[LLD][COFF] Reduce chance of symbol name collision with delay-load

[alvinhochun]

Summary

Delay-loaded imports creats a load thunk with a symbol name. Before this
change, the name uses a __imp_load_ prefix. On the other hand, normal
import uses the __imp_ prefix for the import address pointer. If an
import symbol named load_func is imported normally and another named
func is imported using delay-load, this can cause a symbol name
collision.

This patch changes delay-load imports to use __imp___load_ prefix.
Because it is less likely for normal imports to have a name starting in
__load_ this should reduce the chance of a name collision.

[alvinhochun]

The code was originally added in https://reviews.llvm.org/D87544.

TO be clear, I only made this patch because I came across this code and it seems to be a potential issue. I don't have an actual failing test case here.

[mstorsjo]

Because it is less likely for normal imports to have a name starting in __load_ this should reduce the chance of a name collision.

Indeed, symbols starting with two underscores are reserved for the toolchain implementation, iirc.

This looks reasonable to me.

lld/COFF/DLL.cpp

for (DefinedImportData *s : syms) {
       thunks.push_back(t);
       StringRef extName = s->getExternalName();
       if (extName.empty()) {
         names.push_back(make<OrdinalOnlyChunk>(s->getOrdinal()));
       } else {
         auto *c = make<HintNameChunk>(extName, 0);
         names.push_back(make<LookupChunk>(c));
         hintNames.push_back(c);
-        // Add a syntentic symbol for this load thunk, using the "__imp_load"
+        // Add a syntentic symbol for this load thunk, using the "__imp___load"
         // prefix, in case this thunk needs to be added to the list of valid
         // call targets for Control Flow Guard.
-        StringRef symName = saver().save("__imp_load_" + extName);
+        StringRef symName = saver().save("__imp___load_" + extName);
         s->loadThunkSym =
             cast<DefinedSynthetic>(ctx.symtab.addSynthetic(symName, t));
       }
     }
     thunks.push_back(tm);
     StringRef tmName =
         saver().save("__tailMerge_" + syms[0]->getDLLName().lower());
     ctx.symtab.addSynthetic(tmName, tm);