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Add comment to explain a non-obvious setting; NFC.
This is paraphrased from Simon Pilgrim's comment in: http://reviews.llvm.org/D7492 llvm-svn: 229566
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@ -657,6 +657,12 @@ X86InstrInfo::X86InstrInfo(X86Subtarget &STI)
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{ X86::VTESTPSYrr, X86::VTESTPSYrm, 0 },
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// AVX2 foldable instructions
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// VBROADCASTS{SD}rr register instructions were an AVX2 addition while the
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// VBROADCASTS{SD}rm memory instructions were available from AVX1.
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// TB_NO_REVERSE prevents unfolding from introducing an illegal instruction
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// on AVX1 targets. The VPBROADCAST instructions are all AVX2 instructions
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// so they don't need an equivalent limitation.
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{ X86::VBROADCASTSSrr, X86::VBROADCASTSSrm, TB_NO_REVERSE },
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{ X86::VBROADCASTSSYrr, X86::VBROADCASTSSYrm, TB_NO_REVERSE },
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{ X86::VBROADCASTSDYrr, X86::VBROADCASTSDYrm, TB_NO_REVERSE },
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