[llvm-dev] ThinLTO Problem

[chenmindong via llvm-dev]

Hi Teresa,

Thanks for the detailed reply!
> How are you creating your bitcode files?
I create the bitcode with `-flto=thin -c` and sure it has a GLOBALVAL_SUMMARY_BLOCK. And there’s no RegularLTO partition only ThinLTO bicode.
> Where is it aborting in the backend?
It aborts at ` report_fatal_error("Failed to setup codegen")` in of codegen() of LTOBackend.cpp. And before that in createTargetMachine() it also warns ‘xxx is not a recognized processor’. But the program should return before run into backend() IIUC since all I want is using lld with `-thinlto-index-only` option to generate *.thinlto.bc and feed it and bitocde to clang to invoke target-specific optimizations(we use gnu as and ld).Do you have any suggestion about this?

Regards,
Mindong

From: Teresa Johnson [mailto:tejohnson at google.com]
Sent: Wednesday, November 27, 2019 11:12 PM
To: chenmindong <chenmindong1 at huawei.com>
Cc: llvm-dev at lists.llvm.org; Yuchao (Michael) <michael.yuchao at huawei.com>
Subject: Re: [llvm-dev] ThinLTO Problem

Hi Mindong,

On Wed, Nov 27, 2019 at 3:29 AM chenmindong via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hi,

I'm working on enabling thinLTO for our custom backend on LLVM-8 with lld to get code size benefits from dead symbol elimination. The code in LTO::run() of LTO.cpp confuses me that, even though thinLTO is specified, runRegularLTO() will be run first and its return value determines whether runThinLTO() will be executed.

My question is if it's clearly known that thinLTO is used, is it still necessary to execute runRegularLTO()?If it is, what's the reason behind?

This is to handle the case where the LTO link is given a mix of regular and thin LTO bitcode - it should do regular LTO on that subset and ThinLTO on the other. The other case this handles is when a bitcode object is split into regular and thin LTO halves - this is enabled for things like CFI but shouldn't be the default currently (you'd have to build with -fsplit-lto-unit to get it unless you are building with CFI).

Whether the bitcode is added to the regular or Thin LTO partition is determined in LTO::addModule, and is based on a flag set when the bitcode is read which is based in turn on whether the bitcode has a summary block, and whether that is a thinlto or regular (full) LTO summary block. How are you creating your bitcode files? If you run llvm-dis on it does it have summary entries? You can also see if you run llvm-bcanalyzer whether it has a GLOBALVAL_SUMMARY_BLOCK or a FULL_LTO_GLOBALVAL_SUMMARY_BLOCK or neither. If you are compiling with -flto=thin you should realistically have the former, which would make it a ThinLTO bitcode. But it sounds like you have some objects that either don't or have the full LTO summary.

For now our custom backend, where distributed thinLTO is adopted, it works fine as I removed the line executing runRegularLTO(). But if I preserve it, the code fails the `if (Conf.PostInternalizeModuleHook &&!Conf.PostInternalizeModuleHook(0, *RegularLTO.CombinedModule))`, which I also don't understand, and fall through to backend() and abort there. I believe something is missed during adding the target support but cannot figure it out. Could anyone help?

By default there should not be a PostInternalizeModuleHook set (it is set to support cases like -save-temps), so it isn't surprising that it would fail that test and fall through to the backend() call, which is what you would want if there was a regular LTO partition. Where is it aborting in the backend?

Teresa


Regards,
Mindong Chen
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Teresa Johnson |

 Software Engineer |

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


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