[water] enforce uniqueness of symbolic tensor dimensions (#776)

Symbols indicate how dimensions are indexed. Repeating the symbol means
we would be indexing two dimensions identically, i.e., their indexes are
always equal, which is not desirable.

Author: ftynse

water/include/water/Dialect/Wave/IR/WaveTypes.td

@@ -23,6 +23,12 @@ def WaveTensorType : TypeDef<WaveDialect, "WaveTensor"> {
     unknown in the earlier stages and will be inferred later. Tensors may have
     an address space indicating whether the data is expected to live in a
     certain location, including local data store (shared memory) and registers.
+
+    When the shape is specified, symbols used in its dimensions indicate which
+    "logical iterators" will be used to index into the tensor. Therefore, all
+    symbols must be unique. (If the same symbol were used twice, it would mean
+    that only a diagonal of the tensor can be indexed since two or more
+    dimensions would be co-indexed).
   }];
 
   let parameters = (ins

water/lib/Dialect/Wave/IR/WaveTypes.cpp

@@ -81,6 +81,12 @@ wave::WaveTensorType::verify(llvm::function_ref<InFlightDiagnostic()> emitError,
     return emitError() << "expected element type to be integer, index or "
                           "floating point scalar";
   }
+  llvm::SmallPtrSet<Attribute, 6> seenSymbols;
+  for (auto symbol : shape) {
+    if (!seenSymbols.insert(symbol).second) {
+      return emitError() << "duplicate symbol " << symbol << " in shape";
+    }
+  }
   return success();
 }
 

water/test/Dialect/Wave/attr-type-invalid.mlir

@@ -127,3 +127,8 @@ module attributes {wave_test.symbol = #wave.symbol<"_A">}
   step = affine_map<()[s0, s1] -> (s0)>,
   stride = affine_map<()[s0, s1] -> (s0)>
 } : () -> ()
+
+// -----
+
+// expected-error @below {{duplicate symbol #wave.symbol<"A"> in shape}}
+"wave_test.create_tensor"() {fully_specified = true, shape = [@A, @B, @A]} : () -> ()