[lld][WebAssemlby] Always enable mutable-globals feature in PIC mode

[sbc100]

Summary

This works around an issue where the feature can be forgotten
in the case of LTO + object file with no functions.

See: https://bugs.llvm.org/show_bug.cgi?id=52339

[sbc100]

• comments

[tlively]

"globala" => "globals" in the descriptions (although globala sounds like a great name for something).

lld/test/wasm/lto/pic-empty.s

+; Test that LTO objects with no function can still be linked as shared
+; libraries.
+; We had a bug where the mutable-globals feature was not being added
+; do the import of `__stack_pointer` (which is currently mandatory for
+; shared libraries) was generating a linker error.
+; See https://bugs.llvm.org/show_bug.cgi?id=52339
+
+; RUN: llvm-as %s -o %t.o
+; RUN: wasm-ld -lto-O2 --experimental-pic -shared --no-gc-sections --export=tls_int %t.o -o %t.so
+; RUN: obj2yaml %t.so  | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-p10:8:8-p20:8:8-i64:64-f128:64-n32:64-S128-ni:1:10:20"
+target triple = "wasm32-unknown-emscripten"
+
+@tls_int = dso_local thread_local global i32 99
+
+; CHECK:  - Type:            CUSTOM
+; CHECK:    Name:            target_features
+; CHECK:    Features:
+; CHECK:      - Prefix:          USED
+; CHECK:        Name:            mutable-globals

lld/wasm/Writer.cpp

 
 void Writer::populateTargetFeatures() {
   StringMap<std::string> used;
   StringMap<std::string> required;
   StringMap<std::string> disallowed;
   SmallSet<std::string, 8> &allowed = out.targetFeaturesSec->features;
   bool tlsUsed = false;
 
+  if (config->isPic) {
+    // This should not be necessary because all PIC objects should
+    // contain the mutable-globals feature.
+    // TODO(https://bugs.llvm.org/show_bug.cgi?id=52339)
+    allowed.insert("mutable-globals");
+  }
+
   // Only infer used features if user did not specify features
   bool inferFeatures = !config->features.hasValue();
 
   if (!inferFeatures) {
     auto &explicitFeatures = config->features.getValue();
     allowed.insert(explicitFeatures.begin(), explicitFeatures.end());
     if (!config->checkFeatures)
       return;