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Commit88ea7a1

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Choose ppc compare_exchange constant path for more operand values.
The implementation uses smaller code when the "expected" operand is asmall constant, but the implementation needlessly defined the set ofacceptable constants more narrowly than the ABI does. Core PostgreSQLand PGXN don't use the constant path at all, so this is future-proofing.Back-patch to v13, where commit30ee5d1introduced this code.Reviewed by Tom Lane. Reported by Christoph Berg.Discussion:https://postgr.es/m/20201009092825.GD889580@msg.df7cb.de
1 parentf5c1167 commit88ea7a1

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2 files changed

+12
-10
lines changed

2 files changed

+12
-10
lines changed

‎src/include/port/atomics/arch-ppc.h

Lines changed: 4 additions & 10 deletions
Original file line numberDiff line numberDiff line change
@@ -72,14 +72,6 @@ typedef struct pg_atomic_uint64
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* the __asm__. (That would remove the freedom to eliminate dead stores when
7373
* the caller ignores "expected", but few callers do.)
7474
*
75-
* The cmpwi variant may be dead code. In gcc 7.2.0,
76-
* __builtin_constant_p(*expected) always reports false.
77-
* __atomic_compare_exchange_n() does use cmpwi when its second argument
78-
* points to a constant. Hence, using this instead of
79-
* __atomic_compare_exchange_n() nominally penalizes the generic.h
80-
* pg_atomic_test_set_flag_impl(). Modern GCC will use the generic-gcc.h
81-
* version, making the penalty theoretical only.
82-
*
8375
* Recognizing constant "newval" would be superfluous, because there's no
8476
* immediate-operand version of stwcx.
8577
*/
@@ -94,7 +86,8 @@ pg_atomic_compare_exchange_u32_impl(volatile pg_atomic_uint32 *ptr,
9486

9587
#ifdefHAVE_I_CONSTRAINT__BUILTIN_CONSTANT_P
9688
if (__builtin_constant_p(*expected)&&
97-
*expected <=PG_INT16_MAX&&*expected >=PG_INT16_MIN)
89+
(int32)*expected <=PG_INT16_MAX&&
90+
(int32)*expected >=PG_INT16_MIN)
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__asm__ __volatile__(
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"sync\n"
10093
"lwarx %0,0,%5\n"
@@ -183,7 +176,8 @@ pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
183176
/* Like u32, but s/lwarx/ldarx/; s/stwcx/stdcx/; s/cmpw/cmpd/ */
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#ifdefHAVE_I_CONSTRAINT__BUILTIN_CONSTANT_P
185178
if (__builtin_constant_p(*expected)&&
186-
*expected <=PG_INT16_MAX&&*expected >=PG_INT16_MIN)
179+
(int64)*expected <=PG_INT16_MAX&&
180+
(int64)*expected >=PG_INT16_MIN)
187181
__asm__ __volatile__(
188182
"sync\n"
189183
"ldarx %0,0,%5\n"

‎src/test/regress/regress.c

Lines changed: 8 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -720,6 +720,14 @@ test_atomic_uint32(void)
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EXPECT_EQ_U32(pg_atomic_read_u32(&var), (uint32)INT_MAX+1);
721721
EXPECT_EQ_U32(pg_atomic_sub_fetch_u32(&var,INT_MAX),1);
722722
pg_atomic_sub_fetch_u32(&var,1);
723+
expected=PG_INT16_MAX;
724+
EXPECT_TRUE(!pg_atomic_compare_exchange_u32(&var,&expected,1));
725+
expected=PG_INT16_MAX+1;
726+
EXPECT_TRUE(!pg_atomic_compare_exchange_u32(&var,&expected,1));
727+
expected=PG_INT16_MIN;
728+
EXPECT_TRUE(!pg_atomic_compare_exchange_u32(&var,&expected,1));
729+
expected=PG_INT16_MIN-1;
730+
EXPECT_TRUE(!pg_atomic_compare_exchange_u32(&var,&expected,1));
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724732
/* fail exchange because of old expected */
725733
expected=10;

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