[Bug target/49244] __sync or __atomic builtins will not emit 'lock bts/btr/btc'

[dhowells at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49244

--- Comment #20 from dhowells at redhat dot com <dhowells at redhat dot com> ---
Here's a further underoptimisation with -Os:

    bool foo_test_and_change_bit(unsigned long *p)
    {
        return test_and_change_bit(83, p);
    }

is compiled to:

0000000000000015 <foo_test_and_change_bit>:
  15:   f0 48 0f ba 7f 08 13    lock btcq $0x13,0x8(%rdi)
  1c:   0f 92 c0                setb   %al
  1f:   c3                      retq   

However, the bit number on BTCQ is an imm8, so the displacement on the memory
operand is unnecessary if the bit number will fit inside the imm8.