[Constant] Allow ConstantAggregateZero a scalable element count

[frasercrmck]

Summary

A ConstantAggregateZero may be created from a scalable vector type.
However, it still assumed fixed number of elements when queried for
them. This patch changes ConstantAggregateZero to correctly report its
element count.

This change fixes a couple of issues. Firstly, it fixes a crash in
Constant::getUniqueValue when called on a scalable-vector
zeroinitializer constant.

Secondly, it fixes a latent bug in GlobalISel's IRTranslator in which
translating a scalable-vector zeroinitializer would hit the assertion in
ConstantAggregateZero::getNumElements when casting to a FixedVectorType,
rather than reporting an error more gracefully. This is currently
hypothetical as the IRTranslator has deeper issues preventing the use of
scalable vector types.

[RKSimon]

LGTM

[frasercrmck]

• rebase
• appease clang-tidy
• remove unnecessary cast to VectorType

[RKSimon]

LGTM

llvm/include/llvm/IR/Constants.h

public:
   /// Return a zero of the right value for the specified GEP index if we can,
   /// otherwise return null (e.g. if C is a ConstantExpr).
   Constant *getElementValue(Constant *C) const;
 
   /// Return a zero of the right value for the specified GEP index.
   Constant *getElementValue(unsigned Idx) const;
 
   /// Return the number of elements in the array, vector, or struct.
-  unsigned getNumElements() const;
+  ElementCount getElementCount() const;
 
   /// Methods for support type inquiry through isa, cast, and dyn_cast:
   ///
   static bool classof(const Value *V) {
     return V->getValueID() == ConstantAggregateZeroVal;
   }
 };
 

llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp

else if (auto CF = dyn_cast<ConstantFP>(&C))
     EntryBuilder->buildFConstant(Reg, *CF);
   else if (isa<UndefValue>(C))
     EntryBuilder->buildUndef(Reg);
   else if (isa<ConstantPointerNull>(C))
     EntryBuilder->buildConstant(Reg, 0);
   else if (auto GV = dyn_cast<GlobalValue>(&C))
     EntryBuilder->buildGlobalValue(Reg, GV);
   else if (auto CAZ = dyn_cast<ConstantAggregateZero>(&C)) {
-    if (!CAZ->getType()->isVectorTy())
+    if (!isa<FixedVectorType>(CAZ->getType()))
       return false;
     // Return the scalar if it is a <1 x Ty> vector.
-    if (CAZ->getNumElements() == 1)
+    unsigned NumElts = CAZ->getElementCount().getFixedValue();
+    if (NumElts == 1)
       return translateCopy(C, *CAZ->getElementValue(0u), *EntryBuilder.get());
     SmallVector<Register, 4> Ops;
-    for (unsigned i = 0; i < CAZ->getNumElements(); ++i) {
+    for (unsigned i = 0; i < NumElts; ++i) {
       Constant &Elt = *CAZ->getElementValue(i);
       Ops.push_back(getOrCreateVReg(Elt));
     }
     EntryBuilder->buildBuildVector(Reg, Ops);
   } else if (auto CV = dyn_cast<ConstantDataVector>(&C)) {
     // Return the scalar if it is a <1 x Ty> vector.
     if (CV->getNumElements() == 1)
       return translateCopy(C, *CV->getElementAsConstant(0),

llvm/lib/IR/Constants.cpp

 
 Constant *Constant::getAggregateElement(unsigned Elt) const {
   assert((getType()->isAggregateType() || getType()->isVectorTy()) &&
          "Must be an aggregate/vector constant");
 
   if (const auto *CC = dyn_cast<ConstantAggregate>(this))
     return Elt < CC->getNumOperands() ? CC->getOperand(Elt) : nullptr;
 
+  if (const auto *CAZ = dyn_cast<ConstantAggregateZero>(this))
+    return Elt < CAZ->getElementCount().getKnownMinValue()
+               ? CAZ->getElementValue(Elt)
+               : nullptr;
+
   // FIXME: getNumElements() will fail for non-fixed vector types.
   if (isa<ScalableVectorType>(getType()))
     return nullptr;
 
-  if (const auto *CAZ = dyn_cast<ConstantAggregateZero>(this))
-    return Elt < CAZ->getNumElements() ? CAZ->getElementValue(Elt) : nullptr;
-
   if (const auto *PV = dyn_cast<PoisonValue>(this))
     return Elt < PV->getNumElements() ? PV->getElementValue(Elt) : nullptr;
 
   if (const auto *UV = dyn_cast<UndefValue>(this))
     return Elt < UV->getNumElements() ? UV->getElementValue(Elt) : nullptr;
 
   if (const auto *CDS = dyn_cast<ConstantDataSequential>(this))
     return Elt < CDS->getNumElements() ? CDS->getElementAsConstant(Elt)

 }
 
 Constant *ConstantAggregateZero::getElementValue(unsigned Idx) const {
   if (isa<ArrayType>(getType()) || isa<VectorType>(getType()))
     return getSequentialElement();
   return getStructElement(Idx);
 }
 
-unsigned ConstantAggregateZero::getNumElements() const {
+ElementCount ConstantAggregateZero::getElementCount() const {
   Type *Ty = getType();
   if (auto *AT = dyn_cast<ArrayType>(Ty))
-    return AT->getNumElements();
+    return ElementCount::getFixed(AT->getNumElements());
   if (auto *VT = dyn_cast<VectorType>(Ty))
-    return cast<FixedVectorType>(VT)->getNumElements();
-  return Ty->getStructNumElements();
+    return cast<VectorType>(VT)->getElementCount();
+  return ElementCount::getFixed(Ty->getStructNumElements());
 }
 
 //===----------------------------------------------------------------------===//
 //                         UndefValue Implementation
 //===----------------------------------------------------------------------===//
 
 UndefValue *UndefValue::getSequentialElement() const {
   if (ArrayType *ATy = dyn_cast<ArrayType>(getType()))

llvm/test/Transforms/InstCombine/scalable-select.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -instcombine -S < %s | FileCheck %s
+
+; This test checks a regression in the select operand folding combine, in which
+; Constant::getUniqueInteger would crash for a scalable-vector zeroinitializer.
+define <vscale x 1 x i32> @select_opt(<vscale x 1 x i32> %b, <vscale x 1 x i1> %m) {
+; CHECK-LABEL: @select_opt(
+; CHECK-NEXT:    [[C:%.*]] = add nsw <vscale x 1 x i32> [[B:%.*]], shufflevector (<vscale x 1 x i32> insertelement (<vscale x 1 x i32> undef, i32 2, i32 0), <vscale x 1 x i32> undef, <vscale x 1 x i32> zeroinitializer)
+; CHECK-NEXT:    [[D:%.*]] = select <vscale x 1 x i1> [[M:%.*]], <vscale x 1 x i32> [[C]], <vscale x 1 x i32> [[B]]
+; CHECK-NEXT:    ret <vscale x 1 x i32> [[D]]
+;
+  %head = insertelement <vscale x 1 x i32> undef, i32 2, i32 0
+  %splat = shufflevector <vscale x 1 x i32> %head, <vscale x 1 x i32> undef, <vscale x 1 x i32> zeroinitializer
+  %c = add nsw <vscale x 1 x i32> %b, %splat
+  %d = select <vscale x 1 x i1> %m, <vscale x 1 x i32> %c, <vscale x 1 x i32> %b
+  ret <vscale x 1 x i32> %d
+}