Add some dllexport tests

[wristow]

Summary

Adds some dllexport tests to verify that:

• Variables in bss are exported appropriately
• Non-dllexport symbols aliased to dllexport symbols are not exported
• Symbols declared as dllexport but are not defined are not exported

We plan to enable dllimport/dllexport support for the PS4, and these
additional tests are for points we noticed in our internal testing.

[silvas]

This would be easier to review as separate patches.

[silvas]

(by that I mean that I can't really review it without it being separate patches with clear explanations; Reid or other people knowledgeable about this stuff can probably make a judgment at a glance)

[wristow]

I'll explain, but maybe Reid or someone else will jump in.

The three items being added, and explanations, are:

• Variables in bss are exported appropriately
  
  Added a definition of an exported new variable WeakVar3, which is identical to the existing (exported) variable WeakVar1, except for the value it is initialized to. WeakVar3 is initialized to 0, making it a candidate for the bss section (WeakVar1 was non-zero, specifically 1, so it could not be in bss). Previously, there weren't any bss variables that were exported, and with this new test, there is one. So we're making sure the dllexport attribute for a variable in bss is handled, by checking that WeakVar3 is in .bss, and that WeakVar3 appears in the export table.

• Non-dllexport symbols aliased to dllexport symbols are not exported
  
  The function f1() is exported, and an alias for f1 has been defined, where the aliasing symbol is _not_ exported, specifically aliasNotExported. The test makes sure that aliasNotExported doesn't appear in the export table. I.e., it makes sure that the dllexport attribute from f1 doesn't incorrectly "propagate through" to aliasNotExported. (Note that the dual of this test already existed, in that there is an unexported function, notExported(), and an alias for notExported was defined (specifically alias3), and alias3 _is_ exported. The test checks that alias3 appears in the export table. With the proposed change, we will also check the other way around.)

• Symbols declared as dllexport but are not defined are not exported
  
  The variable exportedButNotDefinedVariable has been added, and declared as dllexport (and it is used), but no defition of the variable appears. Similarly, the function exportedButNotDefinedFunction() has been declared as dllexport (and used), but there isn't a definition. Even though they are declared as dllexport, since there is no definition, they cannot be exported. So they should not appear in the export table. The test checks that they don't appear in the table.

[rnk]

These are all reasonable test changes to tighten things up. Sean, do you mind applying the patch?

llvm/trunk/test/CodeGen/X86/dllexport-x86_64.ll

 @Var3 = common dllexport global i32 0, align 4
 
 ; CHECK: .globl WeakVar1
 @WeakVar1 = weak_odr dllexport global i32 1, align 4
 
 ; CHECK: .globl WeakVar2
 @WeakVar2 = weak_odr dllexport unnamed_addr constant i32 1
 
+; CHECK: .bss
+; CHECK: .globl WeakVar3
+@WeakVar3 = weak_odr dllexport global i32 0, align 4
+
 
 ; CHECK: .globl alias
 ; CHECK: alias = notExported
 @alias = dllexport alias void(), void()* @notExported
 
+; CHECK: .globl aliasNotExported
+; CHECK: aliasNotExported = f1
+@aliasNotExported = alias void(), void()* @f1
+
 ; CHECK: .globl alias2
 ; CHECK: alias2 = f1
 @alias2 = dllexport alias void(), void()* @f1
 
 ; CHECK: .globl alias3
 ; CHECK: alias3 = notExported
 @alias3 = dllexport alias void(), void()* @notExported
 
 ; CHECK: .weak weak_alias
 ; CHECK: weak_alias = f1
 @weak_alias = weak_odr dllexport alias void(), void()* @f1
 
 @blob = global [6 x i8] c"\B8*\00\00\00\C3", section ".text", align 16
 @blob_alias = dllexport alias i32 (), bitcast ([6 x i8]* @blob to i32 ()*)
 
-; Verify item that should not be exported does not appear in the export table.
+@exportedButNotDefinedVariable = external dllexport global i32
+declare dllexport void @exportedButNotDefinedFunction()
+define void @foo() {
+entry:
+  store i32 4, i32* @exportedButNotDefinedVariable, align 4
+  call void @exportedButNotDefinedFunction()
+  ret void
+}
+
+; Verify items that should not be exported do not appear in the export table.
 ; We use a separate check prefix to avoid confusion between -NOT and -SAME.
 ; NOTEXPORTED: .section .drectve
 ; NOTEXPORTED-NOT: notExported
+; NOTEXPORTED-NOT: aliasNotExported
+; NOTEXPORTED-NOT: exportedButNotDefinedVariable
+; NOTEXPORTED-NOT: exportedButNotDefinedFunction
 
 ; CHECK: .section .drectve
 ; WIN32: /EXPORT:f1
 ; WIN32-SAME: /EXPORT:f2
 ; WIN32-SAME: /EXPORT:lnk1
 ; WIN32-SAME: /EXPORT:lnk2
 ; WIN32-SAME: /EXPORT:weak1
 ; WIN32-SAME: /EXPORT:Var1,DATA
 ; WIN32-SAME: /EXPORT:Var2,DATA
 ; WIN32-SAME: /EXPORT:Var3,DATA
 ; WIN32-SAME: /EXPORT:WeakVar1,DATA
 ; WIN32-SAME: /EXPORT:WeakVar2,DATA
+; WIN32-SAME: /EXPORT:WeakVar3,DATA
 ; WIN32-SAME: /EXPORT:alias
 ; WIN32-SAME: /EXPORT:alias2
 ; WIN32-SAME: /EXPORT:alias3
 ; WIN32-SAME: /EXPORT:weak_alias
 ; WIN32-SAME: /EXPORT:blob_alias
 ; MINGW: -export:f1
 ; MINGW-SAME: -export:f2
 ; MINGW-SAME: -export:lnk1
 ; MINGW-SAME: -export:lnk2
 ; MINGW-SAME: -export:weak1
 ; MINGW-SAME: -export:Var1,data
 ; MINGW-SAME: -export:Var2,data
 ; MINGW-SAME: -export:Var3,data
 ; MINGW-SAME: -export:WeakVar1,data
 ; MINGW-SAME: -export:WeakVar2,data
+; MINGW-SAME: -export:WeakVar3,data
 ; MINGW-SAME: -export:alias
 ; MINGW-SAME: -export:alias2
 ; MINGW-SAME: -export:alias3
 ; MINGW-SAME: -export:weak_alias
 ; MINGW-SAME: -export:blob_alias"