RooNaNPacker.h

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/*
 * Project: RooFit
 * Author:
 *   Stephan Hageboeck, CERN 2020
 *
 * Copyright (c) 2023, CERN
 *
 * Redistribution and use in source and binary forms,
 * with or without modification, are permitted according to the terms
 * listed in LICENSE (http://roofit.sourceforge.net/license.txt)
 */
 
#ifndef ROOFIT_BATCHCOMPUTE_ROONANPACKER_H
#define ROOFIT_BATCHCOMPUTE_ROONANPACKER_H
 
#include <RooBatchComputeTypes.h>
 
#include <TError.h>
 
#include <limits>
#include <cstdint>
#include <cmath>
#include <cassert>
#include <numeric>
#include <cstring>
 
/// Little struct that can pack a float into the unused bits of the mantissa of a
/// NaN double. This can be used to transport information about violation
/// of function definition ranges, negative PDFs or other computation
/// problems in RooFit.
/// To separate NaNs that contain packed floats from regular NaNs, a tag is
/// written into the upper bits of the mantissa. If this tag is found, a payload
/// can be recovered. Otherwise, the NaN is assumed to originate from other sources
/// than a RooFit class that wants to signal to the minimiser.
struct RooNaNPacker {
   double _payload;
 
   // In double-valued NaNs, on can abuse the lowest 51 bit for payloads.
   // We use this to pack a float into the lowest 32 bits, which leaves
   // 19-bit to include a magic tag to tell NaNs with payload from ordinary
   // NaNs:
   static constexpr uint64_t magicTagMask = 0x3ffff00000000;
   static constexpr uint64_t magicTag = 0x321ab00000000;
 
   constexpr RooNaNPacker() : _payload(0.) {}
 
   /// Create NaN with a packed floating point number.
   explicit RooNaNPacker(float value) : _payload(packFloatIntoNaN(value)) {}
 
   /// Pack float into mantissa of NaN.
   void setPayload(float payload)
   {
      _payload = packFloatIntoNaN(payload);
 
      if (!std::isnan(_payload)) {
         // Big-endian machine or other. Just return NaN.
         warn();
         _payload = std::numeric_limits<double>::quiet_NaN();
      }
   }
 
   /// Accumulate a packed float from another NaN into `this`.
   void accumulate(double val) { *this += unpackNaN(val); }
 
   /// Unpack floats from NaNs, and sum the packed values.
   template <class It_t>
   static double accumulatePayloads(It_t begin, It_t end)
   {
      double sum = std::accumulate(begin, end, 0.f, [](float acc, double val) { return acc += unpackNaN(val); });
 
      return packFloatIntoNaN(sum);
   }
 
   /// Add to the packed float.
   RooNaNPacker &operator+=(float val)
   {
      setPayload(getPayload() + val);
      return *this;
   }
 
   /// Multiply the packed float.
   RooNaNPacker &operator*=(float val)
   {
      setPayload(getPayload() * val);
      return *this;
   }
 
   /// Retrieve packed float. Returns zero if number is not NaN
   /// or if float wasn't packed by this class.
   float getPayload() const { return isNaNWithPayload(_payload) ? unpackNaN(_payload) : 0.; }
 
   /// Retrieve a NaN with the current float payload packed into the mantissa.
   double getNaNWithPayload() const { return _payload; }
 
   /// Test if this struct has a float packed into its mantissa.
   bool isNaNWithPayload() const { return isNaNWithPayload(_payload); }
 
   /// Test if `val` has a float packed into its mantissa.
   static bool isNaNWithPayload(double val)
   {
      uint64_t tmp;
      std::memcpy(&tmp, &val, sizeof(uint64_t));
      return std::isnan(val) && (tmp & magicTagMask) == magicTag;
   }
 
   /// Pack float into mantissa of a NaN. Adds a tag to the
   /// upper bits of the mantissa, so a "normal" NaN can be
   /// differentiated from a NaN with a payload.
   __roodevice__ __roohost__ static double packFloatIntoNaN(float payload)
   {
      double result = std::numeric_limits<double>::quiet_NaN();
      uint64_t tmp;
      std::memcpy(&tmp, &result, sizeof(uint64_t));
      tmp |= magicTag;
      std::memcpy(&tmp, &payload, sizeof(float));
      std::memcpy(&result, &tmp, sizeof(uint64_t));
      return result;
   }
 
   /// If `val` is NaN and a this NaN has been tagged as containing
   /// a payload, unpack the float from the mantissa.
   /// Return 0 otherwise.
   static float unpackNaN(double val)
   {
      float tmp;
      std::memcpy(&tmp, &val, sizeof(float));
      return isNaNWithPayload(val) ? tmp : 0.;
   }
 
   /// Warn that packing only works on little-endian machines.
   static void warn()
   {
      static bool haveWarned = false;
      if (!haveWarned)
         Warning("RooNaNPacker",
                 "Fast recovery from undefined function values only implemented for little-endian machines."
                 " If necessary, request an extension of functionality on https://root.cern");
      haveWarned = true;
   }
};
 
#endif /* ROOFIT_ROOFITCORE_INC_ROONANPACKER_H_ */