Change elided large constant syntax to make it more explicit

[mehdi_amini]

Summary

When the printer is requested to elide large constant, we emit an opaque
attribute instead. This patch fills the dialect name with
"elided_large_const" instead of "_" to remove some user confusion when
they later try to consume it.

mlir/lib/IR/AsmPrinter.cpp

 //
 // We choose to use an opaque ElementsAttr literal with conspicuous content to
 // hopefully alert readers to the fact that this has been elided.
 //
 // Unfortunately, neither of the strings of an opaque ElementsAttr literal will
 // accept the string "elided". The first string must be a registered dialect
 // name and the latter must be a hex constant.
 static void printElidedElementsAttr(raw_ostream &os) {
-  os << R"(opaque<"_", "0xDEADBEEF">)";
+  os << R"(opaque<"elided_large_const", "0xDEADBEEF">)";
 }
 
 LogicalResult AsmPrinter::Impl::printAlias(Attribute attr) {
   return success(state && succeeded(state->getAliasState().getAlias(attr, os)));
 }
 
 LogicalResult AsmPrinter::Impl::printAlias(Type type) {
   return success(state && succeeded(state->getAliasState().getAlias(type, os)));

mlir/test/CAPI/ir.c

static void printFirstOfEach(MlirContext ctx, MlirOperation operation) {
   mlirOpPrintingFlagsElideLargeElementsAttrs(flags, 2);
   mlirOpPrintingFlagsPrintGenericOpForm(flags);
   mlirOpPrintingFlagsEnableDebugInfo(flags, /*prettyForm=*/0);
   mlirOpPrintingFlagsUseLocalScope(flags);
   fprintf(stderr, "Op print with all flags: ");
   mlirOperationPrintWithFlags(operation, flags, printToStderr, NULL);
   fprintf(stderr, "\n");
   // clang-format off
-  // CHECK: Op print with all flags: %{{.*}} = "arith.constant"() {elts = opaque<"_", "0xDEADBEEF"> : tensor<4xi32>, value = 0 : index} : () -> index loc(unknown)
+  // CHECK: Op print with all flags: %{{.*}} = "arith.constant"() {elts = opaque<"elided_large_const", "0xDEADBEEF"> : tensor<4xi32>, value = 0 : index} : () -> index loc(unknown)
   // clang-format on
 
   mlirOpPrintingFlagsDestroy(flags);
 }
 
 static int constructAndTraverseIr(MlirContext ctx) {
   MlirLocation location = mlirLocationUnknownGet(ctx);
 

mlir/test/IR/pretty-attributes.mlir

 // RUN: mlir-opt %s -mlir-elide-elementsattrs-if-larger=2 | FileCheck %s
 // Ensure that the elided version is still parseable, although depending on
 // what has been elided, it may not be semantically meaningful.
 // In the typical case where what is being elided is a very large constant
 // tensor which passes don't look at directly, this isn't an issue.
 // RUN: mlir-opt %s -mlir-elide-elementsattrs-if-larger=2 | mlir-opt
 
-// CHECK: opaque<"_", "0xDEADBEEF"> : tensor<3xi32>
+// CHECK: opaque<"elided_large_const", "0xDEADBEEF"> : tensor<3xi32>
 "test.dense_attr"() {foo.dense_attr = dense<[1, 2, 3]> : tensor<3xi32>} : () -> ()
 
 // CHECK: dense<[1, 2]> : tensor<2xi32>
 "test.non_elided_dense_attr"() {foo.dense_attr = dense<[1, 2]> : tensor<2xi32>} : () -> ()
 
-// CHECK: opaque<"_", "0xDEADBEEF"> : vector<1x1x10xf16>
+// CHECK: opaque<"elided_large_const", "0xDEADBEEF"> : vector<1x1x10xf16>
 "test.sparse_attr"() {foo.sparse_attr = sparse<[[0, 0, 5]],  -2.0> : vector<1x1x10xf16>} : () -> ()
 
-// CHECK: opaque<"_", "0xDEADBEEF"> : tensor<100xf32>
-"test.opaque_attr"() {foo.opaque_attr = opaque<"_", "0xEBFE"> : tensor<100xf32> } : () -> ()
+// CHECK: opaque<"elided_large_const", "0xDEADBEEF"> : tensor<100xf32>
+"test.opaque_attr"() {foo.opaque_attr = opaque<"elided_large_const", "0xEBFE"> : tensor<100xf32> } : () -> ()
 
 // CHECK: dense<1> : tensor<3xi32>
 "test.dense_splat"() {foo.dense_attr = dense<1> : tensor<3xi32>} : () -> ()

mlir/test/python/ir/operation.py

def testOperationPrint():
   # CHECK: <class 'bytes'>
   # CHECK: return %arg0 : i32
   module.operation.print(file=f, binary=True)
   bytes_value = f.getvalue()
   print(bytes_value.__class__)
   print(bytes_value)
 
   # Test get_asm with options.
-  # CHECK: value = opaque<"_", "0xDEADBEEF"> : tensor<4xi32>
+  # CHECK: value = opaque<"elided_large_const", "0xDEADBEEF"> : tensor<4xi32>
   # CHECK: "std.return"(%arg0) : (i32) -> () -:4:7
   module.operation.print(
       large_elements_limit=2,
       enable_debug_info=True,
       pretty_debug_info=True,
       print_generic_op_form=True,
       use_local_scope=True)