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++) {
| ^~~
