[mlir] Symbol DCE ignores unknown symbols

Instead of failing when it encounters a reference to an unknown symbol, Symbol DCE should ignore them. References to unknown symbols do not affect the overall function of Symbol DCE, so it should not need to fail when it encounters one.

In general, requiring that symbol references always be valid rather than only when necessary can be overly conservative.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D116047

mlir/lib/Transforms/SymbolDCE.cpp

@@ -124,11 +124,9 @@ LogicalResult SymbolDCE::computeLiveness(Operation *symbolTableOp,
       // Lookup the symbols referenced by this use.
       resolvedSymbols.clear();
       if (failed(symbolTable.lookupSymbolIn(
-              op->getParentOp(), use.getSymbolRef(), resolvedSymbols))) {
-        return use.getUser()->emitError()
-               << "unable to resolve reference to symbol "
-               << use.getSymbolRef();
-      }
+              op->getParentOp(), use.getSymbolRef(), resolvedSymbols)))
+        // Ignore references to unknown symbols.
+        continue;
 
       // Mark each of the resolved symbols as live.
       for (Operation *resolvedSymbol : resolvedSymbols)

mlir/test/Transforms/test-symbol-dce.mlir

@@ -84,7 +84,17 @@ module {
 
 // -----
 
+// Check that unknown symbol references are OK.
 module {
-  // expected-error@+1 {{unable to resolve reference to symbol}}
+  // CHECK-NOT: func private @dead_private_function
+  func private @dead_private_function()
+
+  // CHECK: func private @live_private_function
+  func private @live_private_function()
+
+  // CHECK: "live.user"() {uses = [@live_private_function]} : () -> ()
+  "live.user"() {uses = [@live_private_function]} : () -> ()
+
+  // CHECK: "live.user"() {uses = [@unknown_symbol]} : () -> ()
   "live.user"() {uses = [@unknown_symbol]} : () -> ()
 }