[DAGCombine] Remove SIGN_EXTEND-related inf-loop

The patch's author points out that, despite the function's documentation,
getSetCCResultType is only used to get the SETCC result type (with one
here-removed problematic exception). In one case, getSetCCResultType was being
used to get the predicate type to use for a SELECT node, and then
SIGN_EXTENDing (or truncating) to get the input predicate to match that type.
Unfortunately, this was happening inside visitSIGN_EXTEND, and creating new
SIGN_EXTEND nodes was causing an infinite loop. In addition, this behavior was
wrong if a target was not using ZeroOrNegativeOneBooleanContent. Lastly, the
extension/truncation seems unnecessary here: SELECT is defined as:

  Select(COND, TRUEVAL, FALSEVAL). If the type of the boolean COND is not i1
  then the high bits must conform to getBooleanContents.

So here we remove this use of getSetCCResultType and update
getSetCCResultType's documentation to reflect its actual uses.

Patch by deadal nix!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219141 91177308-0d34-0410-b5e6-96231b3b80d8

Author: Hal Finkel

include/llvm/Target/TargetLowering.h

@@ -275,10 +275,7 @@ class TargetLoweringBase {
     return false;
   }
 
-  /// Return the ValueType of the result of SETCC operations.  Also used to
-  /// obtain the target's preferred type for the condition operand of SELECT and
-  /// BRCOND nodes.  In the case of BRCOND the argument passed is MVT::Other
-  /// since there are no other operands to get a type hint from.
+  /// Return the ValueType of the result of SETCC operations.
   virtual EVT getSetCCResultType(LLVMContext &Context, EVT VT) const;
 
   /// Return the ValueType for comparison libcalls. Comparions libcalls include

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -5208,14 +5208,10 @@ SDValue DAGCombiner::visitSIGN_EXTEND(SDNode *N) {
       if (!LegalOperations || TLI.isOperationLegal(ISD::SETCC, SetCCVT)) {
         SDLoc DL(N);
         ISD::CondCode CC = cast<CondCodeSDNode>(N0.getOperand(2))->get();
-        SDValue SetCC = DAG.getSetCC(DL,
-                                     SetCCVT,
+        SDValue SetCC = DAG.getSetCC(DL, SetCCVT,
                                      N0.getOperand(0), N0.getOperand(1), CC);
-        EVT SelectVT = getSetCCResultType(VT);
-        return DAG.getSelect(DL, VT,
-                             DAG.getSExtOrTrunc(SetCC, DL, SelectVT),
+        return DAG.getSelect(DL, VT, SetCC,
                              NegOne, DAG.getConstant(0, VT));
-
       }
     }
   }

test/CodeGen/X86/sext-i1.ll

@@ -61,3 +61,36 @@ if.end:                                           ; preds = %if.then, %entry
   %xor27 = xor i32 undef, %cond                   ; <i32> [#uses=0]
   ret i32 0
 }
+
+define i32 @t4(i64 %x) nounwind readnone ssp {
+entry:
+; 32-LABEL: t4:
+; 32: movl
+; 32: orl
+; 32: movl
+; 32: je
+; 32: xorl
+
+; 64-LABEL: t4:
+; 64: cmpq $1
+; 64: sbbl
+  %0 = icmp eq i64 %x, 0
+  %1 = sext i1 %0 to i32
+  ret i32 %1
+}
+
+define i64 @t5(i32 %x) nounwind readnone ssp {
+entry:
+; 32-LABEL: t5:
+; 32: cmpl $1
+; 32: sbbl
+; 32: movl
+
+; 64-LABEL: t5:
+; 64: cmpl $1
+; 64: sbbq
+  %0 = icmp eq i32 %x, 0
+  %1 = sext i1 %0 to i64
+  ret i64 %1
+}
+