[piadodjanho]

I looked at the asm generate from my original example and they generate very different codes, gcc applies other optimization when compiled with -O1.

I've been fighting the compiler to generate a minimal working example of the subnormals, but didn't have any success.

Some things take need to be taken in account (from the top of my head):

- Rounding. You don't want to get stuck in the same number. - The FPU have some accumulator register that are larger than the floating point register. - Using more register than the architecture has it not trivial because the register renaming and code reordering. The CPU might optimize in a way that the data never leaves those register.

Trying to make a mwe, I found this code:

    #include <stdio.h>
    
    int
    main ()
    {
        double x = 5e-324;
        double acc = x;
    
        for (size_t i; i < (1ul<<46); i++) {
                acc += x;
    
        }
    
        printf ("%e\n", acc);
    
        return 0;
    }

Runs is fraction of seconds with -O0:

    gcc double.c -o double -O0

But takes forever (killed after 5 minutes) with -O1:

    gcc double.c -o double -O1

I'm using gcc (Arch Linux 9.3.0-1) 9.3.0 on i7-8700

I also manage to create a code that sometimes run in 1s, but in others would take 30s. Didn't matter if I recompiled.

Floating point is hard.

[sigjuice]

Shouldn't i be initialized?

[piadodjanho]

lol, how the compiler didn't warn about uninitialized variable

[sigjuice]

Compiler warnings are not straightforward and depend on many things; compiler version, optimization settings, warning settings.

  $ gcc-9 -Wuninitialized -O0 float.c  # NO WARNINGS!!!

  $ gcc-9 -O1 float.c  # NO WARNINGS!!!
  
  $ gcc-9 -Wuninitialized -O1 float.c
  float.c: In function 'main':
  float.c:9:5: warning: 'i' is used uninitialized in this function [-Wuninitialized]
      9 |     for (size_t i; i < (1ul << 33); i++) {
        |     ^~~

[piadodjanho]

Weirdly, those two codes behaves differently:

    $ gcc hn.c -O0 -o hn

    for (size_t i; i < (1ul<<46); i++) {
        printf ("%zd\n",i);
        acc += x;
        break;
    }

Without break:

    $ gcc hn.c -O0 -o hn

    for (size_t i; i < (1ul<<46); i++) {
        printf ("%zd\n",i);
        acc += x;
    }