[PATCH] D26769: [IR] Remove the DIExpression field from DIGlobalVariable.

[Duncan P. N. Exon Smith via llvm-commits]

No comment on whether we need an offset/etc.; but assuming we do want that some day, it might be more efficient to add an offset field to DIGlobalVariableExpr:

DIGlobalVariableExpr(value: DW_OP_const 42, offset: 32 bits) -> DIGlobalVariable(name: foo)


> On 2016-Nov-18, at 09:51, David Blaikie <dblaikie at gmail.com> wrote:
> 
> Would it be reasonable/useful to add another level of indirection while we're here - that would describe which part of the variable is being described by this location:
> 
> DIGlobalVariableExpr(value: DW_OP_const 42) -> DIGlobalVariableFragment(offset: 32 bits) -> DIGlobalVariable(name: foo)
> 
> (just guessing/pretending there's a DW_OP_const for constant values - I forget how it all works, but purely for demonstration purposes)
> 
> That way code manipulating DIGlobalVariableExprs wouldn't need to have explicit knowledge of which part of the variable is being described here. (I suppose that could end up with us creating a bottom-up expression tree in metadata, which probably isn't ideal (too heavyweight))
> 
> Perhaps keeping the bit_piece parts out of the DWARF expression bytes and in a separate field of the DIGlobalVariableExpr?
> 
> DIGlobalVariableExpr(offset: 32, value: DW_OP_const 42)
> 
> Then at least when a variable moves around (does that happen? maybe? - I suppose something like asan, smooshing everything into a single alloca would move something around without splitting it up) the expression can be updated relatively simply without unpacking through a DW_OP_bit_piece, etc?
> 
> Maybe this doesn't matter - Ih aven't looked at much of the expression/location handling code.
> 
> On Fri, Nov 18, 2016 at 9:32 AM Adrian Prantl <aprantl at apple.com> wrote:
> aprantl removed rL LLVM as the repository for this revision.
> aprantl updated this revision to Diff 78544.
> aprantl added a comment.
> 
> Improved type-safety by adding a DIGlobalVarExpr pointer union.
> 
> 
> https://reviews.llvm.org/D26769
> 
> Files:
>   include/llvm/Bitcode/LLVMBitCodes.h
>   include/llvm/IR/DIBuilder.h
>   include/llvm/IR/DebugInfo.h
>   include/llvm/IR/DebugInfoMetadata.h
>   include/llvm/IR/GlobalVariable.h
>   include/llvm/IR/Metadata.def
>   lib/Analysis/ModuleDebugInfoPrinter.cpp
>   lib/AsmParser/LLParser.cpp
>   lib/Bitcode/Reader/BitcodeReader.cpp
>   lib/Bitcode/Writer/BitcodeWriter.cpp
>   lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
>   lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
>   lib/CodeGen/AsmPrinter/DwarfCompileUnit.h
>   lib/CodeGen/AsmPrinter/DwarfDebug.cpp
>   lib/IR/AsmWriter.cpp
>   lib/IR/DIBuilder.cpp
>   lib/IR/DebugInfo.cpp
>   lib/IR/DebugInfoMetadata.cpp
>   lib/IR/LLVMContextImpl.h
>   lib/IR/Metadata.cpp
>   lib/IR/Verifier.cpp
>   lib/Transforms/IPO/StripSymbols.cpp
>   lib/Transforms/Instrumentation/AddressSanitizer.cpp
>   test/Assembler/diglobalvariable.ll
>   test/Assembler/diglobalvariableexpression.ll
>   test/Bitcode/DIGlobalVariableExpr.ll
>   test/Bitcode/DIGlobalVariableExpr.ll.bc
>   test/Bitcode/diglobalvariable-3.8.ll
>   test/Bitcode/diglobalvariable-3.8.ll.bc
>   test/DebugInfo/X86/multiple-at-const-val.ll
>   test/DebugInfo/X86/pr12831.ll
>   test/DebugInfo/X86/split-global.ll
>   test/DebugInfo/X86/stack-value-dwarf4.ll
>   test/DebugInfo/X86/unattached-global.ll
>   test/Transforms/GlobalMerge/debug-info.ll
>   unittests/IR/MetadataTest.cpp
>