[PATCH] Promote GlobalMerge pass to a ModulePass to ease reuse

[Quentin Colombet]

Ping?

-Quentin

On Apr 19, 2013, at 11:08 AM, Quentin Colombet <qcolombet at apple.com> wrote:

> Hi Eric,
> 
> I’ve looked closer on the debug information were generated for the merged variables.
> It appeared that at the moment GlobalMerge has to perform its modifications in doInitialization, because it relies on how the global variables will be interpreted in DwarfDebug::beginModule which is called from the doInitialization of the AsmPrinter pass.
> More specifically, CompileUnit::createGlobalVariableDIE has a special case for MergedGlobals:
> […]
> if (const ConstantExpr *CE = getMergedGlobalExpr(N->getOperand(11)))
> 
> After a (very) quick look to the debug interfaces, I didn’t see a way to reproduce this behavior a posteriori.
> 
> Any ideas?
> 
> Thanks,
> 
> -Quentin
> 
> On Apr 17, 2013, at 9:31 AM, Quentin Colombet <qcolombet at apple.com> wrote:
> 
>> Thanks Eric.
>> 
>> Do you think it would be possible/desirable to fix the debug information within GlobalMerge?
>> 
>> Currently GlobalMerge pass is added to the pass manager in code that is ARM specific, thus, I don’t have much latitude to move it. In particular, if I move it at the earliest I can in target specific code (i.e., using addIRPasses) I didn’t reproduce the old behavior.
>> 
>> -Quentin
>> 
>> On Apr 16, 2013, at 7:18 PM, Eric Christopher <echristo at gmail.com> wrote:
>> 
>>> On Tue, Apr 16, 2013 at 5:51 PM, Jim Grosbach <grosbach at apple.com> wrote:
>>>> 
>>>> On Apr 16, 2013, at 5:48 PM, Quentin Colombet <qcolombet at apple.com> wrote:
>>>> 
>>>> Oh, that’s true the pass is not run where it was running.
>>>> Thanks Jim.
>>>> 
>>>> I’ll have to check if we can reproduce the old behavior since the pass may
>>>> rely on running before the doInitialization of another pass.
>>>> 
>>>> Regarding the test cases, the debug information have changed.
>>>> MergedGlobals is no more referenced in the debug information. I’m fine with
>>>> that since it was not in the program initially (we give insight on something
>>>> the developer never wrote!), but it seems to be the expected behavior.
>>>> 
>>>> 
>>>> Interesting. Well, it’s entirely possible that expected != correct here. If
>>>> we’re referring back (correctly, of course) to the user’s variables and not
>>>> referencing the merged globals variable at all, that seems an improvement to
>>>> me. Eric, any thoughts on that?
>>>> 
>>> 
>>> The new code is, unfortunately, incorrect. The addresses of the
>>> various objects are relocations for undefined symbols in the output
>>> _x1, _x2, etc rather than offsets off of the new _MergedGlobal.
>>> Basically those static local symbols no longer exist and all uses of
>>> them in the program have been replaced.
>>> 
>>> -eric
>>> 
>>> 
>>>> -Jim
>>>> 
>>>> 
>>>> -Quentin
>>>> 
>>>> On Apr 16, 2013, at 5:31 PM, Jim Grosbach <grosbach at apple.com> wrote:
>>>> 
>>>> Hi Quentin,
>>>> 
>>>> Hmmmm…. Perhaps we should consider whether to move the pass to where it was
>>>> actually really running before. We’ll want to consider whether the old
>>>> behavior was a bug, though, and it really belongs where it’s actually
>>>> running now (later). A quick glance suggests that the test case may just
>>>> need to be updated (i.e., the new order is fine, just different). We should
>>>> understand a bit more of what’s going on before assuming that, though.
>>>> 
>>>> -Jim
>>>> 
>>>> On Apr 16, 2013, at 4:27 PM, Quentin Colombet <qcolombet at apple.com> wrote:
>>>> 
>>>> Hi,
>>>> 
>>>> I’ve noticed that all the work performed by the GlobalMerge pass is done
>>>> within the doInitialization method.
>>>> It’s unclear to me if it is a valid design or not, because according to
>>>> http://llvm.org/docs/WritingAnLLVMPass.html#the-functionpass-class:
>>>> - On one hand:
>>>> FunctionPass subclasses are not allowed to:
>>>> […]
>>>> 3. Add or remove global variables from the current Module.
>>>> - On the other hand:
>>>> The doInitialization method is allowed to do most of the things that
>>>> FunctionPasses are not allowed to do.
>>>> 
>>>> What is clear however is: This design prevents other passes to take
>>>> advantage of this pass. Let’s say I write a pass A that creates some global
>>>> variables and I’d like to see these variables merged. Thus, I’ll add
>>>> GlobalMerge pass after A. This does not work unless I push all the work of
>>>> pass A into doInitialization…
>>>> 
>>>> The proposed patch turns GlobalMerge into a ModulePass and move all its work
>>>> in runOnModule method.
>>>> Although this patch does not intend to make any functionality change it
>>>> seems that the phase ordering implied by this design was expected!
>>>> 
>>>> Indeed, with this patch I have two make check failures related to debug
>>>> info:
>>>> test/CodeGen/ARM/2011-01-19-MergedGlobalDbg.ll:26:14: error: expected string
>>>> not found in input
>>>> ;CHECK-NEXT: .long __MergedGlobals
>>>>             ^
>>>> <stdin>:233:2: note: scanning from here
>>>> .long _x2
>>>> ^
>>>> --
>>>> and
>>>> test/CodeGen/ARM/2011-08-02-MergedGlobalDbg.ll:17:14: error: expected string
>>>> not found in input
>>>> ;CHECK-NEXT: .long __MergedGlobals
>>>>             ^
>>>> <stdin>:205:2: note: scanning from here
>>>> .long _x2
>>>> ^
>>>> —
>>>> 
>>>> Any ideas on how I can fix those?
>>>> 
>>>> -Quentin
>>>> <GlobalMergeToModulePass.patch>
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