[llvm-dev] Always running Regular LTO before ThinLTO?

[Arthur Eubanks via llvm-dev]

Looks like the secondary split LTO units (e.g. due to
-fwhole-program-vtables) are being added together into one large module
which is then monolithically LTO'd, which is where the extra passes were
being run.

splitAndWriteThinLTOBitcode():
  // Mark the merged module as requiring full LTO. We still want an index
for
  // it though, so that it can participate in summary-based dead stripping.


On Mon, May 24, 2021 at 12:32 PM Arthur Eubanks <aeubanks at google.com> wrote:

> Ah I messed something up locally, removing the regular LTO run
> causes undefined hidden symbol errors for vtables.
> The `runRegularLTO()` name is confusing though, it apparently sets up some
> things necessary for ThinLTO.
>
> On Mon, May 24, 2021 at 11:58 AM Arthur Eubanks <aeubanks at google.com>
> wrote:
>
>> While debugging a ThinLTO issue, I noticed that passes were running more
>> often than I expected. This is being invoked pretty normally, something like
>> $ clang++ -fuse-ld=lld -flto=thin
>>
>> Looks like at [1] we try to first run regular LTO before running ThinLTO.
>> Removing the first line makes the link go from 208s to 115s. Is this
>> expected behavior?
>>
>> [1]:
>> https://github.com/llvm/llvm-project/blob/a64ebb8637277998f77e55d335faca6fdcf5859b/llvm/lib/LTO/LTO.cpp#L1033
>>
>
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