[ELF] Refine ctx.arg.exportDynamic condition

--export-dynamic should be a no-op when ctx.hasDynsym is false.

* Drop unneeded ctx.hasDynsym checks.
* Static linking with --export-dynamic does not prevent devirtualization.

Author: MaskRay

lld/ELF/Driver.cpp

@@ -2617,8 +2617,7 @@ void LinkerDriver::compileBitcodeFiles(bool skipLinkedOutput) {
       for (Symbol *sym : obj->getGlobalSymbols()) {
         if (!sym->isDefined())
           continue;
-        if (ctx.hasDynsym && ctx.arg.exportDynamic &&
-            sym->computeBinding(ctx) != STB_LOCAL)
+        if (ctx.arg.exportDynamic && sym->computeBinding(ctx) != STB_LOCAL)
           sym->isExported = true;
         if (sym->hasVersionSuffix)
           sym->parseSymbolVersion(ctx);
@@ -2965,6 +2964,7 @@ template <class ELFT> void LinkerDriver::link(opt::InputArgList &args) {
 
   // Create dynamic sections for dynamic linking and static PIE.
   ctx.hasDynsym = !ctx.sharedFiles.empty() || ctx.arg.isPic;
+  ctx.arg.exportDynamic &= ctx.hasDynsym;
 
   // If an entry symbol is in a static archive, pull out that file now.
   if (Symbol *sym = ctx.symtab->find(ctx.arg.entry))

lld/ELF/Symbols.cpp

@@ -361,13 +361,10 @@ void elf::parseVersionAndComputeIsPreemptible(Ctx &ctx) {
   // can contain versions in the form of <name>@<version>.
   // Let them parse and update their names to exclude version suffix.
   // In addition, compute isExported and isPreemptible.
-  bool hasDynsym = ctx.hasDynsym;
   bool maybePreemptible = ctx.sharedFiles.size() || ctx.arg.shared;
   for (Symbol *sym : ctx.symtab->getSymbols()) {
     if (sym->hasVersionSuffix)
       sym->parseSymbolVersion(ctx);
-    if (!hasDynsym)
-      continue;
     if (sym->computeBinding(ctx) == STB_LOCAL) {
       sym->isExported = false;
       continue;
@@ -377,7 +374,7 @@ void elf::parseVersionAndComputeIsPreemptible(Ctx &ctx) {
     } else if (ctx.arg.exportDynamic &&
                (sym->isUsedInRegularObj || !sym->ltoCanOmit)) {
       sym->isExported = true;
-      sym->isPreemptible = computeIsPreemptible(ctx, *sym);
+      sym->isPreemptible = maybePreemptible && computeIsPreemptible(ctx, *sym);
     }
   }
 }

lld/ELF/Writer.cpp

@@ -284,7 +284,6 @@ static void demoteDefined(Defined &sym, DenseMap<SectionBase *, size_t> &map) {
 static void demoteSymbolsAndComputeIsPreemptible(Ctx &ctx) {
   llvm::TimeTraceScope timeScope("Demote symbols");
   DenseMap<InputFile *, DenseMap<SectionBase *, size_t>> sectionIndexMap;
-  bool hasDynsym = ctx.hasDynsym;
   bool maybePreemptible = ctx.sharedFiles.size() || ctx.arg.shared;
   for (Symbol *sym : ctx.symtab->getSymbols()) {
     if (auto *d = dyn_cast<Defined>(sym)) {
@@ -301,9 +300,8 @@ static void demoteSymbolsAndComputeIsPreemptible(Ctx &ctx) {
       }
     }
 
-    if (hasDynsym)
-      sym->isPreemptible = maybePreemptible &&
-                           (sym->isUndefined() || sym->isExported) &&
+    if (maybePreemptible)
+      sym->isPreemptible = (sym->isUndefined() || sym->isExported) &&
                            computeIsPreemptible(ctx, *sym);
   }
 }
@@ -1853,7 +1851,7 @@ template <class ELFT> void Writer<ELFT>::finalizeSections() {
 
   // If the previous code block defines any non-hidden symbols (e.g.
   // __global_pointer$), they may be exported.
-  if (ctx.hasDynsym && ctx.arg.exportDynamic)
+  if (ctx.arg.exportDynamic)
     for (Symbol *sym : ctx.synthesizedSymbols)
       if (sym->computeBinding(ctx) != STB_LOCAL)
         sym->isExported = true;

lld/test/ELF/lto/devirt_vcall_vis_export_dynamic.ll

@@ -9,6 +9,10 @@
 ; RUN: ld.lld %t2.o -o %t3 -save-temps --lto-whole-program-visibility \
 ; RUN:   -mllvm -pass-remarks=. 2>&1 | FileCheck %s --check-prefix=REMARK
 ; RUN: llvm-dis %t2.o.4.opt.bc -o - | FileCheck %s --check-prefix=CHECK-IR
+;; --export-dynamic without .dynsym does not prevent devirtualization.
+; RUN: ld.lld %t2.o -o %t3 -save-temps --lto-whole-program-visibility \
+; RUN:   -mllvm -pass-remarks=. \
+; RUN:   --export-dynamic 2>&1 | FileCheck %s --check-prefix=REMARK
 
 ;; Hybrid WPD
 ;; Generate split module with summary for hybrid Thin/Regular LTO WPD.
@@ -33,19 +37,20 @@
 ; RUN: ld.lld -shared -soname=ta %ta.o -o %ta.so
 
 ;; Index based WPD
-; RUN: ld.lld %t2.o -o %t3 -save-temps --lto-whole-program-visibility \
+
+; RUN: ld.lld %t2.o %ta.so -o %t3 -save-temps --lto-whole-program-visibility \
 ; RUN:   -mllvm -pass-remarks=. \
 ; RUN:   --export-dynamic 2>&1 | FileCheck /dev/null --implicit-check-not single-impl --allow-empty
 ; RUN: llvm-dis %t2.o.4.opt.bc -o - | FileCheck %s --check-prefix=CHECK-NODEVIRT-IR
 
 ;; Hybrid WPD
-; RUN: ld.lld %t.o -o %t3 -save-temps --lto-whole-program-visibility \
+; RUN: ld.lld %t.o %ta.so -o %t3 -save-temps --lto-whole-program-visibility \
 ; RUN:   -mllvm -pass-remarks=. \
 ; RUN:   --export-dynamic 2>&1 | FileCheck /dev/null --implicit-check-not single-impl --allow-empty
 ; RUN: llvm-dis %t.o.4.opt.bc -o - | FileCheck %s --check-prefix=CHECK-NODEVIRT-IR
 
 ;; Regular LTO WPD
-; RUN: ld.lld %t4.o -o %t3 -save-temps --lto-whole-program-visibility \
+; RUN: ld.lld %t4.o %ta.so -o %t3 -save-temps --lto-whole-program-visibility \
 ; RUN:   -mllvm -pass-remarks=. \
 ; RUN:   --export-dynamic 2>&1 | FileCheck /dev/null --implicit-check-not single-impl --allow-empty
 ; RUN: llvm-dis %t3.0.4.opt.bc -o - | FileCheck %s --check-prefix=CHECK-NODEVIRT-IR