[llvm-dev] [EXTERNAL] Re: Question about -flto behavior

[Snider, Todd via llvm-dev]

Yes, if the end user application is building with LTO, then we are going through code generation unnecessarily during -flto compiles.

However, one of the motivations for embedding the IR in object code is so that the libc and other runtime libraries can be pre-built and shipped with embedded bitcode IR. The runtime libraries can then be linked in whether the end user application chooses to built with LTO or not.

I'm speculating that this tradeoff becomes less cost effective as the user application gets very big, but it is probably a reasonable tradeoff for embedded applications.

~ Todd

From: Teresa Johnson <tejohnson at google.com>
Sent: Tuesday, October 26, 2021 6:04 PM
To: Snider, Todd <t-snider at ti.com>
Cc: David Blaikie <dblaikie at gmail.com>; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] [EXTERNAL] Re: Question about -flto behavior



On Mon, Oct 18, 2021 at 3:55 PM Snider, Todd via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

David,

Thanks for the reply. That makes sense.

A couple of further thoughts ... In the LTO implementation that I am working on, when -flto is specified to the compiler, the compiler embeds the IR in the compiler generated object file. The linker can then read the IR out of the incoming object file if LTO is enabled at link time or just ignore the IR if LTO is disabled at link time.

I guess this approach gives flexibility to the build at the cost of extra compile time to go through the whole optimization pipeline including code generation potentially unnecessarily. Normally in LLVM we don't go through code generation during an -flto -S or -c compile, and only part of the optimization pipeline.


I would agree that having -S write out the IR content for -flto provides a good way to see what is being fed into the LTO link in a human readable form.

For our LTO implementation, the linker can be told to keep the IR that it extracts from the incoming object files. You can then run llvm-dis over the extracted IR to see the .ll version.

Note you can emit the final machine code when building with -flto and linking with lld via -Wl,--lto-emit-asm, in which case the a.out or specified output file will contain assembly instead of ELF.

Teresa


~ Todd

From: David Blaikie <dblaikie at gmail.com<mailto:dblaikie at gmail.com>>
Sent: Monday, October 18, 2021 4:21 PM
To: Snider, Todd <t-snider at ti.com<mailto:t-snider at ti.com>>
Cc: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
Subject: [EXTERNAL] Re: [llvm-dev] Question about -flto behavior

Yeah, sounds expected to me - flto produces object files that aren't really object files - instead they're LLVM IR (bitcode) that the linker identifies, then calls back into LLVM to link the IR, optimize on that IR, then produce object code/assembly/whatever).

So the "assembly" form of an "object" (really LLVM bitcode) file is LLVM textual IR.

On Mon, Oct 18, 2021 at 1:56 PM Snider, Todd via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hi All,

When -flto is combined with -S on the clang command line, the output .s file contains IR content instead of target assembly language.

Is this expected/correct behavior? I was anticipating that the output .s file would contain target assembly code.

~ Todd Snider
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--
Teresa Johnson |

 Software Engineer |

 tejohnson at google.com<mailto:tejohnson at google.com> |


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