Do I have a gcc optimization bug or a C code problem?

Question

Test the following code:

#include 
#include 
main()
{
    const char *yytext=\"0\";
    const float f=(float)atof(yytext);
    siz         


        

Answer 1

Use the compiler flag -fno-strict-aliasing.

With strict aliasing enabled, as it is by default for at least -O3, in the line:

size_t t = *((size_t*)&f);

the compiler assumes that the size_t* does NOT point to the same memory area as the float*. As far as I know, this is standards-compliant behaviour (adherence with strict aliasing rules in the ANSI standard start around gcc-4, as Thomas Kammeyer pointed out).

If I recall correctly, you can use an intermediate cast to char* to get around this. (compiler assumes char* can alias anything)

In other words, try this (can't test it myself right now but I think it will work):

size_t t = *((size_t*)(char*)&f);