[llvm-dev] Unify debug and optimized variable locations with llvm.dbg.addr [was: DW_OP_LLVM_memory]

Adrian Prantl via llvm-dev llvm-dev at lists.llvm.org
Fri Sep 8 10:32:56 PDT 2017

For my understanding,

based on this:

> On Sep 7, 2017, at 2:00 PM, Reid Kleckner <rnk at google.com> wrote:
>> On Thu, Sep 7, 2017 at 12:01 PM, Adrian Prantl <aprantl at apple.com> wrote:
>> What exactly is allowed as the first parameter of a dbg.addr? Only allocas? Anything else?
> Any pointer value. Maybe we could allow pointer-sized integers, but that anything else is probably indescribable. If the base pointer is in XMM0, how would we describe that with DWARF?

and this:

> On Sep 7, 2017, at 2:18 PM, Reid Kleckner <rnk at google.com> wrote:
> On Thu, Sep 7, 2017 at 11:11 AM, Robinson, Paul <paul.robinson at sony.com> wrote:
>> Different intrinsics sounds like a good solution to me. J
>> So what happens with the case where a variable is registerized but later we decide to spill it?  Presumably we'd have a dbg.addr to point to the spill slot.  In past compilers I've used, spill slots were treated analogous to register allocation, i.e. some effort was made to minimize the number of spill slots and a variable might be spilled to different slots at different points.  If LLVM does that, then dbg.addr will have to be allowed to associated different addresses with the variable.  On the other hand, if LLVM allocates a unique memory "home" for each spilled variable, then dbg.addr can retain the property you suggest, that the address expression is always the same.
> dbg.addr is really IR only. Machine DBG_VALUE instructions can already represent addresses or values depending on their second argument. At this point, I don't see any reason to change that.

If we can write a verifier to check the validity of a dbg.addr's address, why do we need the separate intrinsic? I guess the answer is that while every address must be a pointer value, not every pointer value is an address. Is this correct?

-- adrian

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