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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

// ---
// Author: Craig Silverstein
//
// Produce stack trace.  I'm guessing (hoping!) the code is much like
// for x86.  For apple machines, at least, it seems to be; see
//    http://developer.apple.com/documentation/mac/runtimehtml/RTArch-59.html
//    http://www.linux-foundation.org/spec/ELF/ppc64/PPC-elf64abi-1.9.html#STACK
// Linux has similar code: http://patchwork.ozlabs.org/linuxppc/patch?id=8882

#include <stdint.h>   // for uintptr_t
#include <stdlib.h>   // for NULL
#include <google/stacktrace.h>

// Given a pointer to a stack frame, locate and return the calling
// stackframe, or return NULL if no stackframe can be found. Perform sanity
// checks (the strictness of which is controlled by the boolean parameter
// "STRICT_UNWINDING") to reduce the chance that a bad pointer is returned.
template<bool STRICT_UNWINDING>
static void **NextStackFrame(void **old_sp) {
  void **new_sp = (void **) *old_sp;

  // Check that the transition from frame pointer old_sp to frame
  // pointer new_sp isn't clearly bogus
  if (STRICT_UNWINDING) {
    // With the stack growing downwards, older stack frame must be
    // at a greater address that the current one.
    if (new_sp <= old_sp) return NULL;
    // Assume stack frames larger than 100,000 bytes are bogus.
    if ((uintptr_t)new_sp - (uintptr_t)old_sp > 100000) return NULL;
  } else {
    // In the non-strict mode, allow discontiguous stack frames.
    // (alternate-signal-stacks for example).
    if (new_sp == old_sp) return NULL;
    // And allow frames upto about 1MB.
    if ((new_sp > old_sp)
        && ((uintptr_t)new_sp - (uintptr_t)old_sp > 1000000)) return NULL;
  }
  if ((uintptr_t)new_sp & (sizeof(void *) - 1)) return NULL;
  return new_sp;
}

// If you change this function, also change GetStackFrames below.
int GetStackTrace(void** result, int max_depth, int skip_count) {
  void **sp;
  // Apple OS X uses an old version of gnu as -- both Darwin 7.9.0 (Panther)
  // and Darwin 8.8.1 (Tiger) use as 1.38.  This means we have to use a
  // different asm syntax.  I don't know quite the best way to discriminate
  // systems using the old as from the new one; I've gone with __APPLE__.
  // TODO(csilvers): use autoconf instead, to look for 'as --version' == 1 or 2
#ifdef __APPLE__
  __asm__ volatile ("mr %0,r1" : "=r" (sp));
#else
  __asm__ volatile ("mr %0,1" : "=r" (sp));
#endif

  int n = 0;
  while (sp && n < max_depth) {
    if (skip_count > 0) {
      skip_count--;
    } else {
      // sp[2] holds the "Link Record", according to RTArch-59.html.
      // On PPC, the Link Record is the return address of the
      // subroutine call (what instruction we run after our function
      // finishes).  This is the same as the stack-pointer of our
      // parent routine, which is what we want here.  We believe that
      // the compiler will always set up the LR for subroutine calls.
      //
      // It may be possible to get the stack-pointer of the parent
      // routine directly.  In my experiments, this code works:
      //    result[n++] = NextStackFrame(sp)[-18]
      // But I'm not sure what this is doing, exactly, or how reliable it is.
      result[n++] = *(sp+2);  // sp[2] holds the Link Record (return address)
    }
    // Use strict unwinding rules.
    sp = NextStackFrame<true>(sp);
  }
  return n;
}

// If you change this function, also change GetStackTrace above:
//
// This GetStackFrames routine shares a lot of code with GetStackTrace
// above. This code could have been refactored into a common routine,
// and then both GetStackTrace/GetStackFrames could call that routine.
// There are two problems with that:
//
// (1) The performance of the refactored-code suffers substantially - the
//     refactored needs to be able to record the stack trace when called
//     from GetStackTrace, and both the stack trace and stack frame sizes,
//     when called from GetStackFrames - this introduces enough new
//     conditionals that GetStackTrace performance can degrade by as much
//     as 50%.
//
// (2) Whether the refactored routine gets inlined into GetStackTrace and
//     GetStackFrames depends on the compiler, and we can't guarantee the
//     behavior either-way, even with "__attribute__ ((always_inline))"
//     or "__attribute__ ((noinline))". But we need this guarantee or the
//     frame counts may be off by one.
//
// Both (1) and (2) can be addressed without this code duplication, by
// clever use of template functions, and by defining GetStackTrace and
// GetStackFrames as macros that expand to these template functions.
// However, this approach comes with its own set of problems - namely,
// macros and  preprocessor trouble - for example,  if GetStackTrace
// and/or GetStackFrames is ever defined as a member functions in some
// class, we are in trouble.
int GetStackFrames(void** pcs, int *sizes, int max_depth, int skip_count) {
  void **sp;
  // Apple OS X uses an old version of gnu as -- both Darwin 7.9.0 (Panther)
  // and Darwin 8.8.1 (Tiger) use as 1.38.  This means we have to use a
  // different asm syntax.  I don't know quite the best way to discriminate
  // systems using the old as from the new one; I've gone with __APPLE__.
#ifdef __APPLE__
  __asm__ volatile ("mr %0,r1" : "=r" (sp));
#else
  __asm__ volatile ("mr %0,1" : "=r" (sp));
#endif

  int n = 0;
  while (sp && n < max_depth) {
    // The GetStackFrames routine is called when we are in some
    // informational context (the failure signal handler for example).
    // Use the non-strict unwinding rules to produce a stack trace
    // that is as complete as possible (even if it contains a few bogus
    // entries in some rare cases).
    void **next_sp = NextStackFrame<false>(sp);
    if (skip_count > 0) {
      skip_count--;
    } else {
      // sp[2] holds the "Link Record", according to RTArch-59.html.
      // On PPC, the Link Record is the return address of the
      // subroutine call (what instruction we run after our function
      // finishes).  This is the same as the stack-pointer of our
      // parent routine, which is what we want here.  We believe that
      // the compiler will always set up the LR for subroutine calls.
      //
      // It may be possible to get the stack-pointer of the parent
      // routine directly.  In my experiments, this code works:
      //    pcs[n] = NextStackFrame(sp)[-18]
      // But I'm not sure what this is doing, exactly, or how reliable it is.
      pcs[n] = *(sp+2);  // sp[2] holds the Link Record (return address)
      if (next_sp > sp) {
        sizes[n] = (uintptr_t)next_sp - (uintptr_t)sp;
      } else {
        // A frame-size of 0 is used to indicate unknown frame size.
        sizes[n] = 0;
      }
      n++;
    }
    sp = next_sp;
  }
  return n;
}