[cfe-dev] clang::CodeGen::CodeGenModule::GetOrCreateLLVMGlobal

Vassil Vassilev vasil.georgiev.vasilev at cern.ch
Mon Mar 17 03:59:46 PDT 2014

On 03/11/2014 01:05 PM, Rafael Espíndola wrote:
>> I am not sure I understand the intended semantics of the UnnamedAddr argument to GetOrCreateLLVMGlobal.  Is it “this use does not care about the specific address”?  Because, if so, the (existing) logic is completely backwards: we need to be *clearing* the flag on the global if the argument is ever *false*.  And that seems like the most reasonable semantics, frankly.
>> Also, the idea in the existing code that all runtime variables are unnamed_addr seems bogus to me.  __dso_handle is a specific example of a variable whose address value *is* semantically important.  This sort of thing would be important if there were any plausible transformations at all that involve unnamed_addr on external variable declarations.
>> Also, the assertion is wrong: if something introduces a “runtime" declaration (e.g. by just declaring and using a global variable with the right name) before CodeGen emits its first intrinsic use of it, the declaration will not be marked unnamed_addr.
>> So, basically, right now, this entire code path is useless and wrong.  Is there a plan to apply it in situations where it wouldn’t be useless, i.e. to non-runtime user declarations?
> So, on how the patch is organized:
> * The structure the patch uses is that when the variable is first
> created, its unnamed_addr is set to the value of the argument.
> * We then check that no user expect that a variable is unnamed_addr
> when in fact it isn't. I tried to make the check more strict by
> asserting that Entry->getUnnamedAddr() == UnnamedAddr, but that fails
> deep down in some objc cases that create runtime variables and access
> them as regular ones.
> But to the more important point: Should we be adding unnamed_addr in
> the first place? I am not sure. Back in
> http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20110110/037796.html
> Chris asked if we should. I could not find the entire discussion (the
> patch that ended up being committed is simpler) and I am still not
> sure exactly when CreateRuntimeVariable should be used.
> An alternative patch that simply never sets it in
> CreateRuntimeVariable is attached.
> Cheers,
> Rafael

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