RFR: 8319850: PrintInlining should print which methods are late inlines

[theoweidmannoracle]

On Tue, 5 Nov 2024 17:10:08 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> In https://github.com/openjdk/jdk/pull/16595 @caojoshua previously suggested changes to indicate which calls were inlined late, when printing inlines. This PR re-introduces the changes from the previously closed PR and fixes a minor issue where asserts were triggered.
>> 
>> Concerns were raised by @rwestrel in the previous PR:
>> 
>>> When InlineTree::ok_to_inline() is called, some diagnostic message is recorded for the call site. Do I understand right that with this patch, if the call is inlined late, then that message is dropped and replaced by a new "late inline.." message? If that's the case, isn't it the case that sometimes the InlineTree::ok_to_inline() has some useful information that's lost when late inlining happens?
>> 
>> As already pointed out in the PR by @caojoshua, this does not matter for string/methodhandle/vector/boxing late inlines, as they are [only performed if ok_to_inline() returns true](https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/doCall.cpp#L189). This is also the only call to ok_to_inline().  
>> 
>> The only other location, where late inline call generators are created, are calls to CallGenerator::for_late_inline_virtual(), which creates a LateInlineVirtualCallGenerator. LateInlineVirtualCallGenerator (introduced in https://github.com/openjdk/jdk/pull/1550) does not really perform inlining but rather performs strength reduction from virtual to static calls. As can be verified by running the according test `test/hotspot/jtreg/compiler/c2/irTests/TestPostParseCallDevirtualization.java`, this does not affect the printing for inlining:
>> 
>> 
>> 5022 1026       3       compiler.c2.irTests.TestPostParseCallDevirtualization::callHelper (7 bytes)   made not entrant
>>                               @ 1   compiler.c2.irTests.TestPostParseCallDevirtualization$I::method (0 bytes)   failed to inline: virtual call
>> 
>> 
>> Thus, as far as I can tell, the proposed changes by @caojoshua do not lose any useful information about why no inlining prior to the late inlining occurred.
>
>> As already pointed out in the PR by @caojoshua, this does not matter for string/methodhandle/vector/boxing late inlines, as they are [only performed if ok_to_inline() returns true](https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/doCall.cpp#L189). This is also the only call to ok_to_inline().
> 
>> The only other location, where late inline call generators are created, are calls to CallGenerator::for_late_inline_virtual(), which creates a LateInlineVirtualCallGenerator.
> 
> What about regular calls that fail to inline initially because the compiler ran out of inlining budget but are inlined later on?

@rwestrel I think in case the inlining budget has been exceeded (i.e. try_to_inline and subsequently ok_to_inline fail),  there's only two code locations where we would create a late inlining code generator: [here](https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/doCall.cpp#L380) and [here](https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/doCall.cpp#L292). Both are calls to CallGenerator::for_late_inline_virtual() that create a LateInlineVirtualCallGenerator, which only performs strength reduction AFAIK. 

There might be something I'm missing, though, since I've only been working on the C2 compiler for three days 🙂 So please feel free to point me other cases of late inlining and I will investigate.

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PR Comment: https://git.openjdk.org/jdk/pull/21899#issuecomment-2457866376