8000 Remove duplicate reads from the inner loops in generic atomic ops. · postgres/postgres@e530be9 · GitHub
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Remove duplicate reads from the inner loops in generic atomic ops.
The pg_atomic_compare_exchange_xxx functions are defined to update *expected to whatever they read from the target variable. Therefore, there's no need to do additional explicit reads after we've initialized the "old" variable. The actual benefit of this is somewhat debatable, but it seems fairly unlikely to hurt anything, especially since we will override the generic implementations in most performance-sensitive cases. Yura Sokolov, reviewed by Jesper Pedersen and myself Discussion: https://postgr.es/m/7f65886daca545067f82bf2b463b218d@postgrespro.ru
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‎src/include/port/atomics/generic.h

Lines changed: 24 additions & 48 deletions
Original file line numberDiff line numberDiff line change
@@ -170,12 +170,9 @@ static inline uint32
170170
pg_atomic_exchange_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 xchg_)
171171
{
172172
uint32 old;
173-
while (true)
174-
{
175-
old = pg_atomic_read_u32_impl(ptr);
176-
if (pg_atomic_compare_exchange_u32_impl(ptr, &old, xchg_))
177-
break;
178-
}
173+
old = pg_atomic_read_u32_impl(ptr);
174+
while (!pg_atomic_compare_exchange_u32_impl(ptr, &old, xchg_))
175+
/* skip */;
179176
return old;
180177
}
181178
#endif
@@ -186,12 +183,9 @@ static inline uint32
186183
pg_atomic_fetch_add_u32_impl(volatile pg_atomic_uint32 *ptr, int32 add_)
187184
{
188185
uint32 old;
189-
while (true)
190-
{
191-
old = pg_atomic_read_u32_impl(ptr);
192-
if (pg_atomic_compare_exchange_u32_impl(ptr, &old, old + add_))
193-
break;
194-
}
186+
old = pg_atomic_read_u32_impl(ptr);
187+
while (!pg_atomic_compare_exchange_u32_impl(ptr, &old, old + add_))
188+
/* skip */;
195189
return old;
196190
}
197191
#endif
@@ -211,12 +205,9 @@ static inline uint32
211205
pg_atomic_fetch_and_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 and_)
212206
{
213207
uint32 old;
214-
while (true)
215-
{
216-
old = pg_atomic_read_u32_impl(ptr);
217-
if (pg_atomic_compare_exchange_u32_impl(ptr, &old, old & and_))
218-
break;
219-
}
208+
old = pg_atomic_read_u32_impl(ptr);
209+
while (!pg_atomic_compare_exchange_u32_impl(ptr, &old, old & and_))
210+
/* skip */;
220211
return old;
221212
}
222213
#endif
@@ -227,12 +218,9 @@ static inline uint32
227218
pg_atomic_fetch_or_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 or_)
228219
{
229220
uint32 old;
230-
while (true)
231-
{
232-
old = pg_atomic_read_u32_impl(ptr);
233-
if (pg_atomic_compare_exchange_u32_impl(ptr, &old, old | or_))
234-
break;
235-
}
221+
old = pg_atomic_read_u32_impl(ptr);
222+
while (!pg_atomic_compare_exchange_u32_impl(ptr, &old, old | or_))
223+
/* skip */;
236224
return old;
237225
}
238226
#endif
@@ -261,12 +249,9 @@ static inline uint64
261249
pg_atomic_exchange_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 xchg_)
262250
{
263251
uint64 old;
264-
while (true)
265-
{
266-
old = ptr->value;
267-
if (pg_atomic_compare_exchange_u64_impl(ptr, &old, xchg_))
268-
break;
269-
}
252+
old = ptr->value;
253+
while (!pg_atomic_compare_exchange_u64_impl(ptr, &old, xchg_))
254+
/* skip */;
270255
return old;
271256
}
272257
#endif
@@ -357,12 +342,9 @@ static inline uint64
357342
pg_atomic_fetch_add_u64_impl(volatile pg_atomic_uint64 *ptr, int64 add_)
358343
{
359344
uint64 old;
360-
while (true)
361-
{
362-
old = pg_atomic_read_u64_impl(ptr);
363-
if (pg_atomic_compare_exchange_u64_impl(ptr, &old, old + add_))
364-
break;
365-
}
345+
old = pg_atomic_read_u64_impl(ptr);
346+
while (!pg_atomic_compare_exchange_u64_impl(ptr, &old, old + add_))
347+
/* skip */;
366348
return old;
367349
}
368350
#endif
@@ -382,12 +364,9 @@ static inline uint64
382364
pg_atomic_fetch_and_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 and_)
383365
{
384366
uint64 old;
385-
while (true)
386-
{
387-
old = pg_atomic_read_u64_impl(ptr);
388-
if (pg_atomic_compare_exchange_u64_impl(ptr, &old, old & and_))
389-
break;
390-
}
367+
old = pg_atomic_read_u64_impl(ptr);
368+
while (!pg_atomic_compare_exchange_u64_impl(ptr, &old, old & and_))
369+
/* skip */;
391370
return old;
392371
}
393372
#endif
@@ -398,12 +377,9 @@ static inline uint64
398377
pg_atomic_fetch_or_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 or_)
399378
{
400379
uint64 old;
401-
while (true)
402-
{
403-
old = pg_atomic_read_u64_impl(ptr);
404-
if (pg_atomic_compare_exchange_u64_impl(ptr, &old, old | or_))
405-
break;
406-
}
380+
old = pg_atomic_read_u64_impl(ptr);
381+
while (!pg_atomic_compare_exchange_u64_impl(ptr, &old, old | or_))
382+
/* skip */;
407383
return old;
408384
}
409385
#endif

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