[RISCV][TTI] Cost non-power-of-two size changing casts (#101047)

For a cast with src and destination size being unequal, we were costing
the cast as if it were being scalarized, when in fact we can often
promote such cases to a wider legal type.

Note that for casts with equal size (i.e. bitcast, some fp<->i, and
ptrtoint) the generic logic in BasicTTI already assumed promotion. It
just doesn't handle the cast where source and destination are both
promoted to non-equal types.

This is analogous to d3fd28a, but with the same reasoning applied to
casts instead.

Author: preames

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -1030,17 +1030,28 @@ InstructionCost RISCVTTIImpl::getCastInstrCost(unsigned Opcode, Type *Dst,
   if (!IsVectorType)
     return BaseT::getCastInstrCost(Opcode, Dst, Src, CCH, CostKind, I);
 
-  bool IsTypeLegal = isTypeLegal(Src) && isTypeLegal(Dst) &&
-                     (Src->getScalarSizeInBits() <= ST->getELen()) &&
-                     (Dst->getScalarSizeInBits() <= ST->getELen());
-
-  // FIXME: Need to compute legalizing cost for illegal types.
-  if (!IsTypeLegal)
+  // FIXME: Need to compute legalizing cost for illegal types.  The current
+  // code handles only legal types and those which can be trivially
+  // promoted to legal.
+  if (!ST->hasVInstructions() || Src->getScalarSizeInBits() > ST->getELen() ||
+      Dst->getScalarSizeInBits() > ST->getELen())
     return BaseT::getCastInstrCost(Opcode, Dst, Src, CCH, CostKind, I);
 
   std::pair<InstructionCost, MVT> SrcLT = getTypeLegalizationCost(Src);
   std::pair<InstructionCost, MVT> DstLT = getTypeLegalizationCost(Dst);
 
+  // Our actual lowering for the case where a wider legal type is available
+  // uses promotion to the wider type.  This is reflected in the result of
+  // getTypeLegalizationCost, but BasicTTI assumes the widened cases are
+  // scalarized if the legalized Src and Dst are not equal sized.
+  const DataLayout &DL = this->getDataLayout();
+  if (!SrcLT.second.isVector() || !DstLT.second.isVector() ||
+      !TypeSize::isKnownLE(DL.getTypeSizeInBits(Src),
+                           SrcLT.second.getSizeInBits()) ||
+      !TypeSize::isKnownLE(DL.getTypeSizeInBits(Dst),
+                           DstLT.second.getSizeInBits()))
+    return BaseT::getCastInstrCost(Opcode, Dst, Src, CCH, CostKind, I);
+
   int ISD = TLI->InstructionOpcodeToISD(Opcode);
   assert(ISD && "Invalid opcode");
 

llvm/test/Analysis/CostModel/RISCV/cast.ll

@@ -4317,24 +4317,24 @@ define void @uitofp() {
 
 define void @oddvec_sizes() {
 ; CHECK-LABEL: 'oddvec_sizes'
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 9 for instruction: %1 = sext <3 x i8> undef to <3 x i16>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 21 for instruction: %2 = sext <7 x i8> undef to <7 x i32>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 45 for instruction: %3 = sext <15 x i8> undef to <15 x i32>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 9 for instruction: %4 = zext <3 x i8> undef to <3 x i16>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 21 for instruction: %5 = zext <7 x i8> undef to <7 x i32>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 45 for instruction: %6 = zext <15 x i8> undef to <15 x i32>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 6 for instruction: %7 = trunc <3 x i32> undef to <3 x i8>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 14 for instruction: %8 = trunc <7 x i32> undef to <7 x i8>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 30 for instruction: %9 = trunc <15 x i32> undef to <15 x i8>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %1 = sext <3 x i8> undef to <3 x i16>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 2 for instruction: %2 = sext <7 x i8> undef to <7 x i32>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 4 for instruction: %3 = sext <15 x i8> undef to <15 x i32>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %4 = zext <3 x i8> undef to <3 x i16>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 2 for instruction: %5 = zext <7 x i8> undef to <7 x i32>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 4 for instruction: %6 = zext <15 x i8> undef to <15 x i32>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 2 for instruction: %7 = trunc <3 x i32> undef to <3 x i8>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 2 for instruction: %8 = trunc <7 x i32> undef to <7 x i8>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 3 for instruction: %9 = trunc <15 x i32> undef to <15 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %10 = bitcast <3 x i32> undef to <3 x float>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %11 = bitcast <7 x i32> undef to <7 x float>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %12 = bitcast <15 x i32> undef to <15 x float>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 9 for instruction: %13 = sitofp <3 x i32> undef to <3 x float>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 21 for instruction: %14 = sitofp <7 x i32> undef to <7 x float>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 45 for instruction: %15 = sitofp <15 x i32> undef to <15 x float>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 9 for instruction: %16 = uitofp <3 x i32> undef to <3 x float>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 21 for instruction: %17 = uitofp <7 x i32> undef to <7 x float>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 45 for instruction: %18 = uitofp <15 x i32> undef to <15 x float>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %13 = sitofp <3 x i32> undef to <3 x float>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %14 = sitofp <7 x i32> undef to <7 x float>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %15 = sitofp <15 x i32> undef to <15 x float>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %16 = uitofp <3 x i32> undef to <3 x float>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %17 = uitofp <7 x i32> undef to <7 x float>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %18 = uitofp <15 x i32> undef to <15 x float>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %19 = fptosi <3 x float> undef to <3 x i32>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %20 = fptosi <7 x float> undef to <7 x i32>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %21 = fptosi <15 x float> undef to <15 x i32>