[WebAssembly] Avoid unused function imports in PIC mode

[sbc100]

Summary

In PIC mode we import function address via GOT.mem imports but for
direct function calls we still import the first class function.
However, if the function is never directly called we can avoid the first
class import completely.

[sbc100]

• fix bug

lld/test/wasm/shared.s

 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         GlobalType:      I32
 # CHECK-NEXT:         GlobalMutable:   false
 # CHECK-NEXT:       - Module:          env
 # CHECK-NEXT:         Field:           __table_base
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         GlobalType:      I32
 # CHECK-NEXT:         GlobalMutable:   false
-# CHECK-NEXT:       - Module:          env
-# CHECK-NEXT:         Field:           func_external
-# CHECK-NEXT:         Kind:            FUNCTION
-# CHECK-NEXT:         SigIndex:        1
 # CHECK-NEXT:       - Module:          GOT.mem
 # CHECK-NEXT:         Field:           indirect_func
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         GlobalType:      I32
 # CHECK-NEXT:         GlobalMutable:   true
 # CHECK-NEXT:       - Module:          GOT.func
 # CHECK-NEXT:         Field:           func_external
 # CHECK-NEXT:         Kind:            GLOBAL

 # CHECK-NEXT:         GlobalType:      I32
 # CHECK-NEXT:         GlobalMutable:   true
 # CHECK-NEXT:   - Type:            FUNCTION
 
 # CHECK:        - Type:            EXPORT
 # CHECK-NEXT:     Exports:
 # CHECK-NEXT:       - Name:            __wasm_call_ctors
 # CHECK-NEXT:         Kind:            FUNCTION
-# CHECK-NEXT:         Index:           1
+# CHECK-NEXT:         Index:           0
 
 # check for elem segment initialized with __table_base global as offset
 
 # CHECK:        - Type:            ELEM
 # CHECK-NEXT:     Segments:
 # CHECK-NEXT:       - Offset:
 # CHECK-NEXT:           Opcode:          GLOBAL_GET
 # CHECK-NEXT:           Index:           2
-# CHECK-NEXT:         Functions:       [ 4, 3 ]
+# CHECK-NEXT:         Functions:       [ 3, 2 ]
 
 # check the generated code in __wasm_call_ctors and __wasm_apply_data_relocs functions
 # TODO(sbc): Disassemble and verify instructions.
 
 # CHECK:        - Type:            CODE
 # CHECK-NEXT:     Functions:
-# CHECK-NEXT:       - Index:           1
+# CHECK-NEXT:       - Index:           0
 # CHECK-NEXT:         Locals:          []
-# CHECK-NEXT:         Body:            10020B
-# CHECK-NEXT:       - Index:           2
+# CHECK-NEXT:         Body:            10010B
+# CHECK-NEXT:       - Index:           1
 # CHECK-NEXT:         Locals:          []
 # CHECK-NEXT:         Body:            230141046A2304360200230141086A230241016A3602002301410C6A230141006A360200230141106A2305360200230141146A230641046A3602000B
 
 # check the data segment initialized with __memory_base global as offset
 
 # CHECK:        - Type:            DATA
 # CHECK-NEXT:     Segments:
 # CHECK-NEXT:       - SectionOffset:   6
 # CHECK-NEXT:         InitFlags:       0
 # CHECK-NEXT:         Offset:
 # CHECK-NEXT:           Opcode:          GLOBAL_GET
 # CHECK-NEXT:           Index:           1
 # CHECK-NEXT:         Content:         '020000000000000001000000000000000000000000000000'

lld/test/wasm/shared64.s

 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         GlobalType:      I64
 # CHECK-NEXT:         GlobalMutable:   false
 # CHECK-NEXT:       - Module:          env
 # CHECK-NEXT:         Field:           __table_base32
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         GlobalType:      I32
 # CHECK-NEXT:         GlobalMutable:   false
-# CHECK-NEXT:       - Module:          env
-# CHECK-NEXT:         Field:           func_external
-# CHECK-NEXT:         Kind:            FUNCTION
-# CHECK-NEXT:         SigIndex:        1
 # CHECK-NEXT:       - Module:          GOT.mem
 # CHECK-NEXT:         Field:           indirect_func
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         GlobalType:      I64
 # CHECK-NEXT:         GlobalMutable:   true
 # CHECK-NEXT:       - Module:          GOT.func
 # CHECK-NEXT:         Field:           func_external
 # CHECK-NEXT:         Kind:            GLOBAL

