Allow common linkage global var have non-zero initializer

[yaxunl]

Summary

In C99 tentative definition of file-scope variable results in common linkage, e.g.

private char *p;
private char *p;

which are initialized by default initializer. For pointer type global variable, the default initializer is null pointer, which does not necessarily have zero value. If a structure contains pointer type member for which null pointer does not have zero value, the strucutre cannot be initialized with zeroinitializer.

Currently IR Verifier asserts when a global variable with common linkage has an initializer which is not zeroinitializer, which it should not. This patch fixes that.

[yaxunl]

This is not right solution since global variable with common linkage does not have explicit section. Use weak linkage for global variables with non-zero initializer instead.

lib/IR/Verifier.cpp

     // If the global has common linkage, it must have a zero initializer and
     // cannot be constant.
     if (GV.hasCommonLinkage()) {
-      Assert(GV.getInitializer()->isNullValue(),
-             "'common' global must have a zero initializer!", &GV);
       Assert(!GV.isConstant(), "'common' global may not be marked constant!",
              &GV);
       Assert(!GV.hasComdat(), "'common' global may not be in a Comdat!", &GV);

test/Verifier/common-linkage-initializer.ll

+; REQUIRES: asserts
+; RUN: llvm-as < %s -o /dev/null 2>&1
+;
+; Test the IR verifier does not assert when a global variable with common linkage
+; has an initializer which is not zeroinitializer.
+;
+; In C99 tentative definition of file-scope variable results in common linkage,
+; e.g.
+;   private char *p;
+;   private char *p;
+; which are initialized by default initializer. For pointer type global variable,
+; the default initializer is null pointer, which does not necessarily have zero
+; value. If a structure contains pointer type member for which null pointer does
+; not have zero value, the strucutre cannot be initialized with zeroinitializer.
+ 
+target datalayout = "e-p:32:32-p1:64:64-p2:64:64-p3:32:32-p4:64:64-p5:32:32-i64:64-v16:16-v24:32-v32:32-v48:64-v96:128-v192:256-v256:256-v512:512-v1024:1024-v2048:2048-n32:64"
+target triple = "amdgcn"
+
+%struct.ST = type <{ i8*, i8 addrspace(1)*, i8 addrspace(2)*, i8 addrspace(3)*, i8 addrspace(4)* }>
+
+@st = common addrspace(1) global %struct.ST <{ i8* addrspacecast (i8 addrspace(4)* null to i8*), i8 addrspace(1)* null, i8 addrspace(2)* null, i8 addrspace(3)* addrspacecast (i8 addrspace(4)* null to i8 addrspace(3)*), i8 addrspace(4)* null }>, align 4