Consider following toy example:

__attribute__ ((target ("default")))
static uint32_t foo(const char *buf, size_t size) {
  return 1;
}

__attribute__ ((target ("avx")))
static uint32_t foo(const char *buf, size_t size) {
  return 2;
}

__attribute__ ((target ("default")))
uint32_t bar() {
  char buf[4096];
  uint32_t acc = 0;
  for (int i = 0; i < sizeof(buf); i++) {
    acc += foo(&buf[i], 1);
  }
  return acc;
}

__attribute__ ((target ("avx")))
uint32_t bar() {
  char buf[4096];
  uint32_t acc = 0;
  for (int i = 0; i < sizeof(buf); i++) {
    acc += foo(&buf[i], 1);
  }
  return acc;
}

bar.avx is correctly optimized to mov:

bar() [clone .avx]:
        movl    $8192, %eax
        ret

However even though default bar could be optimized to mov as well it goes through loop and dispatch:

bar():
        pushq   %r12
        pushq   %rbp
        xorl    %ebp, %ebp
        pushq   %rbx
        subq    $4096, %rsp
        leaq    4096(%rsp), %r12
        movq    %rsp, %rbx
.L10:
        movq    %rbx, %rdi
        movl    $1, %esi
        addq    $1, %rbx
        call    _ZL3fooPKcm._GLOBAL____tmp_compiler_explorer_compiler117919_59_b8onwy.b8iqhyqfr_example.cpp_00000000_0x82e640d209aabe90.ifunc(char const*, unsigned long)
        addl    %eax, %ebp
        cmpq    %r12, %rbx
        jne     .L10
        addq    $4096, %rsp
        movl    %ebp, %eax
        popq    %rbx
        popq    %rbp
        popq    %r12
        ret

Possibly overlapping with bug #71990.