cpp

// dumb_alloc.cc: Test this C++ mechanism as a lower bound on performance.

#include "leaky_dumb_alloc.h"

#include <stdio.h>

// 400 MiB of memory

char kMem[400 << 20];

int gMemPos = 0;

int gNumNew = 0;

int gNumDelete = 0;

// I'm not sure why this matters but we get crashes when aligning to 8 bytes.

// That is annoying.

// Example: we get a crash in cpp/frontend_flag_spec.cc

// auto out = new flag_spec::_FlagSpecAndMore();

//

// https://stackoverflow.com/questions/52531695/int128-alignment-segment-fault-with-gcc-o-sse-optimize

constexpr int kMask = alignof(max_align_t) - 1;  // e.g. 15 or 7

// Align returned pointers to the worst case of 8 bytes (64-bit pointers)

inline size_t aligned(size_t n) {

  // https://stackoverflow.com/questions/2022179/c-quick-calculation-of-next-multiple-of-4

  // return (n + 7) & ~7;

  return (n + kMask) & ~kMask;

}

// This global interface is silly ...

#ifdef DUMB_ALLOC

void* operator new(size_t size) {

  char* p = &(kMem[gMemPos]);

#ifdef ALLOC_LOG

  printf("new %zu\n", size);

#endif

  gMemPos += aligned(size);

  ++gNumNew;

  return p;

}

// noexcept fixes Clang warning

void operator delete(void* p) noexcept {

  // fprintf(stderr, "\tdelete %p\n", p);

  ++gNumDelete;

}

#endif

char kMem2[400 << 20];

int gMemPos2 = 0;

int gNumMalloc = 0;

int gNumFree = 0;

void* dumb_malloc(size_t size) noexcept {

  char* p = &(kMem2[gMemPos2]);

#ifdef ALLOC_LOG

  printf("malloc %zu\n", size);

#endif

  gMemPos2 += aligned(size);

  ++gNumMalloc;

  return p;

}

void dumb_free(void* p) noexcept {

  // fprintf(stderr, "free\n");

  ++gNumFree;

}

namespace dumb_alloc {

void Summarize() {

#ifdef DUMB_ALLOC_VERBOSE

  fprintf(stderr, "\n");

  fprintf(stderr, "dumb_alloc:\n");

  fprintf(stderr, "\tgNumNew = %d\n", gNumNew);

  fprintf(stderr, "\tgNumDelete = %d\n", gNumDelete);

  fprintf(stderr, "\tgMemPos = %d\n", gMemPos);

  fprintf(stderr, "\n");

  fprintf(stderr, "\tgNumMalloc = %d\n", gNumMalloc);

  fprintf(stderr, "\tgNumFree = %d\n", gNumFree);

  fprintf(stderr, "\tgMemPos2 = %d\n", gMemPos2);

#endif

}

}  // namespace dumb_alloc