[llvm-dev] getting nowhere with thinLTO

Fri Nov 10 12:09:54 PST 2017

Hi Alan,

I am seeing the inline of t3 into t2 after it is imported. Look at the
post-opt bitcode file generated by -thinlto-save-temps:

llvm-dis -o - .0.4.opt.bc
; ModuleID = '.0.4.opt.bc'
source_filename = "t2.c"
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

; Function Attrs: norecurse nounwind readnone uwtable
define i32 @t2() local_unnamed_addr #0 {
entry:
  ret i32 1
}
...

Also, if you do -debug-only=inline instead of -debug-only=function-import,
you will see:

llvm-lto t2.o t3.o --exported-symbol=t2 --thinlto-action=run
-debug-only=inline -thinlto-save-temps=.Inliner visiting SCC: t3: 0 call
sites.
Inliner visiting SCC: t3: 0 call sites.
Inliner visiting SCC: INDIRECTNODE: 0 call sites.
Inliner visiting SCC: t2: 1 call sites.
    Inlining: cost=-30, thres=337, Call:   %call = tail call i32 @t3() #2
Inliner visiting SCC: INDIRECTNODE: 0 call sites.

Are you seeing something different?
Teresa

On Fri, Nov 10, 2017 at 9:12 AM, Davis, Alan <adavis at ti.com> wrote:

> Teresa, thanks, that’s helpful.
>
>
>
> I discovered the resolution-based API and am now converting to that. I can
> also see that the summaries are in the bitcode files, via llvm-bcanalyzer.
>
>
>
> I hope the llvm-dis support will happen. Also it would be nice if the
> documentation were more up-to-date. But this will definitely help get me
> moving again.
>
>
>
> For whatever reason in this simple example t3 is not being inlined, but I
> can set that aside for now.
>
>
>
> // t2.c:
>
> int t3();
>
>
>
> int t2()
>
> {
>
>    if (t3()) return 1;
>
>    else return 0;
>
> }
>
>
>
> // t3.c:
>
> int t3()
>
> {
>
>    return 10;
>
> }
>
>
>
> % clang -c -O2 -flto=thin t2.c t3.c
>
> % llvm-lto t2.o t3.o --exported-symbol=t2 --thinlto-action=run
> -debug-only=function-import -thinlto-save-temps=.
>
> Live root: 11079814748834529807
>
> 2 symbols Live, and 1 symbols Dead
>
> Computing import for Module 't3.o'
>
> Ignores Dead GUID: 8059958135287337232
>
> Initialize import for 14314392591860533259
>
> Computing import for Module 't2.o'
>
> Initialize import for 11079814748834529807
>
> Import/Export lists for 2 modules:
>
> * Module t3.o exports 0 functions. Imports from 0 modules.
>
> * Module t2.o exports 0 functions. Imports from 0 modules.
>
> Starting import for Module t2.o
>
> Imported 0 functions for Module t2.o
>
> Starting import for Module t3.o
>
> Imported 0 functions for Module t3.o
>
>
>
> -Alan
>
>
>
> *From:* Teresa Johnson [mailto:tejohnson at google.com]
> *Sent:* Thursday, November 09, 2017 10:33 AM
> *To:* Davis, Alan
> *Cc:* llvm-dev at lists.llvm.org; Peter Collingbourne
> *Subject:* [EXTERNAL] Re: [llvm-dev] getting nowhere with thinLTO
>
>
>
>
>
>
>
> On Wed, Nov 8, 2017 at 2:31 PM, Davis, Alan via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> I’m trying to incorporate thinLTO into our proprietary linker. I can’t
> seem to make it work at all (although I do have the ‘thick’ form working).
>
>
>
> Sorry to hear that, hopefully we can help get that sorted out.
>
>
>
> When I try to invoke the ThinGenerator API it crashes somewhere way down
> in the bowels of IPO. So stepping away from the linker, I am just trying to
> explore the processing steps using standalone off-the shelf llvm built from
> tip-of-tree.
>
>
>
> For a new implementation I would highly recommend using the new LTO API,
> which utilizes linker resolution info and is better supported at this
> point. See http://llvm-cs.pcc.me.uk/include/llvm/LTO/LTO.h#234.
>
> See also gold-plugin.cpp and the lld sources - both utilize this API. To
> test it use the llvm-lto2 tool instead of llvm-lto. Because this interface
> uses linker resolution info, invoking via llvm-lto2 is more tedious since
> resolutions must be specified for all symbols in the input files. See
> invocations in the llvm tests for examples.
>
>
>
>
>
> 1. When I do ‘clang -c -flto=thin t1.c’ on a simple input file, the
> resulting .o file is a bitcode file as expected, but when I llvm-dis it
> there is no evidence of the module summary information. Is there a way to
> view the summary index for a given module?
>
>
>
> Unfortunately not. This has come up a few times and the long-term plan is
> to serialize it through llvm assembly, but no one has had the bandwidth to
> completely flesh this out yet, and there was disagreement about what to do
> in the short term, so nothing went in. For now, you have to use
> llvm-bcanalyzer -dump, but that is a raw output format, although it will at
> least allow you to confirm the summary sections are there.
>
>
>
>
>
> 2. When I take two or three .o files generated using -flto=thin and pass
> them to the standalone llvm-lto tool, as in:
>
>    llvm-lto --thinlto t2.o t3.o --exported-symbol=t2 -o index
>
> the combined index is (apparently) empty.
>
>
>
> Use llvm-bcanalyzer -dump to see the contents of the output index. It
> shouldn't be empty.
>
>
>
> 3. When I take a program and pass all the .o files to llvm-lto
> --thinlto-action=run, I get recompiled .o files for each but there seems to
> be no IPO (no inlining, etc).
>
>
>
> How do you know? BTW you can invoke llvm-lto with the -thinlto-save-temps=<pathprefix>
> option to get bitcode files emitted from the backends after the major
> phases of ThinLTO backend compilation. I.e. to see what was imported.
>
> You can also pass internal llvm options to llvm-lto,
> i.e. -debug-only=function-import will give you debug info from the
> function importing done both during the thin link and during the thinlto
> backends.
>
>
>
> Let me know what you find.
>
>
>
> Teresa
>
>
>
>
>
> What am I missing?
>
>
>
> -Alan
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
>
>
> --
>
> Teresa Johnson |
>
>  Software Engineer |
>
>  tejohnson at google.com |
>
>  408-460-2413 <(408)%20460-2413>
>
>
>

-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |  408-460-2413
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