[PATCH] SelectionDAG: mimic InstCombine when comparing to undef

[Mehdi AMINI]

Hi hfinkel,

This implements the same folding for SelectionDAG than what is present in InstCombine.

I haven't put a test yet, and it breaks the validation, but before fixing this I seek opinion on NaN handling here. InstCombine does not care and I don't really see why the same IR would be handled differently in the DAG than in InstCombine.

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D7602

Files:
  lib/CodeGen/SelectionDAG/SelectionDAG.cpp

Index: lib/CodeGen/SelectionDAG/SelectionDAG.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -1912,6 +1912,18 @@
     }
   }
 
+  // InstCombine implements:
+  //
+  // icmp X, X -> true/false
+  // X icmp undef -> true/false.  For example, icmp ugt %X, undef -> false
+  // because X could be 0.
+  //
+  // fcmp pred X, undef -> undef
+  //
+  // Do the same here
+  if (N1 == N2 || N1.getOpcode() == ISD::UNDEF || N1.getOpcode() == ISD::UNDEF)
+    return getConstant(isTrueWhenEqual(Cond), VT);
+
   // Could not fold it.
   return SDValue();
 }

EMAIL PREFERENCES
  http://reviews.llvm.org/settings/panel/emailpreferences/
-------------- next part --------------
A non-text attachment was scrubbed...
Name: D7602.19866.patch
Type: text/x-patch
Size: 669 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150213/7c5c0732/attachment.bin>