[Bug target/85683] New: [8 Regression] GCC 8 stopped using RMW (Read Modify Write) instructions on x86[_64]

[dmitry at zend dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85683

            Bug ID: 85683
           Summary: [8 Regression] GCC 8 stopped using RMW (Read Modify
                    Write) instructions on x86[_64]
           Product: gcc
           Version: 8.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dmitry at zend dot com
  Target Milestone: ---

GCC 8 stopped generation of RMW instructions and uses 3 instructions instead of
one. Although, this shouldn't affect executed uops, this increases code size
(pressure to instruction decoder and instruction cache) and requires an extra
register (that may cause extra spills and related degradations). 

=== test_rmw.c ===
typedef struct _refcounted {
        unsigned int refcount;
} refcounted;

typedef struct _value {
        refcounted *val;
} value;

extern void value_dtor_func(refcounted *val);

void ptr_dtor(value *ptr)
{
        refcounted *ref = ptr->val;

        --ref->refcount;
        if (ref->refcount == 0) {
                value_dtor_func(ref);
        }
}

===

$ gcc -O2 -S test_rmw.c
$ cat test_rmw.s

        movq    (%rdi), %rdi
        movl    (%rdi), %eax
        subl    $1, %eax
        movl    %eax, (%rdi)
        je      .L4
        ret
.L4:
        jmp     value_dtor_func