[ValueTracking] Handle shufflevector constants in ComputeNumSignBits

[huihuiz]

Looks fine with me.

llvm/test/Transforms/InstCombine/nsw.ll

138

Not needed, just shuffle(insertelement) + mul will get this crash.
But not very important.

[efriedma]

llvm/test/Transforms/InstCombine/nsw.ll

138

Since I'm here, might as well add a testcase where we could, in theory, mark the mul as nsw/nuw.

llvm/lib/Analysis/ValueTracking.cpp

case Instruction::ExtractElement:
       // skip tracking the specific element. But at least we might find
       // information valid for all elements of the vector (for example if vector
       // is sign extended, shifted, etc).
       return ComputeNumSignBits(U->getOperand(0), Depth + 1, Q);
 
     case Instruction::ShuffleVector: {
       // Collect the minimum number of sign bits that are shared by every vector
       // element referenced by the shuffle.
-      auto *Shuf = cast<ShuffleVectorInst>(U);
+      auto *Shuf = dyn_cast<ShuffleVectorInst>(U);
+      if (!Shuf) {
+        // FIXME: Add support for shufflevector constant expressions.
+        return 1;
+      }
       APInt DemandedLHS, DemandedRHS;
       // For undef elements, we don't know anything about the common state of
       // the shuffle result.
       if (!getShuffleDemandedElts(Shuf, DemandedElts, DemandedLHS, DemandedRHS))
         return 1;
       Tmp = std::numeric_limits<unsigned>::max();
       if (!!DemandedLHS) {
         const Value *LHS = Shuf->getOperand(0);

llvm/test/Transforms/InstCombine/nsw.ll

 ; CHECK-NEXT:    ret <3 x i32> [[T3]]
 ;
   %t2 = zext <2 x i8> %x to <2 x i32>
   %shuf = shufflevector <2 x i32> %t2, <2 x i32> undef, <3 x i32> <i32 1, i32 undef, i32 0>
   %t3 = shl <3 x i32> %shuf, <i32 17, i32 17, i32 17>
   ret <3 x i32> %t3
 }
 
+; Make sure we don't crash on a ConstantExpr shufflevector
+define <vscale x 2 x i64> @mul_nuw_nsw_shuffle_constant_expr(<vscale x 2 x i8> %z) {
+; CHECK-LABEL: @mul_nuw_nsw_shuffle_constant_expr(
+; CHECK-NEXT:    [[XX:%.*]] = zext <vscale x 2 x i8> [[Z:%.*]] to <vscale x 2 x i64>
+; CHECK-NEXT:    [[T3:%.*]] = mul <vscale x 2 x i64> [[XX]], shufflevector (<vscale x 2 x i64> insertelement (<vscale x 2 x i64> undef, i64 3, i32 0), <vscale x 2 x i64> zeroinitializer, <vscale x 2 x i32> zeroinitializer)
+; CHECK-NEXT:    ret <vscale x 2 x i64> [[T3]]
+;
+  %xx = zext <vscale x 2 x i8> %z to <vscale x 2 x i64>
+  %shuf = shufflevector <vscale x 2 x i64> insertelement (<vscale x 2 x i64> undef, i64 3, i32 0), <vscale x 2 x i64> zeroinitializer, <vscale x 2 x i32> zeroinitializer
+  %t3 = mul <vscale x 2 x i64> %shuf, %xx
+  ret <vscale x 2 x i64> %t3
+}