[llvm-commits] [llvm] r62790 - /llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

[Bob Wilson]

Author: bwilson
Date: Thu Jan 22 16:05:48 2009
New Revision: 62790

URL: http://llvm.org/viewvc/llvm-project?rev=62790&view=rev
Log:
Fix a minor bug in DAGCombiner's folding of SELECT.  Folding "select C, 0, 1"
to "C ^ 1" is only valid when C is known to be either 0 or 1.  Most of the
similar foldings in this function only handle "i1" types, but this one appears
intentionally written to handle larger integer types.  If C has an integer
type larger than "i1", this needs to check if the high bits of a boolean
are known to be zero.  I also changed the comment to describe this folding as
"C ^ 1" instead of "~C", since that is what the code does and since the latter
would only be valid for "i1" types.  The good news is that most LLVM targets
use TargetLowering::ZeroOrOneBooleanContent so this change will not disable
the optimization; the bad news is that I've been unable to come up with a
testcase to demonstrate the problem.

I have also removed a "FIXME" comment for folding "select C, X, 0" to "C & X",
since the code looks correct to me.  It could be made more aggressive by not
limiting the type to "i1", but that would then require checking for
TargetLowering::ZeroOrNegativeOneBooleanContent.  Similar changes could be
done for the other SELECT foldings, but it was decided to be not worth the
trouble and complexity (see e.g., r44663).

Modified:
    llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

Modified: llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp?rev=62790&r1=62789&r2=62790&view=diff

==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp Thu Jan 22 16:05:48 2009
@@ -2746,8 +2746,11 @@
   // fold select C, 1, X -> C | X
   if (VT == MVT::i1 && N1C && N1C->getAPIntValue() == 1)
     return DAG.getNode(ISD::OR, VT, N0, N2);
-  // fold select C, 0, 1 -> ~C
-  if (VT.isInteger() && VT0.isInteger() &&
+  // fold select C, 0, 1 -> C ^ 1
+  if (VT.isInteger() &&
+      (VT0 == MVT::i1 ||
+       (VT0.isInteger() &&
+        TLI.getBooleanContents() == TargetLowering::ZeroOrOneBooleanContent)) &&
       N1C && N2C && N1C->isNullValue() && N2C->getAPIntValue() == 1) {
     SDValue XORNode = DAG.getNode(ISD::XOR, VT0, N0, DAG.getConstant(1, VT0));
     if (VT == VT0)
@@ -2770,7 +2773,6 @@
     return DAG.getNode(ISD::OR, VT, NOTNode, N1);
   }
   // fold select C, X, 0 -> C & X
-  // FIXME: this should check for C type == X type, not i1?
   if (VT == MVT::i1 && N2C && N2C->isNullValue())
     return DAG.getNode(ISD::AND, VT, N0, N1);
   // fold  X ? X : Y --> X ? 1 : Y --> X | Y