We would emit a libcall for a 64-bit atomic on x86 after SVN r212119. This was
due to the misuse of hasCmpxchg16 to indicate if cmpxchg8b was supported on a
32-bit target. They were added at different times and would result in the
border condition being mishandled.
This fixes the border case to emit the cmpxchg8b instruction for 64-bit atomic
operations on x86 at the cost of restoring a long-standing bug in the codegen.
We emit a cmpxchg8b on all x86 targets even where the CPU does not support this
instruction (pre-Pentium CPUs). Although this bug should be fixed, this was
present prior to SVN r212119 and this change, so this is not really introducing
a regression.
llvm-svn: 212956
On targets without cmpxchg16b or cmpxchg8b, the borderline atomic
operations were slipping through the gaps.
X86AtomicExpand.cpp was delegating to ISelLowering. Generic
ISelLowering was delegating to X86ISelLowering and X86ISelLowering was
asserting. The correct behaviour is to expand to a libcall, preferably
in generic ISelLowering.
This can be achieved by X86ISelLowering deciding it doesn't want the
faff after all.
llvm-svn: 212134
