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

Snider, Todd via llvm-dev llvm-dev at lists.llvm.org
Mon Oct 18 15:55:37 PDT 2021


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 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.

~ Todd

From: David Blaikie <dblaikie at gmail.com>
Sent: Monday, October 18, 2021 4:21 PM
To: Snider, Todd <t-snider at ti.com>
Cc: 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
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
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