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 // PythiaFpe.h is a part of the PYTHIA event generator.
 // Copyright (C) 2025 Christian Bierlich and Torbjorn Sjostrand.
 // PYTHIA is licenced under the GNU GPL v2 or later, see COPYING for details.
 // Please respect the MCnet Guidelines, see GUIDELINES for details.
 
 // Definition of behaviour for debugging flag -DGCCFPDEBUG. This
 // snippet should, when enabled, cause programs to fail at runtime if
 // they contain a floating point exception. Note that underflow and
 // inexact are not caught, see comment in raisefpe() to enable
 // them. This is not recommended, as FE_UNDERFLOW would also catch
 // things like small exponents, and FE_INEXACT would catch things like
 // 1.0 / 3.0. Expected output is "Caught SIGFPE (Floating Point
 // Exception)" followed by a stack trace. The program will exit with
 // error. Note that this behaviour is both compiler dependent and
 // platform dependent. It works only with gcc on x86/x86_64 (with
 // SSE). On other platforms (ARM, PPC, etc.), SSE intrinsics won't be
 // valid. This includes x86_64 emulated on ARM e.g. in docker.
 
 // Run ./configure with --obj-common=-DGCCFPDEBUG to enable.
 // For better stack trace symbols add -O0 -g -rdynamic:
 // --obj-common='-g -O0 -rdynamic -DGCCFPDEBUG'
 // Consider also -lexecinfo (if you see "undefined reference to
 // backtrace...") and -fno-omit-frame-pointer.
 // Should never be combined with compile flags such as:
 // -ffast-math -Ofast -fno-trapping-math
 // or other fast math flags, as they may optimize away the
 // trapping.
 
 #ifndef Pythia8_PythiaFpe_H
 #define Pythia8_PythiaFpe_H
 
 // Check the GCCFPDEBUG flag.
 #ifdef GCCFPDEBUG
 
 // Catch compilation on ARM platforms.
 #ifdef __aarch64__
 #error "GCCDEBUG unsupported on ARM64. Disable -DGCCDEBUG or compile on x86."
 #endif
 
 #ifndef __ENABLE_FP_DEBUG__
 #define __ENABLE_FP_DEBUG__
 #include <csignal>     // Provides sigaction, siginfo_t.
 #include <fenv.h>      // Provides feenableexcept().
 #include <xmmintrin.h> // Provides _MM_GET/SET_EXCEPTION_MASK().
 #include <execinfo.h>  // Provides backtrace(), backtrace_symbols_fd().
 #include <unistd.h>    // Provides STDERR_FILENO.
 
 // Implement a signal handler. This will catch the action raised by
 // the FPE trigger, print a stack trace and exit. This makes it easier
 // to find the location in the code where the FPE happened. Handle
 // the signal.
 static void fpeSignalHandler(int sig, siginfo_t* info, void* context) {
 
   // Suppress compiler warnings from -Wunused-parameter.
   (void)sig;
   (void)info;
   (void)context;
 
   // Print an error message. Avoid C++ I/O for async safety.
   fprintf(stderr, "\n*************************************************\n");
   fprintf(stderr,  "** Caught SIGFPE (Floating Point Exception)    **\n");
   fprintf(stderr,  "** Printing stack trace (compile with -O0 -g)  **\n");
   fprintf(stderr,  "** For better symbols, also consider -rdynamic.**\n");
   fprintf(stderr,  "*************************************************\n");
 
   // Obtain a backtrace.
   void* buffer[32];
   int n = backtrace(buffer, 32);
 
   // Print the backtrace.
   backtrace_symbols_fd(buffer, n, STDERR_FILENO);
   fprintf(stderr,  "*************************************************\n");
 
   // Exit with error. Use low-level _exit to avoid calling destructors.
   _exit(1);
 
 }
 
 // Setup a handler to catch the signal allowing an action before the
 // program exits.
 static void __attribute__((constructor)) setupFpeHandler() {
 
   // Setup a sigaction for the handler.
   struct sigaction sa;
   // Define the action.
   sa.sa_sigaction = fpeSignalHandler;
   // Empty the handler.
   sigemptyset(&sa.sa_mask);
   // Set the action we want: siginfo_t
   sa.sa_flags = SA_SIGINFO;
   // Install the handler
   sigaction(SIGFPE, &sa, nullptr);
 
 }
 
 // Enable raising of the FPE by unmasking FPU exceptions. This
 // includes both x87 FPU (feenableexcept) and SSE exceptions.
 static void __attribute__((constructor)) raisefpe() {
 
   // Enable x87 FPU exceptions. To catch all exceptions, add
   // FE_UNDERFLOW and FE_INEXACT.
   feenableexcept (FE_DIVBYZERO | FE_OVERFLOW | FE_INVALID);
 
   // Enable SSE exceptions. To catch all exceptions, add
   // _MM_MASK_UNDERFLOW and _MM_MASK_INEXACT.
   unsigned int cw = _MM_GET_EXCEPTION_MASK();
   cw &= ~(_MM_MASK_DIV_ZERO | _MM_MASK_INVALID | _MM_MASK_OVERFLOW);
   _MM_SET_EXCEPTION_MASK(cw);
 
 }
 
 //==========================================================================
 
 #endif // GCCFPDEBUG
 
 #endif // __ENABLE_FP_DEBUG__
 
 #endif // Pythia8_PythiaFpe_H

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