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

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.