Don't emit structors for available_externally globals (PR19933)

[hans]

Summary

We would previously assert here when trying to figure out the section
for the global.

This makes us handle the situation more gracefully since the IR isn't
malformed.

[rnk]

lgtm

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

1354

This should briefly explain why, with something like:
// If the associated variable is available_externally, some other TU will provide its dynamic initializer.

[hans]

Closed by commit rL210215 (authored by @hans).

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

void AsmPrinter::EmitXXStructorList(const Constant *List, bool isCtor) {
   unsigned Align = Log2_32(DL->getPointerPrefAlignment());
   std::stable_sort(Structors.begin(), Structors.end(),
                    [](const Structor &L,
                       const Structor &R) { return L.Priority < R.Priority; });
   for (Structor &S : Structors) {
     const TargetLoweringObjectFile &Obj = getObjFileLowering();
     const MCSymbol *KeySym = nullptr;
     const MCSection *KeySec = nullptr;
-    if (S.ComdatKey) {
-      KeySym = getSymbol(S.ComdatKey);
-      KeySec = getObjFileLowering().SectionForGlobal(S.ComdatKey, *Mang, TM);
+    if (GlobalValue *GV = S.ComdatKey) {
+      if (GV->hasAvailableExternallyLinkage())
+        // Don't dynamically initialize available_externally vars.
+        continue;
+
+      KeySym = getSymbol(GV);
+      KeySec = getObjFileLowering().SectionForGlobal(GV, *Mang, TM);
     }
     const MCSection *OutputSection =
         (isCtor ? Obj.getStaticCtorSection(S.Priority, KeySym, KeySec)
                 : Obj.getStaticDtorSection(S.Priority, KeySym, KeySec));
     OutStreamer.SwitchSection(OutputSection);
     if (OutStreamer.getCurrentSection() != OutStreamer.getPreviousSection())
       EmitAlignment(Align);
     EmitXXStructor(S.Func);

test/MC/COFF/global_ctors_dtors.ll

 ; Test that global ctors are emitted into the proper COFF section for the
 ; target. Mingw uses .ctors, whereas MSVC uses .CRT$XC*.
 ; RUN: llc < %s -mtriple i686-pc-win32 | FileCheck %s --check-prefix WIN32
 ; RUN: llc < %s -mtriple x86_64-pc-win32 | FileCheck %s --check-prefix WIN32
 ; RUN: llc < %s -mtriple i686-pc-mingw32 | FileCheck %s --check-prefix MINGW32
 ; RUN: llc < %s -mtriple x86_64-pc-mingw32 | FileCheck %s --check-prefix MINGW32
 
 @.str = private unnamed_addr constant [13 x i8] c"constructing\00", align 1
 @.str2 = private unnamed_addr constant [12 x i8] c"destructing\00", align 1
 @.str3 = private unnamed_addr constant [5 x i8] c"main\00", align 1
 
 %ini = type { i32, void()*, i8* }
 
[e-mail]_ctors = appending global [2 x %ini ] [
[e-mail]_ctors = appending global [3 x %ini ] [
   %ini { i32 65535, void ()* @a_global_ctor, i8* null },
-  %ini { i32 65535, void ()* @b_global_ctor, i8* bitcast (i32* @b to i8*) }
+  %ini { i32 65535, void ()* @b_global_ctor, i8* bitcast (i32* @b to i8*) },
+  %ini { i32 65535, void ()* @c_global_ctor, i8* bitcast (i32* @c to i8*) }
 ]
 @llvm.global_dtors = appending global [1 x %ini ] [%ini { i32 65535, void ()* @a_global_dtor, i8* null }]
 
 declare i32 @puts(i8*)
 
 define void @a_global_ctor() nounwind {
   %1 = call i32 @puts(i8* getelementptr inbounds ([13 x i8]* @.str, i32 0, i32 0))
   ret void
 }
 
 @b = global i32 zeroinitializer
 
+@c = available_externally dllimport global i32 zeroinitializer
+
 define void @b_global_ctor() nounwind {
   store i32 42, i32* @b
   ret void
 }
 
+define void @c_global_ctor() nounwind {
+  store i32 42, i32* @c
+  ret void
+}
+
 define void @a_global_dtor() nounwind {
   %1 = call i32 @puts(i8* getelementptr inbounds ([12 x i8]* @.str2, i32 0, i32 0))
   ret void
 }
 
 define i32 @main() nounwind {
   %1 = call i32 @puts(i8* getelementptr inbounds ([5 x i8]* @.str3, i32 0, i32 0))
   ret i32 0
 }
 
 ; WIN32: .section .CRT$XCU,"rd"
 ; WIN32: a_global_ctor
 ; WIN32: .section .CRT$XCU,"rd",associative .bss,{{_?}}b
 ; WIN32: b_global_ctor
+; WIN32-NOT: c_global_ctor
 ; WIN32: .section .CRT$XTX,"rd"
 ; WIN32: a_global_dtor
 ; MINGW32: .section .ctors,"wd"
 ; MINGW32: a_global_ctor
 ; MINGW32: .section .ctors,"wd",associative .bss,{{_?}}b
 ; MINGW32: b_global_ctor
+; MINGW32-NOT: c_global_ctor
 ; MINGW32: .section .dtors,"wd"
 ; MINGW32: a_global_dtor