Bug#412617: gcc-4.1: -O2 optimises away necessary code with no warning

[Anders Johansson <ajh@ares.watri.org.au>]

Package: gcc-4.1
Version: 4.1.1-21
Severity: normal


The below code should return a random number between 0 and 1, but does
not do that when -O2 or higher is enabled. Uncomment the printf line
to make the code work properly.

Cheers,
//Anders


#include <stdlib.h>
#include <math.h>
#include <inttypes.h>
#include <stdio.h>

uint32_t rngi(uint32_t* ws)
{
  /* Dummy code in place of more advanced RNG */
  return rand() | ((rand() & 0x1)<<31);
}

static inline double rng(uint32_t* ws) 
{
  /* Create 53 bit random number */
  uint64_t r = 0ULL;
  void* rp = &r;
  ((uint32_t*)rp)[1] = rngi(ws) >> 5;
  ((uint32_t*)rp)[0] = rngi(ws);
  r >>= 6ULL;
  
  // Uncomment this line to make the proram work properly
  //printf("0x%08X%08X\n", ((uint32_t*)rp)[1], ((uint32_t*)rp)[0]);

  /* Create double precision floating point random number in the range [0,1) 
     The constant is generated as:
     uint64_t g  = 0x0020000000000000ULL;
     printf("%0.24g \n",1.0/(*((double*)(&g))));
  */  
  return *((double*)rp) * 2.24711641857789488466163e+307;
}

int main(void)
{
  static uint32_t* ws = NULL;
  double r = rng(ws);
  printf("%g\n",r);

  return 0;
}




-- System Information:
Debian Release: 4.0
  APT prefers testing
  APT policy: (500, 'testing')
Architecture: i386 (i686)
Shell:  /bin/sh linked to /bin/bash
Kernel: Linux 2.6.20
Locale: LANG=en_GB, LC_CTYPE=sv_SE (charmap=ANSI_X3.4-1968) (ignored: LC_ALL set to C)

Versions of packages gcc-4.1 depends on:
ii  binutils                    2.17-3       The GNU assembler, linker and bina
ii  cpp-4.1                     4.1.1-21     The GNU C preprocessor
ii  gcc-4.1-base                4.1.1-21     The GNU Compiler Collection (base 
ii  libc6                       2.3.6.ds1-11 GNU C Library: Shared libraries
ii  libgcc1                     1:4.1.1-21   GCC support library
ii  libssp0                     4.1.1-21     GCC stack smashing protection libr

Versions of packages gcc-4.1 recommends:
ii  libc6-dev                   2.3.6.ds1-11 GNU C Library: Development Librari
pn  libmudflap0-dev             <none>       (no description available)

-- no debconf information