[DAGCombiner] guard against an oversized shift crash

[spatel]

Summary

This change prevents the crash noted in the post-commit comments for rL347478 :
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20181119/605166.html

We can't guarantee that an oversized shift amount is folded away, so we have to check for it.

Note that I committed an incomplete fix for that crash with:
rL347502

But as discussed here:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20181126/605679.html
...we have to try harder.

So I'm not sure how to expose the bug now (and apparently no fuzzers have found a way yet either).

On the plus side, we have discovered that we're missing real optimizations by not simplifying nodes sooner, so the earlier fix still has value, and there's likely more value in extending that so we can simplify more opcodes and simplify when doing RAUW and/or putting nodes on the combiner worklist.

[efriedma]

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

6105

I think you need to use getLimitedValue() here?

[spatel]

Patch updated:
Fixed to use "getLimitedValue" (make sure that really big values don't escape the check).

[efriedma]

LGTM

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

if (N0Opcode == ISD::AND && N0.hasOneUse() && N0->getOperand(1) == N1) {
     SDValue NotX = DAG.getNOT(SDLoc(X), X, VT);
     AddToWorklist(NotX.getNode());
     return DAG.getNode(ISD::AND, DL, VT, NotX, N1);
   }
 
   if ((N0Opcode == ISD::SRL || N0Opcode == ISD::SHL) && N0.hasOneUse()) {
     ConstantSDNode *XorC = isConstOrConstSplat(N1);
     ConstantSDNode *ShiftC = isConstOrConstSplat(N0.getOperand(1));
+    unsigned BitWidth = VT.getScalarSizeInBits();
     if (XorC && ShiftC) {
-      APInt Ones = APInt::getAllOnesValue(VT.getScalarSizeInBits());
-      Ones = N0Opcode == ISD::SHL ? Ones.shl(ShiftC->getZExtValue())
-                                  : Ones.lshr(ShiftC->getZExtValue());
+      // Don't crash on an oversized shift. We can not guarantee that a bogus
+      // shift has been simplified to undef.
+      unsigned ShiftAmt = ShiftC->getZExtValue();
+      if (ShiftAmt < BitWidth) {
+        APInt Ones = APInt::getAllOnesValue(BitWidth);
+        Ones = N0Opcode == ISD::SHL ? Ones.shl(ShiftAmt) : Ones.lshr(ShiftAmt);
         if (XorC->getAPIntValue() == Ones) {
           // If the xor constant is a shifted -1, do a 'not' before the shift:
           // xor (X << ShiftC), XorC --> (not X) << ShiftC
           // xor (X >> ShiftC), XorC --> (not X) >> ShiftC
           SDValue Not = DAG.getNOT(DL, N0.getOperand(0), VT);
           return DAG.getNode(N0Opcode, DL, VT, Not, N0.getOperand(1));
         }
       }
     }
+  }
 
   // fold Y = sra (X, size(X)-1); xor (add (X, Y), Y) -> (abs X)
   if (TLI.isOperationLegalOrCustom(ISD::ABS, VT)) {
     SDValue A = N0Opcode == ISD::ADD ? N0 : N1;
     SDValue S = N0Opcode == ISD::SRA ? N0 : N1;
     if (A.getOpcode() == ISD::ADD && S.getOpcode() == ISD::SRA) {
       SDValue A0 = A.getOperand(0), A1 = A.getOperand(1);
       SDValue S0 = S.getOperand(0);