[llvm-dev] [RFC][ThinLTO] llvm-dis ThinLTO summary dump format

[David Blaikie via llvm-dev]

I know there's been a bunch of discussion here already, but I was wondering
if perhaps someone (probably Teresa? Peter?) could:

1) summarize the current state
2) describe the end-goal
3) describe what steps (& how this patch relates) are planned to get to (2)

My naive thoughts, not being intimately familiar with any of this: Usually
bitcode and textual IR support go in together or around the same time, and
designed that way from the start (take r211920 for examaple, which added an
explicit representation of COMDATs to the IR). This seems to have been an
oversight in the implementation of IR summaries (is that an accurate
representation/statement?) & now there's an effort to correct that.

So it seems like that would start with a discussion of what the right
end-state would be: What the syntax in textual IR should be, then
implementing it. I can understand implementing such a thing in steps - it's
perhaps more involved than the COMDAT situation. In that case starting on
either side seems fine - implementing the emission first (hidden behind a
flag, so as not to break round-tripping in the interim) or the parsing
first (no need to hide it behind any flags - manually written examples can
be used as input tests).

(& it sounds like there's some partially implemented functionality using a
YAML format that was intended to address how some test cases could be
written? & this might be a good basis for the syntax - but seems to me like
it might be a bit disjointed/out of place in the textual IR format that's
not otherwise YAML-based?)

- Dave

On Fri, Jun 2, 2017 at 8:46 AM Charles Saternos via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hey all,
>
> Below is the proposed format for the dump of the ThinLTO module summary in
> the llvm-dis utility:
>
> > ../build/bin/llvm-dis t.o && cat t.o.ll
> ; ModuleID = '2.o'
> source_filename = "2.ll"
> target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
> target triple = "x86_64-unknown-linux-gnu"
>
> @X = constant i32 42, section "foo", align 4
>
> @a = weak alias i32, i32* @X
>
> define void @afun() {
>   %1 = load i32, i32* @a
>   ret void
> }
>
> define void @testtest() {
>   tail call void @boop()
>   ret void
> }
>
> declare void @boop()
>
> ; Module summary:
> ;  testtest (External linkage)
> ;    Function (2 instructions)
> ;    Calls: boop
> ;  X (External linkage)
> ;    Global Variable
> ;  afun (External linkage)
> ;    Function (2 instructions)
> ;    Refs:
> ;      a
> ;  a (Weak any linkage)
> ;    Alias (aliasee X)
>
> I've implemented the above format in the llvm-dis utility, since there
> currently isn't really a way of getting ThinLTO summaries in a
> human-readable format.
>
> Let me know what you think of this format, and what information you think
> should be added/removed.
>
> Thanks,
> Charles
>
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