[mlir] Symbol DCE ignores unknown symbols

[Mogball]

Summary

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.

[mehdi_amini]

LGTM, but let's wait for River to double check this.

[mehdi_amini]

Ping @rriddle ?

[rriddle]

mlir/lib/Transforms/SymbolDCE.cpp

126–130

nit: Try to avoid "floating" control flow statements.

[rriddle]

In D116047#3220580, @mehdi_amini wrote:

Ping @rriddle ?

Sorry, coming back from OOO.

mlir/lib/Transforms/SymbolDCE.cpp

if (!uses) {
              << "operation contains potentially unknown symbol table, "
                 "meaning that we can't reliable compute symbol uses";
     }
 
     SmallVector<Operation *, 4> resolvedSymbols;
     for (const SymbolTable::SymbolUse &use : *uses) {
       // 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)
         if (liveSymbols.insert(resolvedSymbol).second)
           worklist.push_back(resolvedSymbol);
     }
   }
 
   return success();
 }
 
 std::unique_ptr<Pass> mlir::createSymbolDCEPass() {
   return std::make_unique<SymbolDCE>();
 }

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

module {
 
   // expected-error@+1 {{contains potentially unknown symbol table}}
   "foo.possibly_unknown_symbol_table"() ({
   }) : () -> ()
 }
 
 // -----
 
+// 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]} : () -> ()
 }