[MooreToCore] Lower empty string constant to expected bit width (#8688)

Fix type bit width mismatch when lowering empty string constants.

Author: mvpant

lib/Conversion/MooreToCore/MooreToCore.cpp

@@ -604,23 +604,23 @@ struct StringConstantOpConv : public OpConversionPattern<StringConstantOp> {
   LogicalResult
   matchAndRewrite(moore::StringConstantOp op, OpAdaptor adaptor,
                   ConversionPatternRewriter &rewriter) const override {
-    const auto str = op.getValue();
-    const unsigned byteWidth = str.size() * 8;
     const auto resultType =
         typeConverter->convertType(op.getResult().getType());
-    if (const auto intType = mlir::dyn_cast<IntegerType>(resultType)) {
-      if (intType.getWidth() < byteWidth) {
-        return rewriter.notifyMatchFailure(op,
-                                           "invalid string constant type size");
-      }
-    } else {
-      return rewriter.notifyMatchFailure(op, "invalid string constant type");
-    }
+    const auto intType = mlir::cast<IntegerType>(resultType);
+
+    const auto str = op.getValue();
+    const unsigned byteWidth = intType.getWidth();
     APInt value(byteWidth, 0);
-    for (size_t i = 0; i < str.size(); ++i) {
-      const auto asciiChar = static_cast<uint8_t>(str[i]);
-      value |= APInt(byteWidth, asciiChar) << (8 * (str.size() - 1 - i));
+
+    // Pack ascii chars from the end of the string, until it fits.
+    const size_t maxChars =
+        std::min(str.size(), static_cast<size_t>(byteWidth / 8));
+    for (size_t i = 0; i < maxChars; i++) {
+      const size_t pos = str.size() - 1 - i;
+      const auto asciiChar = static_cast<uint8_t>(str[pos]);
+      value |= APInt(byteWidth, asciiChar) << (8 * i);
     }
+
     rewriter.replaceOpWithNewOp<hw::ConstantOp>(
         op, resultType, rewriter.getIntegerAttr(resultType, value));
     return success();

test/Conversion/MooreToCore/basic.mlir

@@ -1047,8 +1047,18 @@ moore.module @StringConstant() {
   moore.procedure initial {
     // CHECK: hw.constant 1415934836 : i32
     %str = moore.string_constant "Test" : i32
+    // CHECK: hw.constant 1415934836 : i36
+    %str1 = moore.string_constant "Test" : i36
+    // CHECK: hw.constant 116 : i8
+    %str2 = moore.string_constant "Test" : i8
+    // CHECK: hw.constant 0 : i7
+    %str_trunc = moore.string_constant "Test" : i7
+    // CHECK: hw.constant 29556 : i17
+    %str_trunc1 = moore.string_constant "Test" : i17
     // CHECK: hw.constant 0 : i0
     %str_empty = moore.string_constant "" : i0
+    // CHECK: hw.constant 0 : i8
+    %str_empty_zext = moore.string_constant "" : i8
     moore.return
   }
 }

test/Conversion/MooreToCore/errors.mlir

@@ -6,12 +6,3 @@ func.func @invalidType() {
 
   return
 }
-
-// -----
-
-func.func @invalidStringContant() {
-  // expected-error @below {{failed to legalize operation 'moore.string_constant'}}
-  %str = moore.string_constant "Test" : i8
-
-  return
-}