 # CHECK-NEXT:         GlobalType:      I64
 # CHECK-NEXT:         GlobalMutable:   true
 # CHECK-NEXT:   - Type:            FUNCTION
 
 # CHECK:        - Type:            EXPORT
 # CHECK-NEXT:     Exports:
 # CHECK-NEXT:       - Name:            __wasm_call_ctors
 # CHECK-NEXT:         Kind:            FUNCTION
-# CHECK-NEXT:         Index:           1
+# CHECK-NEXT:         Index:           0
 
 # check for elem segment initialized with __table_base global as offset
 
 # CHECK:        - Type:            ELEM
 # CHECK-NEXT:     Segments:
 # CHECK-NEXT:       - Offset:
 # CHECK-NEXT:           Opcode:          GLOBAL_GET
 # CHECK-NEXT:           Index:           3
-# CHECK-NEXT:         Functions:       [ 4, 3 ]
+# CHECK-NEXT:         Functions:       [ 3, 2 ]
 
 # check the generated code in __wasm_call_ctors and __wasm_apply_data_relocs functions
 # TODO(sbc): Disassemble and verify instructions.
 
 # CHECK:        - Type:            CODE
 # CHECK-NEXT:     Functions:
-# CHECK-NEXT:       - Index:           1
+# CHECK-NEXT:       - Index:           0
 # CHECK-NEXT:         Locals:          []
-# CHECK-NEXT:         Body:            10020B
-# CHECK-NEXT:       - Index:           2
+# CHECK-NEXT:         Body:            10010B
+# CHECK-NEXT:       - Index:           1
 # CHECK-NEXT:         Locals:          []
 # CHECK-NEXT:         Body:            230142047C23053702002301420C7C230242017C370200230142147C230141006A360200230142187C2306370200230142207C230741046A3602000B
 
 # check the data segment initialized with __memory_base global as offset
 
 # CHECK:        - Type:            DATA
 # CHECK-NEXT:     Segments:
 # CHECK-NEXT:       - SectionOffset:   6
 # CHECK-NEXT:         InitFlags:       0
 # CHECK-NEXT:         Offset:
 # CHECK-NEXT:           Opcode:          GLOBAL_GET
 # CHECK-NEXT:           Index:           1
 # CHECK-NEXT:         Content:         '020000000000000000000000010000000000000000000000000000000000000000000000'

lld/wasm/SymbolTable.cpp

if (isCalledDirectly && !signatureMatches(existingFunction, sig)) {
       // If the existing undefined functions is not called directly then let
       // this one take precedence.  Otherwise the existing function is either
       // directly called or defined, in which case we need a function variant.
       if (existingUndefined && !existingUndefined->isCalledDirectly)
         replaceSym();
       else if (getFunctionVariant(s, sig, file, &s))
         replaceSym();
     }
-    if (existingUndefined)
+    if (existingUndefined) {
       setImportAttributes(existingUndefined, importName, importModule, flags,
                           file);
+      if (isCalledDirectly)
+        existingUndefined->isCalledDirectly = true;
+    }
   }
 
   return s;
 }
 
 Symbol *SymbolTable::addUndefinedData(StringRef name, uint32_t flags,
                                       InputFile *file) {
   LLVM_DEBUG(dbgs() << "addUndefinedData: " << name << "\n");

lld/wasm/Writer.cpp

static bool shouldImport(Symbol *sym) {
   if (!sym->isUsedInRegularObj)
     return false;
 
   // We don't generate imports for data symbols. They however can be imported
   // as GOT entries.
   if (isa<DataSymbol>(sym))
     return false;
 
+  // In PIC mode we only need to import functions when they are called directly.
+  // Indirect usage all goes via GOT imports.
+  if (config->isPic) {
+    if (auto *f = dyn_cast<UndefinedFunction>(sym))
+      if (!f->isCalledDirectly)
+        return false;
+  }
+
   if (config->isPic || config->relocatable || config->importUndefined)
     return true;
   if (config->allowUndefinedSymbols.count(sym->getName()) != 0)
     return true;
 
   return sym->importName.hasValue();
 }