https://godbolt.org/z/hdWKn4bjc

All three functions write to the same variable (b). Clang is able to optimize this. GCC -O3 cannot always do this.
Assembly for foo is writing to **d instead to directly write 1 to b. There is similar behavior for bar and baz. Clang optimizes the code to directly write to b.
Baz is fully optimized only if the foo and bar are removed.

int b = 0;
static int *c = &b;
static int **d = &c;
static int ***e = &d;

void foo() {***e = 1;}
void bar() {**d = 1;}
void baz() {*c = 1;}

Assembly code:
foo:
	movq	d(%rip), %rax
	movq	(%rax), %rax
	movl	$1, (%rax)
	ret