[DAGCombiner] Fix a crash visiting `AND` nodes.

Instead of asserting that the shift count is != 0 we just bail out
as it's not profitable trying to optimize a node which will be
removed anyway.

Differential Revision:  https://reviews.llvm.org/D26098

llvm-svn: 285480

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -3046,6 +3046,11 @@ SDValue DAGCombiner::visitANDLike(SDValue N0, SDValue N1,
         unsigned Size = VT.getSizeInBits();
         const APInt &AndMask = CAnd->getAPIntValue();
         unsigned ShiftBits = CShift->getZExtValue();
+
+        // Bail out, this node will probably disappear anyway.
+        if (ShiftBits == 0)
+          return SDValue();
+
         unsigned MaskBits = AndMask.countTrailingOnes();
         EVT HalfVT = EVT::getIntegerVT(*DAG.getContext(), Size / 2);
 
@@ -3064,7 +3069,7 @@ SDValue DAGCombiner::visitANDLike(SDValue N0, SDValue N1,
           // extended to handle extensions mixed in.
 
           SDValue SL(N0);
-          assert(ShiftBits != 0 && MaskBits <= Size);
+          assert(MaskBits <= Size);
 
           // Extracting the highest bit of the low half.
           EVT ShiftVT = TLI.getShiftAmountTy(HalfVT, DAG.getDataLayout());

llvm/test/CodeGen/X86/pr30813.s

@@ -0,0 +1,20 @@
+	.text
+	.file	"/home/davide/work/llvm/test/CodeGen/X86/visitand-shift.ll"
+	.globl	patatino
+	.p2align	4, 0x90
+	.type	patatino,@function
+patatino:                               # @patatino
+	.cfi_startproc
+# BB#0:
+                                        # implicit-def: %RAX
+	movzwl	(%rax), %ecx
+	movl	%ecx, %eax
+                                        # implicit-def: %RDX
+	movq	%rax, (%rdx)
+	retq
+.Lfunc_end0:
+	.size	patatino, .Lfunc_end0-patatino
+	.cfi_endproc
+
+
+	.section	".note.GNU-stack","",@progbits