[PATCH] x86 NaCl: Emit LAHF/SAHF instead of PUSHF/POPF

[JF Bastien]

Hi t.p.northover, jvoung,

NaCl's sandbox doesn't allow PUSHF/POPF out of security concerns (priviledged emulators have forgotten to mask system bits in the past, and EFLAGS's DF bit is a constant source of hilarity). Commit r220529 fixed PR20376 by saving cmpxchg's falgs result using EFLAGS, this commit now generated LAHF/SAHF instead of NaCl.

As with the previous patch this code generation is pretty bad because it occurs very later, after register allocation, and in many cases it rematerializes flags which were already available (e.g. already in a register through SETE). Fortunately it's somewhat rare that this code needs to fire.

http://reviews.llvm.org/D6629

Files:
  lib/Target/X86/X86InstrInfo.cpp
  test/CodeGen/X86/cmpxchg-clobber-flags.ll

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