[ConstantFold][SVE] Fold bitcast into splat value for splat vector.

[huihuiz]

Summary

Split from D71389, as it depends on D71637.

I am not 100% confident about the general impact of D71637, if we
support constexpr in all OneOps_match, TwoOps_match and ThreeOps_match.

Split first to unblock D71389.

llvm/lib/IR/ConstantFold.cpp

       for (unsigned i = 1, e = CE->getNumOperands(); i != e; ++i)
         if (!CE->getOperand(i)->isNullValue()) {
           isAllNull = false;
           break;
         }
       if (isAllNull)
         // This is casting one pointer type to another, always BitCast
         return ConstantExpr::getPointerCast(CE->getOperand(0), DestTy);
+    } else if (CE->getType()->isVectorTy() && DestTy->isVectorTy() &&
+               CE->getType()->getVectorElementCount() ==
+                   DestTy->getVectorElementCount() &&
+               CE->getOpcode() == Instruction::ShuffleVector &&
+               opc == Instruction::BitCast) {
+      // For splat vector, fold bitcast to splat value.
+      // BitCast(ShuffleVector(InsertElement(C1, SplatV, Zero), C2, Zero)) to NewType
+      // into
+      // ShuffleVector(InsertElement(C1, BitCast(SplatV) to NewType, Zero), C2, Zero)
+      Constant *SplatV, *C1, *C2, *ZeroIdx, *ZeroMask;
+      if (match(CE, m_ShuffleVector(
+                        m_InsertElement(
+                            m_Constant(C1), m_Constant(SplatV),
+                            m_CombineAnd(m_Zero(), m_Constant(ZeroIdx))),
+                        m_Constant(C2),
+                        m_CombineAnd(m_Zero(), m_Constant(ZeroMask))))) {
+        auto *CastedSplatV =
+            ConstantExpr::getCast(opc, SplatV, DestTy->getScalarType());
+        return ConstantExpr::getShuffleVector(
+            ConstantExpr::getInsertElement(
+                ConstantExpr::getCast(opc, C1, DestTy), CastedSplatV, ZeroIdx),
+            ConstantExpr::getCast(opc, C2, DestTy), ZeroMask);
+      }
     }
   }
 
   // If the cast operand is a constant vector, perform the cast by
   // operating on each element. In the cast of bitcasts, the element
   // count may be mismatched; don't attempt to handle that here.
   if ((isa<ConstantVector>(V) || isa<ConstantDataVector>(V)) &&
       DestTy->isVectorTy() &&

llvm/test/Analysis/ConstantFolding/bitcast.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -constprop -S -verify | FileCheck %s
 
 define <vscale x 4 x float> @bitcast_scalable_constant() {
 ; CHECK-LABEL: @bitcast_scalable_constant(
-; CHECK-NEXT:    ret <vscale x 4 x float> bitcast (<vscale x 4 x i32> shufflevector (<vscale x 4 x i32> insertelement (<vscale x 4 x i32> undef, i32 1, i32 0), <vscale x 4 x i32> undef, <vscale x 4 x i32> zeroinitializer) to <vscale x 4 x float>)
+; CHECK-NEXT:    ret <vscale x 4 x float> shufflevector (<vscale x 4 x float> insertelement (<vscale x 4 x float> undef, float 0x36A0000000000000, i32 0), <vscale x 4 x float> undef, <vscale x 4 x i32> zeroinitializer)
 ;
   %i1 = insertelement <vscale x 4 x i32> undef, i32 1, i32 0
   %i2 = shufflevector <vscale x 4 x i32> %i1, <vscale x 4 x i32> undef, <vscale x 4 x i32> zeroinitializer
   %i3 = bitcast <vscale x 4 x i32> %i2 to <vscale x 4 x float>
   ret <vscale x 4 x float> %i3
 }