[WebAssembly][lld] --importTable flag only imports table if needed

[wingo]

Summary

Before, --importTable forced the creation of an indirect function table,
whether it was needed or not. Now it only imports a table if needed.

[sbc100]

Assuming all the tests pass this lgtm!

[sbc100]

I'm trying to think how this could effect either wasi-sdk or emscripten and I can't think of any ways.. but I will run the emscripten test suite with this change just in case.

[sbc100]

lld/wasm/Driver.cpp

834

I'm seeing `Driver.cpp:802:21: warning: unused function 'createUndefinedIndirectFunctionTable' [-Wunused-function]
static TableSymbol *createUndefinedIndirectFunctionTable(StringRef name) {`

[wingo]

Remove unused function

[wingo]

lld/wasm/Driver.cpp

834

Heh yeah, just fixed.

[sbc100]

Emscripten test suite passes. lgtm.

lld/test/wasm/import-table.test

 # RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %p/Inputs/start.s -o %t.start.o
-# RUN: wasm-ld --import-table -o %t.wasm %t.start.o
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %s -o %t.o
+# RUN: wasm-ld --export-all --import-table -o %t.wasm %t.start.o %t.o
 # RUN: obj2yaml %t.wasm | FileCheck %s
 
+.globl require_function_table
+require_function_table:
+.functype require_function_table () -> ()
+          i32.const 1
+          call_indirect () -> ()
+          end_function
+
 # Verify the --import-table flag creates a table import
 
 # CHECK:       - Type:            IMPORT
 # CHECK-NEXT:    Imports:
 # CHECK-NEXT:      - Module:          env
 # CHECK-NEXT:        Field:           __indirect_function_table
 # CHECK-NEXT:        Kind:            TABLE
 # CHECK-NEXT:        Table:
 # CHECK-NEXT:          Index:           0
 # CHECK-NEXT:          ElemType:        FUNCREF
 # CHECK-NEXT:          Limits:
 # CHECK-NEXT:            Initial:         0x1
 

lld/wasm/Driver.cpp

if (existing) {
     if (existing->isDefined()) {
       error(Twine("reserved symbol must not be defined in input files: `") +
             functionTableName + "`");
       return nullptr;
     }
   }
 
   if (config->importTable) {
-    if (existing)
-      return cast<TableSymbol>(existing);
-    else
-      return createUndefinedIndirectFunctionTable(functionTableName);
+    return cast_or_null<TableSymbol>(existing);
   } else if ((existing && existing->isLive()) || config->exportTable) {
     // A defined table is required.  Either because the user request an exported
     // table or because the table symbol is already live.  The existing table is
     // guaranteed to be undefined due to the check above.
     return createDefinedIndirectFunctionTable(functionTableName);
   }
 
   // An indirect function table will only be present in the symbol table if
   // needed by a reloc; if we get here, we don't need one.