilationCallback);
  }

  return MCE.finishFunctionStub(F);
}

/// relocate - Before the JIT can run a block of code that has been emitted,
/// it must rewrite the code to contain the actual addresses of any
/// referenced global symbols.
void ARMJITInfo::relocate(void *Function, MachineRelocation *MR,
                          unsigned NumRelocs, unsigned char* GOTBase) {
  for (unsigned i = 0; i != NumRelocs; ++i, ++MR) {
    void *RelocPos = (char*)Function + MR->getMachineCodeOffset();
    intptr_t ResultPtr = (intptr_t)MR->getResultPointer();
    switch ((ARM::RelocationType)MR->getRelocationType()) {
    case ARM::reloc_arm_relative: {
      // It is necessary to calculate the correct PC relative value. We
      // subtract the base addr from the target addr to form a byte offset.
      ResultPtr = ResultPtr-(intptr_t)RelocPos-8;
      // If the result is positive, set bit U(23) to 1.
      if (ResultPtr >= 0)
        *((unsigned*)RelocPos) |= 1 << 23;
      else {
      // otherwise, obtain the absolute value and set
      // bit U(23) to 0.
        ResultPtr *= -1;
        *((unsigned*)RelocPos) &= 0xFF7FFFFF;
      }
      // set the immed value calculated
      *((unsigned*)RelocPos) |= (unsigned)ResultPtr;
      // set register Rn to PC
      *((unsigned*)RelocPos) |= 0xF << 16;
      break;
    }
    case ARM::reloc_arm_branch: {
      // It is necessary to calculate the correct value of signed_immed_24
      // field. We subtract the base addr from the target addr to form a
      // byte offset, which must be inside the range -33554432 and +33554428.
      // Then, we set the signed_immed_24 field of the instruction to bits
      // [25:2] of the byte offset. More details ARM-ARM p. A4-11.
      ResultPtr = ResultPtr-(intptr_t)RelocPos-8;
      ResultPtr = (ResultPtr & 0x03FFFFFC) >> 2;
      assert(ResultPtr >= -33554432 && ResultPtr <= 33554428);
      *((unsigned*)RelocPos) |= ResultPtr;
      break;
    }
    }
  }
}