On Thu, 11 Mar 2021 at 16:30, Hugo Musso Gualandi <hgualandi@xxxxxxxxxxxxxx> wrote: > > Hi everyone, > > Does anyone know if there is a way to tell the C compiler that a > pointer's contents won't change after a function call? For example, if > I compile the following code with -O2 then GCC 10 can optimize away the > second pointer dereference in "f" but not in "g". > https://godbolt.org/z/5P3rxn > > void reader(const int *p); > > int f(int *p) { > int x = *p; > int y = *p; > return x + y; > } > > int g(int *p) { > int x = *p; > reader(p); > int y = *p; > return x + y; > } > > Note how there is a single "movl" in the first case but two in the > second: > > f: > movl (%rdi), %eax > addl %eax, %eax > ret > g: > pushq %rbp > pushq %rbx > movq %rdi, %rbx > subq $8, %rsp > movl (%rdi), %ebp > call reader > movl (%rbx), %eax > addq $8, %rsp > popq %rbx > addl %ebp, %eax > popq %rbp > ret > > I also tried using restrict but the result was still the same. > > // The generated ASM was still the same even with restrict. > int g(int * restrict p) { The restrict qualifier says that no other function arguments alias with that one, so it doesn't mean anything on a single argument. > Is there a way to tell the optimizer that the "reader" function won't > modify the contents of "p"? The context of this is that I'm working > with a compiler that uses C as a compilation target. We can rely on GCC > to optimize most things I can't figure out how help it optimize this > one... Did you try the access attribute? https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html
