[HW] Canonicalize array get on uniform arrays (#3842)

Add a `getUniformElement()` helper function to `ArrayCreateOp` which
checks if all elements of the created array are identical and returns
that uniform value. Add a folder for `ArrayGetOp` that simply forwards
this uniform value if it exists.

Fixes #3841.

include/circt/Dialect/HW/HWAggregates.td

@@ -39,6 +39,14 @@ def ArrayCreateOp : HWOp<"array_create", [NoSideEffect, SameTypeOperands]> {
     // ValueRange needs to contain at least one element.
     OpBuilder<(ins "ValueRange":$input)>
   ];
+
+  let extraClassDeclaration = [{
+    /// If the all elements of the array are identical, returns that element
+    /// value. Otherwise returns a null value.
+    Value getUniformElement();
+    /// Returns true if all array elements are identical.
+    bool isUniform() { return !!getUniformElement(); }
+  }];
 }
 
 def ArrayConcatOp : HWOp<"array_concat", [NoSideEffect]> {

lib/Dialect/HW/HWOps.cpp

@@ -1710,6 +1710,12 @@ LogicalResult ArrayCreateOp::verify() {
   return success();
 }
 
+Value ArrayCreateOp::getUniformElement() {
+  if (!getInputs().empty() && llvm::all_equal(getInputs()))
+    return getInputs()[0];
+  return {};
+}
+
 static Optional<uint64_t> getUIntFromValue(Value value) {
   auto idxOp = dyn_cast_or_null<ConstantOp>(value.getDefiningOp());
   if (!idxOp)
@@ -2287,13 +2293,18 @@ void ArrayGetOp::build(OpBuilder &builder, OperationState &result, Value input,
 }
 
 // An array_get of an array_create with a constant index can just be the
-// array_create operand at the constant index.
+// array_create operand at the constant index. If the array_create has a single
+// uniform value for each element, just return that value regardless of the
+// index.
 OpFoldResult ArrayGetOp::fold(ArrayRef<Attribute> operands) {
   auto inputCreate =
       dyn_cast_or_null<ArrayCreateOp>(getInput().getDefiningOp());
   if (!inputCreate)
     return {};
 
+  if (auto uniformValue = inputCreate.getUniformElement())
+    return uniformValue;
+
   IntegerAttr constIdx = operands[1].dyn_cast_or_null<IntegerAttr>();
   if (!constIdx || constIdx.getValue().getBitWidth() > 64)
     return {};

test/Dialect/HW/canonicalization.mlir

@@ -1467,3 +1467,11 @@ hw.module @SliceOfCreate(%a0: i1, %a1: i1, %a2: i1, %a3: i1) -> (out: !hw.array<
 
   hw.output %slice : !hw.array<2xi1>
 }
+
+// CHECK-LABEL: hw.module @GetOfUniformArray
+hw.module @GetOfUniformArray(%in: i42, %address: i2) -> (out: i42) {
+  // CHECK: hw.output %in : i42
+  %0 = hw.array_create %in, %in, %in, %in : i42
+  %1 = hw.array_get %0[%address] : !hw.array<4xi42>
+  hw.output %1 : i42
+}