[llvm] [llvm-profgen] Add an option to force marking the profile as preinlined (PR #156501)

[via llvm-commits]

WenleiHe wrote:

> > surprised that we don't have flag for this already. how did we do probe only (implies replay inlining) experiments earlier?
> 
> Good question! Let me first confirm we’re aligned on term: the previous "probe-only" refers to using `-ignore-stack-samples` on llvm-profgen and no extra flags on compiler side, right? If so:
> 
> yes, we have done something to make it behave like "replay inlining", there are some settings on compiler side to remove the limits: https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/IPO/SampleProfile.cpp#L2053-L2056, set the max/min inlinelimit to int::max, so it then works as it will replay without any limit.
> 
> However, there are still some important and subtle differences:
> 
> one major is the inlining callsite threshold: https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/IPO/SampleProfile.cpp#L1386-L1406
> 
> * In the previous probe-only, the cost is compared against a `SampleThreshold` which can be  `SampleColdCallSiteThreshold`(45) and `SampleHotCallSiteThreshold`(3000) depending on the hotness.
> * In preinlined setting(this PR), it use the "always_inline" cost(`getAlways("preinliner")`).
> 
> Another one is for the external/importing functions: https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/IPO/SampleProfile.cpp#L1060-L1062 In preinliner setting, the threshold is zero while in previous probe-only, it's the `HotCountThreshold`.
> 
> Therefore, the previous "probe-only" was not a pure replay.
> 
> I don't remember whether this is a "bug"(that we intentionally want a pure reply but missed the above setting) or it's just by design as we may just want it to do more like the classic line-number based AutoFDO which has those limits.
> 
> Yeah, alternatively we can have a "fix" in compiler to remove the limits for all pseudo-probe based profile if it's a "bug".

I was thinking about `csspgo-replay-preinline`: https://github.com/llvm/llvm-project/blame/main/llvm/tools/llvm-profgen/CSPreInliner.cpp#L50 

I thought we run pre-inliner as long as `csspgo-preinliner` is on, and that flag is on by default and also not affected by whether we ignore stack samples? 

Judging from your change, it seems like we don't run pre-inliner for probe-only (`-ignore-stack-samples`)? 

https://github.com/llvm/llvm-project/pull/156501