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Author SHA1 Message Date
Tim Northover 51472bc600 X86: lower ATOMIC_CMP_SWAP_WITH_SUCCESS directly 
Lowering this new node allows us to fold the almost universal
comparison for success before it's even formed. Instead we can create
a copy from EFLAGS and an X86ISD::SETCC operation since all "cmpxchg"
instructions set the zero-flag to the correct value.

rdar://problem/13201607

llvm-svn: 210923
 2014-06-13 17:29:39 +00:00
Tim Northover 7b9f86da5d Revert "X86: elide comparisons after cmpxchg instructions." 
This reverts commit r210523. It was committed prematurely without waiting for
review.

llvm-svn: 210524
 2014-06-10 10:50:11 +00:00
Tim Northover 84ad29ca1f X86: elide comparisons after cmpxchg instructions. 
The C++ and C semantics of the compare_and_swap operations actually
require us to return a boolean "success" value. In LLVM terms this
means a second comparison of the output of "cmpxchg" against the input
desired value.

However, x86's "cmpxchg" instruction sets all flags for the comparison
formed, so we can skip any secondary comparison. (N.b. this isn't true
for cmpxchg8b/16b, which only set ZF).

rdar://problem/13201607

llvm-svn: 210523
 2014-06-10 10:49:07 +00:00