[PATCH] D20348: IR: Introduce local_unnamed_addr attribute.

Tue May 17 18:40:49 PDT 2016

On Tue, May 17, 2016 at 6:07 PM, Chandler Carruth <chandlerc at gmail.com>
wrote:

> Thanks for the detailed write-up, and sorry to Rafael and Mehdi that it's
> on a new thread. =/
>
> On Tue, May 17, 2016 at 5:59 PM Peter Collingbourne via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
>> pcc created this revision.
>> pcc added reviewers: rafael, joker.eph, chandlerc, majnemer.
>> pcc added a subscriber: llvm-commits.
>> Herald added a reviewer: tstellarAMD.
>> Herald added subscribers: jfb, mzolotukhin, joker.eph, arsenm.
>>
>> If a local_unnamed_addr attribute is attached to a global, the address
>> is known to be insignificant within the module. It is distinct from the
>> existing unnamed_addr attribute in that it only describes a local property
>> of the module rather than a global property of the symbol.
>>
>> This attribute is intended to be used by the code generator and LTO to
>> allow
>> the linker to decide whether the global needs to be in the symbol table.
>> It is
>> possible to exclude a global from the symbol table if three things are
>> true:
>> - This attribute is present on every instance of the global (which means
>> that
>>   the normal rule that the global must have a unique address can be
>> broken without
>>   being observable by the program by performing comparisons against the
>> global's
>>   address)
>> - The global has linkonce_odr linkage (which means that each linkage unit
>> must have
>>   its own copy of the global if it requires one, and the copy in each
>> linkage unit
>>   must be the same)
>> - It is a constant or a function (which means that the program cannot
>> observe that
>>   the unique-address rule has been broken by writing to the global)
>>
>> Although this attribute could in principle be computed from the module
>> contents, LTO clients (i.e. linkers) will normally need to be able to
>> compute
>> this property as part of symbol resolution, and it would be inefficient to
>> materialize every module just to compute it.
>>
>
> Cool, this last part is really key.
>
>
> My real problem with adding this as a normal attribute is that I'm not
> sure what it really means. Is it just a "cache" of some local analysis? Do
> we expect things to invalidate it if they make the address significant
> within a module? Is this something that would be "blessed" by some
> frontends?
>
> I feel like, from your description, this really is just intended to solve
> the problem of materializing all of the module. It would seem that for that
> purpose something more akin to the "summary" information used by ThinLTO
> would be a better tool than an attribute which has to have a semantic
> contract for the IR.
>
> What do you think?
>

To a certain extent this is a summary of the module contents, and in most
cases I'd expect the property to simply summarize the module.

However, it is also a property that should be preserved if a pass
introduces an address comparison. Modulo bugs, the new comparison should be
"benign" -- if the original program could not observe the address, the
optimized program shouldn't be able to observe it either.

One example of this would be a comparison of a vptr against a vtable or
function address for speculative devirtualization, or in general any form
of PGO that relies on global addresses. The introduction of such a
comparison doesn't invalidate the unnamed_addr property (which we do
currently apply to vtables), as the program would still have the same
semantics if we, say, merged two identical vtables. The same applies to the
local_unnamed_addr property.

 Regarding frontends, yes, I'd expect that if a frontend knows that all
address comparisons within a module are benign, it could apply this
property.

Thanks,
Peter

>
>>
>> See:
>>
>> http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160509/356401.html
>>
>> http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160516/356738.html
>> for earlier discussion.
>>
>> Part of the fix for PR27553.
>>
>> http://reviews.llvm.org/D20348
>>
>> Files:
>>   docs/BitCodeFormat.rst
>>   docs/LangRef.rst
>>   include/llvm/IR/GlobalValue.h
>>   lib/Analysis/InstructionSimplify.cpp
>>   lib/AsmParser/LLLexer.cpp
>>   lib/AsmParser/LLParser.cpp
>>   lib/AsmParser/LLParser.h
>>   lib/AsmParser/LLToken.h
>>   lib/Bitcode/Reader/BitcodeReader.cpp
>>   lib/Bitcode/Writer/BitcodeWriter.cpp
>>   lib/CodeGen/Analysis.cpp
>>   lib/CodeGen/AsmPrinter/AsmPrinter.cpp
>>   lib/CodeGen/TargetLoweringObjectFileImpl.cpp
>>   lib/ExecutionEngine/Orc/IndirectionUtils.cpp
>>   lib/IR/AsmWriter.cpp
>>   lib/IR/Core.cpp
>>   lib/IR/Globals.cpp
>>   lib/IR/IRBuilder.cpp
>>   lib/IR/Verifier.cpp
>>   lib/Linker/IRMover.cpp
>>   lib/Linker/LinkModules.cpp
>>   lib/Target/AMDGPU/AMDGPUPromoteAlloca.cpp
>>   lib/Target/TargetLoweringObjectFile.cpp
>>   lib/Transforms/IPO/ConstantMerge.cpp
>>   lib/Transforms/IPO/GlobalOpt.cpp
>>   lib/Transforms/IPO/MergeFunctions.cpp
>>   lib/Transforms/Instrumentation/AddressSanitizer.cpp
>>   lib/Transforms/Instrumentation/GCOVProfiling.cpp
>>   lib/Transforms/Instrumentation/InstrProfiling.cpp
>>   lib/Transforms/Scalar/LoopIdiomRecognize.cpp
>>   lib/Transforms/Utils/FunctionImportUtils.cpp
>>   lib/Transforms/Utils/GlobalStatus.cpp
>>   lib/Transforms/Utils/SimplifyCFG.cpp
>>   test/Assembler/local-unnamed-addr.ll
>>   test/Bitcode/compatibility.ll
>>   test/CodeGen/PowerPC/weak_def_can_be_hidden.ll
>>   test/CodeGen/X86/weak_def_can_be_hidden.ll
>>   test/Feature/OperandBundles/pr26510.ll
>>   test/LTO/X86/cfi_endproc.ll
>>   test/LTO/X86/linkonce_odr_func.ll
>>   test/Other/constant-fold-gep.ll
>>   test/Transforms/GlobalOpt/2010-10-19-WeakOdr.ll
>>   test/Transforms/GlobalOpt/alias-used-address-space.ll
>>   test/Transforms/GlobalOpt/alias-used.ll
>>   test/Transforms/GlobalOpt/assume.ll
>>   test/Transforms/GlobalOpt/constantfold-initializers.ll
>>   test/Transforms/GlobalOpt/ctor-list-opt-inbounds.ll
>>   test/Transforms/GlobalOpt/invariant.group.barrier.ll
>>   test/Transforms/GlobalOpt/invoke.ll
>>   test/Transforms/GlobalOpt/pr21191.ll
>>   test/Transforms/GlobalOpt/unnamed-addr.ll
>>   test/tools/gold/X86/coff.ll
>>   test/tools/gold/X86/emit-llvm.ll
>>   tools/gold/gold-plugin.cpp
>>
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>>
>

-- 
-- 
Peter
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