[LLVMdev] RFC: Metadata attachments to function definitions

[Ivan Baev]

>
>> On 2015-Apr-15, at 08:11, Ivan Baev <ibaev at codeaurora.org> wrote:
>>
>>> Date: Tue, 14 Apr 2015 21:33:03 -0700
>>> From: "Duncan P. N. Exon Smith" <dexonsmith at apple.com>
>>> To: LLVM Developers Mailing List <llvmdev at cs.uiuc.edu>
>>> Subject: [LLVMdev] RFC: Metadata attachments to function definitions
>>> Message-ID: <BF4002F0-06DC-4A25-AF84-7D21AD48121A at apple.com>
>>> Content-Type: text/plain; charset=us-ascii
>>>
>>>
>>> `Function` definitions should support `MDNode` attachments, with a
>>> similar syntax to instructions:
>>>
>>>    define void @foo() nounwind !attach !0 {
>>>      unreachable
>>>    }
>>>    !0 = !{}
>>>
>>> Attachments wouldn't be allowed on declarations, just definitions.
>>>
>>> There are two open problems this can help with:
>>>
>>> 1. For PGO, we need somewhere to attach the function entry count.
>>>    Attaching to the function definition is a simple solution.
>>>
>>>        define void @foo() !prof !0 {
>>>          unreachable
>>>        }
>>>        !0 = !{i32 987}
>>
>>
>> That will be great. Would it be better if the type is i64? We really
>> want
>> to avoid an overflow if possible. What is the type for the raw profile
>> data for region(0)?
>
> Right, of course; `i64` makes more sense for entry counts.  BTW, I'm not
> volunteering to implement this one myself, just to provide the metadata
> infrastructure.
>

Providing the metadata infrastructure - API for accessing the function
entry count profile metadata - will be more than enough. Thank you for
doing that.

The type in clang/lib/CodeGen/CodeGenPGO.h is a 64 bit:
  uint64_t getRegionCount(unsigned Counter)

Ivan