[PATCH] [DAGCombiner] Fix & simplify constant folding of sext/zext.

[Paweł Bylica]

Trunc the constant value first as it might be wider than the scalar type. Thanks @RKSimon for the tip.


http://reviews.llvm.org/D10607

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/X86/fold-vector-sext-crash2.ll

Index: lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -5560,12 +5560,12 @@
                        SDLoc(N));
 }
 
-// tryToFoldExtendOfConstant - Try to fold a sext/zext/aext
-// dag node into a ConstantSDNode or a build_vector of constants.
-// This function is called by the DAGCombiner when visiting sext/zext/aext
-// dag nodes (see for example method DAGCombiner::visitSIGN_EXTEND).
-// Vector extends are not folded if operations are legal; this is to
-// avoid introducing illegal build_vector dag nodes.
+/// Try to fold a sext/zext/aext dag node into a ConstantSDNode or 
+/// a build_vector of constants.
+/// This function is called by the DAGCombiner when visiting sext/zext/aext
+/// dag nodes (see for example method DAGCombiner::visitSIGN_EXTEND).
+/// Vector extends are not folded if operations are legal; this is to
+/// avoid introducing illegal build_vector dag nodes.
 static SDNode *tryToFoldExtendOfConstant(SDNode *N, const TargetLowering &TLI,
                                          SelectionDAG &DAG, bool LegalTypes,
                                          bool LegalOperations) {
@@ -5595,7 +5595,6 @@
   // We can fold this node into a build_vector.
   unsigned VTBits = SVT.getSizeInBits();
   unsigned EVTBits = N0->getValueType(0).getScalarType().getSizeInBits();
-  unsigned ShAmt = VTBits - EVTBits;
   SmallVector<SDValue, 8> Elts;
   unsigned NumElts = VT.getVectorNumElements();
   SDLoc DL(N);
@@ -5608,14 +5607,13 @@
     }
 
     SDLoc DL(Op);
-    ConstantSDNode *CurrentND = cast<ConstantSDNode>(Op);
-    const APInt &C = APInt(VTBits, CurrentND->getAPIntValue().getZExtValue());
+    // Get the constant value and if needed trunc it to the size of the type.
+    // Nodes like build_vector might have constants wider than the scalar type.
+    APInt C = cast<ConstantSDNode>(Op)->getAPIntValue().zextOrTrunc(EVTBits);
     if (Opcode == ISD::SIGN_EXTEND || Opcode == ISD::SIGN_EXTEND_VECTOR_INREG)
-      Elts.push_back(DAG.getConstant(C.shl(ShAmt).ashr(ShAmt).getZExtValue(),
-                                     DL, SVT));
+      Elts.push_back(DAG.getConstant(C.sext(VTBits), DL, SVT));
     else
-      Elts.push_back(DAG.getConstant(C.shl(ShAmt).lshr(ShAmt).getZExtValue(),
-                                     DL, SVT));
+      Elts.push_back(DAG.getConstant(C.zext(VTBits), DL, SVT));
   }
 
   return DAG.getNode(ISD::BUILD_VECTOR, DL, VT, Elts).getNode();
Index: test/CodeGen/X86/fold-vector-sext-crash2.ll
===================================================================
--- /dev/null
+++ test/CodeGen/X86/fold-vector-sext-crash2.ll
@@ -0,0 +1,17 @@
+; RUN: llc < %s | FileCheck %s
+
+; DAGCombiner crashes during sext folding
+
+; CHECK-LABEL: test_sext1
+define <8 x i256> @test_sext1() {
+  %Se = sext <8 x i8> <i8 -1, i8 -1, i8 -1, i8 -1, i8 -1, i8 -1, i8 -1, i8 -1> to <8 x i256>
+  %Shuff = shufflevector <8 x i256> zeroinitializer, <8 x i256> %Se, <8 x i32> <i32 9, i32 11, i32 13, i32 15, i32 1, i32 3, i32 5, i32 7>
+  ret <8 x i256> %Shuff
+}
+
+; CHECK-LABEL: test_sext2
+define <8 x i256> @test_sext2() {
+  %Se = sext <8 x i128> <i128 -1, i128 -1, i128 -1, i128 -1, i128 -1, i128 -1, i128 -1, i128 -1> to <8 x i256>
+  %Shuff = shufflevector <8 x i256> zeroinitializer, <8 x i256> %Se, <8 x i32> <i32 9, i32 11, i32 13, i32 15, i32 1, i32 3, i32 5, i32 7>
+  ret <8 x i256> %Shuff
+}

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