i386: Relax cmpxchg instruction under -mrelax-cmpxchg-loop [PR103069]

i386: Relax cmpxchg instruction under -mrelax-cmpxchg-loop [PR103069]

For cmpxchg, it is commonly used in spin loop, and several user code
such as pthread directly takes cmpxchg as loop condition, which cause
huge cache bouncing.

This patch extends previous implementation to relax all cmpxchg
instruction under -mrelax-cmpxchg-loop with an extra atomic load,
compare and emulate the failed cmpxchg behavior.

For original spin loop which looks like

loop: mov    %eax,%r8d
      or     $1,%r8d
      lock cmpxchg %r8d,(%rdi)
      jne    loop

It will now truns to

loop: mov    %eax,%r8d
      or     $1,%r8d
      mov    (%r8),%rsi <--- load lock first
      cmp    %rsi,%rax <--- compare with expected input
      jne    .L2 <--- lock ne expected
      lock cmpxchg %r8d,(%rdi)
      jne    loop
  L2: mov    %rsi,%rax <--- perform the behavior of failed cmpxchg
      jne    loop

under -mrelax-cmpxchg-loop.

gcc/ChangeLog:

	PR target/103069
	* config/i386/i386-expand.cc (ix86_expand_atomic_fetch_op_loop):
	Split atomic fetch and loop part.
	(ix86_expand_cmpxchg_loop): New expander for cmpxchg loop.
	* config/i386/i386-protos.h (ix86_expand_cmpxchg_loop): New
	prototype.
	* config/i386/sync.md (atomic_compare_and_swap<mode>): Call new
	expander under TARGET_RELAX_CMPXCHG_LOOP.
	(atomic_compare_and_swap<mode>): Likewise for doubleword modes.

gcc/testsuite/ChangeLog:

	PR target/103069
	* gcc.target/i386/pr103069-2.c: Adjust result check.
	* gcc.target/i386/pr103069-3.c: New test.
	* gcc.target/i386/pr103069-4.c: Likewise.