[llvm] r174345 - Add code to GlobalVariable.h so that global variables marked as

[Michael Gottesman]

Question: is this question answered by my linking to the extensive discussion in your comment on commit r174340?

On Feb 5, 2013, at 1:13 AM, Duncan Sands <baldrick at free.fr> wrote:

> Hi Michael,
> 
> On 05/02/13 07:53, Michael Gottesman wrote:
>> Author: mgottesman
>> Date: Tue Feb  5 00:53:26 2013
>> New Revision: 174345
>> 
>> URL: http://llvm.org/viewvc/llvm-project?rev=174345&view=rev
>> Log:
>> Add code to GlobalVariable.h so that global variables marked as
>> externally_initialized return false for hasDefiniteInitializer and
>> hasUniqueInitializer.
> 
> can you please explain who modifies the initial value, and when exactly?
> 
> Thanks, Duncan.
> 
>> 
>> rdar://12580965.
>> 
>> Added:
>>     llvm/trunk/test/Transforms/GlobalOpt/externally-initialized-global-ctr.ll
>> Modified:
>>     llvm/trunk/include/llvm/IR/GlobalVariable.h
>> 
>> Modified: llvm/trunk/include/llvm/IR/GlobalVariable.h
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/IR/GlobalVariable.h?rev=174345&r1=174344&r2=174345&view=diff
>> ==============================================================================
>> --- llvm/trunk/include/llvm/IR/GlobalVariable.h (original)
>> +++ llvm/trunk/include/llvm/IR/GlobalVariable.h Tue Feb  5 00:53:26 2013
>> @@ -110,7 +110,10 @@ public:
>>      return hasInitializer() &&
>>        // The initializer of a global variable with weak linkage may change at
>>        // link time.
>> -      !mayBeOverridden();
>> +      !mayBeOverridden() &&
>> +      // The initializer of a global variable with the externally_initialized
>> +      // marker may change at runtime before cxx initializers are evaluated.
>> +      !isExternallyInitialized();
>>    }
>> 
>>    /// hasUniqueInitializer - Whether the global variable has an initializer, and
>> @@ -123,7 +126,11 @@ public:
>>        // instead. It is wrong to modify the initializer of a global variable
>>        // with *_odr linkage because then different instances of the global may
>>        // have different initializers, breaking the One Definition Rule.
>> -      !isWeakForLinker();
>> +      !isWeakForLinker() &&
>> +      // It is not safe to modify initializers of global variables with the
>> +      // external_initializer marker since the value may be changed at runtime
>> +      // before cxx initializers are evaluated.
>> +      !isExternallyInitialized();
>>    }
>> 
>>    /// getInitializer - Return the initializer for this global variable.  It is
>> 
>> Added: llvm/trunk/test/Transforms/GlobalOpt/externally-initialized-global-ctr.ll
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/GlobalOpt/externally-initialized-global-ctr.ll?rev=174345&view=auto
>> ==============================================================================
>> --- llvm/trunk/test/Transforms/GlobalOpt/externally-initialized-global-ctr.ll (added)
>> +++ llvm/trunk/test/Transforms/GlobalOpt/externally-initialized-global-ctr.ll Tue Feb  5 00:53:26 2013
>> @@ -0,0 +1,35 @@
>> +; RUN: opt < %s -globalopt -S | FileCheck %s
>> +; rdar://12580965.
>> +; ObjC++ test case.
>> +
>> +%struct.ButtonInitData = type { i8* }
>> +
>> + at _ZL14buttonInitData = internal global [1 x %struct.ButtonInitData] zeroinitializer, align 4
>> +
>> +@"\01L_OBJC_METH_VAR_NAME_40" = internal global [7 x i8] c"print:\00", section "__TEXT,__objc_methname,cstring_literals", align 1
>> +@"\01L_OBJC_SELECTOR_REFERENCES_41" = internal externally_initialized  global i8* getelementptr inbounds ([7 x i8]* @"\01L_OBJC_METH_VAR_NAME_40", i32 0, i32 0), section "__DATA, __objc_selrefs, literal_pointers, no_dead_strip"
>> +
>> + at llvm.global_ctors = appending global [1 x { i32, void ()* }] [{ i32, void ()* } { i32 65535, void ()* @_GLOBAL__I_a }]
>> + at llvm.used = appending global [2 x i8*] [i8* getelementptr inbounds ([7 x i8]* @"\01L_OBJC_METH_VAR_NAME_40", i32 0, i32 0),  i8* bitcast (i8** @"\01L_OBJC_SELECTOR_REFERENCES_41" to i8*)]
>> +
>> +define internal void @__cxx_global_var_init() section "__TEXT,__StaticInit,regular,pure_instructions" {
>> +  %1 = load i8** @"\01L_OBJC_SELECTOR_REFERENCES_41", !invariant.load !2009
>> +  store i8* %1, i8** getelementptr inbounds ([1 x %struct.ButtonInitData]* @_ZL14buttonInitData, i32 0, i32 0, i32 0), align 4
>> +  ret void
>> +}
>> +
>> +define internal void @_GLOBAL__I_a() section "__TEXT,__StaticInit,regular,pure_instructions" {
>> +  call void @__cxx_global_var_init()
>> +  ret void
>> +}
>> +
>> +declare void @test(i8*)
>> +
>> +define void @print() {
>> +; CHECK: %1 = load i8** getelementptr inbounds ([1 x %struct.ButtonInitData]* @_ZL14buttonInitData, i32 0, i32 0, i32 0), align 4
>> +  %1 = load i8** getelementptr inbounds ([1 x %struct.ButtonInitData]* @_ZL14buttonInitData, i32 0, i32 0, i32 0), align 4
>> +  call void @test(i8* %1)
>> +  ret void
>> +}
>> +
>> +!2009 = metadata !{}
>> \ No newline at end of file
>> 
>> 
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