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Fix atomic_*cmpset32 on riscv64 with clang.

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The lr.w instruction used to read the value from memory sign-extends
the value read from memory.  GCC sign-extends the 32-bit comparison
value passed in whereas clang currently does not.  As a result, if the
value being compared has the MSB set, the comparison fails for
matching 32-bit values when compiled with clang.

Use a cast to explicitly sign-extend the unsigned comparison value.
This works with both GCC and clang.

There is commentary in the RISC-V spec that suggests that GCC's
approach is more correct, but it is not clear if the commentary in the
RISC-V spec is binding.

Reviewed by:	mhorne
Obtained from:	Axiado
MFC after:	2 weeks
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D22084
This commit is contained in:
John Baldwin 2019-10-23 16:41:31 +00:00
parent c92f130498
commit b96562eb86
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=353931
1 changed files with 2 additions and 2 deletions
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4
sys/riscv/include/atomic.h
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@ -182,7 +182,7 @@ atomic_cmpset_32(volatile uint32_t *p, uint32_t cmpval, uint32_t newval)
"bnez %1, 0b\n"
"1:"
: "=&r" (tmp), "=&r" (res), "+A" (*p)
: "rJ" (cmpval), "rJ" (newval)
: "rJ" ((long)(int32_t)cmpval), "rJ" (newval)
: "memory");
return (!res);
@ -207,7 +207,7 @@ atomic_fcmpset_32(volatile uint32_t *p, uint32_t *cmpval, uint32_t newval)
"sw %0, %3\n" /* Save old value */
"2:"
: "=&r" (tmp), "=&r" (res), "+A" (*p), "+A" (*cmpval)
: "rJ" (*cmpval), "rJ" (newval)
: "rJ" ((long)(int32_t)*cmpval), "rJ" (newval)
: "memory");
return (!res);