[llvm-dev] RFC: metadata attachments for global variables

Tue May 10 16:55:48 PDT 2016

> On May 10, 2016, at 3:28 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:
> 
> 
> 
> On Tue, May 10, 2016 at 3:03 PM, Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
> 
>> On May 9, 2016, at 4:42 PM, Peter Collingbourne via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> 
>> 
>> On Mon, May 9, 2016 at 4:26 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>> 
>> 
>> On Mon, May 9, 2016 at 3:38 PM, Peter Collingbourne <peter at pcc.me.uk <mailto:peter at pcc.me.uk>> wrote:
>> 
>> 
>> On Mon, May 9, 2016 at 3:17 PM, Peter Collingbourne <peter at pcc.me.uk <mailto:peter at pcc.me.uk>> wrote:
>> 
>> 
>> On Mon, May 9, 2016 at 2:33 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>> 
>> 
>> On Mon, May 9, 2016 at 1:53 PM, Peter Collingbourne <peter at pcc.me.uk <mailto:peter at pcc.me.uk>> wrote:
>> 
>> 
>> On Fri, May 6, 2016 at 2:10 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>> 
>> 
>> On Fri, May 6, 2016 at 2:09 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>> 
>> 
>> On Fri, May 6, 2016 at 1:55 PM, Peter Collingbourne via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> 
>> On Fri, May 6, 2016 at 1:21 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com <mailto:dexonsmith at apple.com>> wrote:
>> 
>> > On 2016-May-06, at 13:17, Peter Collingbourne <peter at pcc.me.uk <mailto:peter at pcc.me.uk>> wrote:
>> >
>> > Hi all,
>> >
>> > I'd like to add support for metadata attachments for global variables in the same way as we did for functions.
>> >
>> > Syntax would be pretty simple:
>> > @foo = global i32 0, !foo !0, !bar !1
>> > (the extra commas are required to disambiguate from a named metadata on the next line)
>> 
>> SGTM.
>> 
>> > Benefits:
>> > 1) Lets us reverse the DIGlobalVariable -> GlobalVariable edge, which should hopefully clear the way for removing the llvm.dbg.cu <http://llvm.dbg.cu/> named metadata node.
>> 
>> A little harder than it sounds (need to somehow support a GlobalVariable that is RAUW'ed with a ConstantInt), but I think this is important to do.
>> 
>> How can a GlobalVariable be replaced with a ConstantInt? Wouldn't that just be an absolute address?
>> 
>> If the global variable has a known constant value & the address is never needed, then we might optimize away the storage and still want debug info describing the constant. 
>> 
>> I'm not sure under what conditions, if any, that currently works - so this may be more of a "nice to have" or "be good to think about how any future design wouldn't preclude fixing this gap" sort of thing.
>> 
>> It looks like this is how debug info for static data members used to look until r172591 landed back in 2013.
>> 
>> Could you explain more what you mean by "<this> is how it worked until" - 'this' being how it used to work and how you're proposing to make it work? I'm not quite following.
>> 
>> Sorry, I meant that prior to r172591 we would represent a static data member with something that looked like what a DIGlobalVariable looks like today, but with a ConstantInt representing the initializer in the variable field (which is where the GlobalVariable is normally stored). The change moved the initializer to an associated "static member type" (yes, it's not really a type), which as a side effect caused us to (as far as I can tell) no longer store ConstantInts in the variable field.
>> 
>> Actually this isn't accurate, we still produce a ConstantInt in the variable field of DIGlobalVariable for constants in the global scope (e.g. test/CodeGen/2010-08-10-DbgConstant.c in clang). I'll have to think more about what the right representation is for that then.
>> 
>> Looks like this might be broken/have regressed at some point:
>> 
>> static const int x = 3;
>> void f1(int);
>> void f2(int*);
>> void f3() {
>>   f1(x);
>>   f2(&x);
>> }
>> 
>> Now we generate two global variables named 'x', one with a constant value, the other with an address.
>> 
>> We probably only want the one with the address. (but, then again, we probably want to turn the one with an address to having a constant value if its storage gets optimized away)
>> 
>> Thanks David. I was thinking about this representational change for DIGlobalVariable:
>> 
>> 1) replace the Variable field with a Value field of type ConstantData, and use it exclusively for constant initializers
> 
> I would prefer a DIExpression field for holding the constant value. Besides it being generally useful; if we use a constant, GlobalOpts will believe the constant to be dead and remove it.
> Such as the problem with ASAN GEPs outlined in http://reviews.llvm.org/D18109 <http://reviews.llvm.org/D18109>.
> 
> Would that work though? Unless I'm misunderstanding something, the DIExpression would appear as a DW_AT_location attribute on the global variable, which would tell the debugger how to calculate its address. But if the global has no address we would instead need a DW_AT_const_value attribute to represent the value, wouldn't we?

What I’m envisioning is that we would have a DIExpression(DW_OP_constu, <the constant value>, DW_OP_stack_value), which we then could either directly emit as DWARF 5 or translate into a DW_AT_const_value in the Dwarf backend for better compatibility and more efficient encoding. This way we don’t need to fear the optimization passes, and get to use complex expressions on global variables.

-- adrian

> 
> Peter
> 
> -- adrian
> 
>> 2) change the logic in the debug info emitter (DwarfCompileUnit::getOrCreateGlobalVariableDIE) to look like this:
>> 
>> if (the DIGlobalVariable was attached to a GlobalVariable) {
>>   // add a location to the variable DIE
>> } else if (the DIGlobalVariable has an associated value) {
>>   // add a constant to the variable DIE
>> }
>> 
>> It seems that this way we would be able to do "emit an address, or a constant if optimized away" if we modify clang to emit a single DIGlobalVariable for constant globals with an appropriate initializer, and add it to the list of globals in the DICompileUnit.
>> 
>> Thanks,
>> -- 
>> -- 
>> Peter
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> 
> 
> 
> -- 
> -- 
> Peter

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