Implement sk_atomic_conditional_inc with sk_atomic_cas.

src/ports/SkAtomics_sync.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkAtomics_sync_DEFINED
 #define SkAtomics_sync_DEFINED
 
 /** GCC/Clang __sync based atomics. */
 
 #include <stdint.h>
 
 static inline __attribute__((always_inline)) int32_t sk_atomic_inc(int32_t* addr) {
     return __sync_fetch_and_add(addr, 1);
 }
 
 static inline __attribute__((always_inline)) int32_t sk_atomic_add(int32_t* addr, int32_t inc) {
     return __sync_fetch_and_add(addr, inc);
 }
 
 static inline __attribute__((always_inline)) int32_t sk_atomic_dec(int32_t* addr) {
     return __sync_fetch_and_add(addr, -1);
 }
 
 static inline __attribute__((always_inline)) void sk_membar_acquire__after_atomic_dec() { }
 
-static inline __attribute__((always_inline)) int32_t sk_atomic_conditional_inc(int32_t* addr) {
-    int32_t value = *addr;
-
-    while (true) {
-        if (value == 0) {
-            return 0;
-        }
-
-        int32_t before = __sync_val_compare_and_swap(addr, value, value + 1);
-
-        if (before == value) {
-            return value;
-        } else {
-            value = before;
-        }
-    }
-}
-
 static inline __attribute__((always_inline)) bool sk_atomic_cas(int32_t* addr,
                                                                 int32_t before,
                                                                 int32_t after) {
     return __sync_bool_compare_and_swap(addr, before, after);
 }
 
 static inline __attribute__((always_inline)) void sk_membar_acquire__after_atomic_conditional_inc() { }
 
 #endif