[Bug rtl-optimization/84083] New: [missed optimization] loop-invariant strlen() not hoisted out of loop

[eyalroz at technion dot ac.il]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84083

            Bug ID: 84083
           Summary: [missed optimization] loop-invariant strlen() not
                    hoisted out of loop
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eyalroz at technion dot ac.il
  Target Milestone: ---

Consider the following code:

#include <string.h>

void bar(char c);

void foo(const char* __restrict__ ss) 
{
    for (int i = 0; i < strlen(ss); ++i) 
    {
        bar(*ss);
    }
}

To my understanding, the fact that ss is __restrict__ed (and the fact that it
isn't written through or that it's a const pointer) is sufficient to allow the
compiler to assume the memory accessible via ss remains constant, and thus that
strlen(ss) will return the same value.

But - that's not what happens (with GCC 7.3):

.L6:
        movsx   edi, BYTE PTR [rbp+0]
        mov     rbx, r12
        call    bar(char)
.L3:
        mov     rdi, rbp
        lea     r12, [rbx+1]
        call    strlen
        cmp     rax, rbx
        ja      .L6

(obtained with https://godbolt.org/g/vdGSBe )

Now, I'm no compiler expert, so maybe there are considerations I'm ignoring,
but it seems to me the compiler should be able to hoist the heavier code up
above the loop.

Cf. https://stackoverflow.com/q/48482003/1593077