[WPD][ELF] Allow whole program devirtualization for version script localized symbols

A `local:` version node in a version script can change the effective symbol binding
to STB_LOCAL. The linker needs to communicate the fact to enable WPD
(otherwise LTO does not know that the `!vcall_visibility` metadata has
effectively changed from VCallVisibilityPublic to VCallVisibilityLinkageUnit).

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D98220

Author: MaskRay

lld/ELF/LTO.cpp

@@ -249,8 +249,9 @@ void BitcodeCompiler::add(BitcodeFile &f) {
                             usedStartStop.count(objSym.getSectionName());
     // Identify symbols exported dynamically, and that therefore could be
     // referenced by a shared library not visible to the linker.
-    r.ExportDynamic = sym->isExportDynamic(sym->kind(), sym->visibility) ||
-                      sym->exportDynamic || sym->inDynamicList;
+    r.ExportDynamic = sym->computeBinding() != STB_LOCAL &&
+                      (sym->isExportDynamic(sym->kind(), sym->visibility) ||
+                       sym->exportDynamic || sym->inDynamicList);
     const auto *dr = dyn_cast<Defined>(sym);
     r.FinalDefinitionInLinkageUnit =
         (isExec || sym->visibility != STV_DEFAULT) && dr &&

lld/test/ELF/lto/devirt_vcall_vis_localize.ll

@@ -0,0 +1,71 @@
+; REQUIRES: x86
+;; If the vtable symbols are localized by a version script, devirtualization
+;; can still happen.
+
+; RUN: opt --thinlto-bc -o %t1.o %s
+; RUN: opt --thinlto-bc -o %t2.o %S/Inputs/devirt_vcall_vis_shared_def.ll
+; RUN: echo '{ global: _start; local: *; };' > %t.ver
+
+; RUN: ld.lld %t1.o %t2.o -o %t.out --save-temps --lto-whole-program-visibility -shared \
+; RUN:   -mllvm -pass-remarks=. 2>&1 | count 0
+
+; RUN: ld.lld %t1.o %t2.o -o %t.out --save-temps --lto-whole-program-visibility -shared \
+; RUN:   --version-script=%t.ver -mllvm -pass-remarks=. 2>&1 | FileCheck %s --check-prefix=REMARK
+; RUN: llvm-dis < %t1.o.4.opt.bc | FileCheck %s --check-prefix=CHECK-IR
+
+; REMARK: single-impl: devirtualized a call to _ZN1A1nEi
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-grtev4-linux-gnu"
+
+%struct.A = type { i32 (...)** }
+%struct.B = type { %struct.A }
+
+@_ZTV1A = available_externally unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* undef, i8* bitcast (i32 (%struct.A*, i32)* @_ZN1A1fEi to i8*), i8* bitcast (i32 (%struct.A*, i32)* @_ZN1A1nEi to i8*)] }, !type !0, !vcall_visibility !2
+@_ZTV1B = linkonce_odr unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* undef, i8* bitcast (i32 (%struct.B*, i32)* @_ZN1B1fEi to i8*), i8* bitcast (i32 (%struct.A*, i32)* @_ZN1A1nEi to i8*)] }, !type !0, !type !1, !vcall_visibility !2
+
+;; Prevent the vtables from being dead code eliminated.
+@llvm.used = appending global [2 x i8*] [ i8* bitcast ( { [4 x i8*] }* @_ZTV1A to i8*), i8* bitcast ( { [4 x i8*] }* @_ZTV1B to i8*)]
+
+; CHECK-IR-LABEL: @_start(
+define i32 @_start(%struct.A* %obj, i32 %a) {
+entry:
+  %0 = bitcast %struct.A* %obj to i8***
+  %vtable = load i8**, i8*** %0
+  %1 = bitcast i8** %vtable to i8*
+  %p = call i1 @llvm.type.test(i8* %1, metadata !"_ZTS1A")
+  call void @llvm.assume(i1 %p)
+  %fptrptr = getelementptr i8*, i8** %vtable, i32 1
+  %2 = bitcast i8** %fptrptr to i32 (%struct.A*, i32)**
+  %fptr1 = load i32 (%struct.A*, i32)*, i32 (%struct.A*, i32)** %2, align 8
+
+  ;; Check that the call was devirtualized.
+  ; CHECK-IR: %call = tail call i32 @_ZN1A1nEi
+  %call = tail call i32 %fptr1(%struct.A* nonnull %obj, i32 %a)
+
+  ret i32 %call
+}
+; CHECK-IR-LABEL: ret i32
+; CHECK-IR-LABEL: }
+
+declare i1 @llvm.type.test(i8*, metadata)
+declare void @llvm.assume(i1)
+
+define available_externally i32 @_ZN1A1fEi(%struct.A* %this, i32 %a) #0 {
+   ret i32 0
+}
+
+define available_externally i32 @_ZN1A1nEi(%struct.A* %this, i32 %a) #0 {
+   ret i32 0
+}
+
+define linkonce_odr i32 @_ZN1B1fEi(%struct.B* %this, i32 %a) #0 {
+   ret i32 0
+}
+
+;; Make sure we don't inline or otherwise optimize out the direct calls.
+attributes #0 = { noinline optnone }
+
+!0 = !{i64 16, !"_ZTS1A"}
+!1 = !{i64 16, !"_ZTS1B"}
+!2 = !{i64 0}