Revise folding for CBitExtractBitOp (#99)

Simplify and add support for the base case of single-bit registers.
While at it, add a verifier for CBitExtractBitOp.

include/Dialect/OQ3/IR/OQ3CBitOps.td

@@ -204,6 +204,9 @@ def OQ3_CBitExtractBitOp : OQ3_Op<"cbit_extractbit", [NoSideEffect]> {
     }];
 
     let hasFolder = 1;
+
+    // TODO in LLVM 15 + this can become just let hasVerifier = 1;
+    let verifier = [{ return ::verify(*this);}];
 }
 
 // -----

lib/Dialect/OQ3/IR/OQ3Ops.cpp

@@ -68,6 +68,10 @@ verifyOQ3VariableOpSymbolUses(SymbolTableCollection &symbolTable,
 static llvm::Optional<mlir::Value>
 findDefiningBitInBitmap(mlir::Value val, mlir::IntegerAttr bitIndex) {
 
+  // for single-bit registers, CBitExtractBitOp is the identity.
+  if (val.getType().isInteger(1))
+    return val;
+
   mlir::Operation *op = val.getDefiningOp();
 
   // follow chains of CBit_InsertBit operations and try to find one matching the
@@ -79,12 +83,9 @@ findDefiningBitInBitmap(mlir::Value val, mlir::IntegerAttr bitIndex) {
     op = insertBitOp.operand().getDefiningOp();
   }
 
-  // is the value defined by an i1 constant? then that would be the bit
-  if (auto constantOp =
-          mlir::dyn_cast_or_null<mlir::arith::ConstantIntOp>(op)) {
-    if (constantOp.getType().isInteger(1))
-      return constantOp.getResult();
-  }
+  // did we identify an op that provides the single bit?
+  if (op && op->getResult(0).getType().isInteger(1))
+    return op->getResult(0);
 
   return llvm::None;
 }
@@ -99,6 +100,20 @@ CBitExtractBitOp::fold(::llvm::ArrayRef<::mlir::Attribute> operands) {
   return nullptr;
 }
 
+static LogicalResult verify(CBitExtractBitOp op) {
+
+  auto t = op.getOperand().getType();
+
+  if (auto cbitType = t.dyn_cast<mlir::quir::CBitType>();
+      cbitType && op.index().ult(cbitType.getWidth()))
+    return success();
+
+  if (t.isIntOrIndex() && op.index().ult(t.getIntOrFloatBitWidth()))
+    return success();
+
+  return op.emitOpError("index must be less than the width of the operand.");
+}
+
 LogicalResult
 CBitAssignBitOp::verifySymbolUses(SymbolTableCollection &symbolTable) {
 

test/Dialect/OQ3/IR/fold-cbit-extractbit.mlir

@@ -0,0 +1,19 @@
+// RUN: qss-opt %s --canonicalize | qss-opt | FileCheck %s --implicit-check-not cbit_extractbit
+// Verify that all oq3.cbit_extractbit operations are eliminated
+
+// CHECK: func @single_bit(%[[ARG0:.*]]: i1) -> i1 {
+func @single_bit(%bit: i1) -> i1 {
+    %2 = oq3.cbit_extractbit(%bit : i1) [0] : i1
+    // CHECK: return %[[ARG0]] : i1
+    return %2 : i1
+}
+
+// CHECK: func @two_bits(%[[ARG0:.*]]: !quir.cbit<2>, %[[ARG1:.*]]: i1, %[[ARG2:.*]]: i1)
+func @two_bits(%cbit: !quir.cbit<2>, %bit1: i1, %bit2: i1) -> i1 {
+   %0 = oq3.cbit_insertbit(%cbit : !quir.cbit<2>)[0] = %bit1 : !quir.cbit<2>
+   %1 = oq3.cbit_insertbit(%cbit : !quir.cbit<2>)[1] = %bit2 : !quir.cbit<2>
+
+   %2 = oq3.cbit_extractbit(%1 : !quir.cbit<2>) [1] : i1
+   // CHECK: return %[[ARG2]] : i1
+   return %2 : i1
+}