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Commit89b4d77

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For PowerPC instruction "addi", use constraint "b".
Without "b", a variant of the tas() code miscompiles on macOS 10.4.This may also fix a compilation failure involving macOS 10.1. Today'scompilers have been allocating acceptable registers with or without thischange, but this future-proofs the code by precisely conveying theacceptable registers. Back-patch to 9.4 (all supported versions).Reviewed by Tom Lane.Discussion:https://postgr.es/m/20191009063900.GA4066266@rfd.leadboat.com
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‎src/include/storage/s_lock.h

Lines changed: 6 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -452,6 +452,11 @@ typedef unsigned int slock_t;
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#defineTAS_SPIN(lock)(*(lock) ? 1 : TAS(lock))
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/*
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* The second operand of addi can hold a constant zero or a register number,
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* hence constraint "=&b" to avoid allocating r0. "b" stands for "address
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* base register"; most operands having this register-or-zero property are
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* address bases, e.g. the second operand of lwax.
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*
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* NOTE: per the Enhanced PowerPC Architecture manual, v1.0 dated 7-May-2002,
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* an isync is a sufficient synchronization barrier after a lwarx/stwcx loop.
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* On newer machines, we can use lwsync instead for better performance.
@@ -488,7 +493,7 @@ tas(volatile slock_t *lock)
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#endif
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"li %1,0\n"
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:"=&r"(_t),"=r"(_res),"+m"(*lock)
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:"=&b"(_t),"=r"(_res),"+m"(*lock)
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:"r"(lock)
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:"memory","cc");
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return_res;

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