Add comment to explain a non-obvious setting; NFC.

This is paraphrased from Simon Pilgrim's comment in:
http://reviews.llvm.org/D7492

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229566 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/X86InstrInfo.cpp b/lib/Target/X86/X86InstrInfo.cpp
index 65f8b74d95cbcf1eed5e96c406735f4e5ce9c8fd..88e454aa0cf0ac0bd8ca55c1f22f5d528f535671 100644 (file)
--- a/lib/Target/X86/X86InstrInfo.cpp
+++ b/lib/Target/X86/X86InstrInfo.cpp
@@ -657,6 +657,12 @@ X86InstrInfo::X86InstrInfo(X86Subtarget &STI)
     { X86::VTESTPSYrr,      X86::VTESTPSYrm,          0 },
 
     // AVX2 foldable instructions
+
+    // VBROADCASTS{SD}rr register instructions were an AVX2 addition while the
+    // VBROADCASTS{SD}rm memory instructions were available from AVX1.
+    // TB_NO_REVERSE prevents unfolding from introducing an illegal instruction
+    // on AVX1 targets. The VPBROADCAST instructions are all AVX2 instructions
+    // so they don't need an equivalent limitation.
     { X86::VBROADCASTSSrr,  X86::VBROADCASTSSrm,      TB_NO_REVERSE },
     { X86::VBROADCASTSSYrr, X86::VBROADCASTSSYrm,     TB_NO_REVERSE },
     { X86::VBROADCASTSDYrr, X86::VBROADCASTSDYrm,     TB_NO_REVERSE },