[WIP] Avoiding Bimodal GC behavior #63408
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Tagging subscribers to this area: @dotnet/gc Issue DetailsWIP
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Thanks, @cshung Confirming that this fixes the bimodal issue I was seeing earlier (dotnet/performance#1992): GC workstation 2gb_pinning benchmark:
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BruceForstall
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Oct 4, 2022
SuperPMI asm diffs tries to ignore constants that can change between multiple replays, such as addresses that the replay engine must generate and not simply hand back from the collected data. Often, addresses have associated relocations generated during replay. SuperPMI can use these relocations to adjust the constants to allow two replays to match. However, there are cases on Arm64 where an address both doesn't report a relocation and is "constructed" using multiple `mov`/`movk` instructions. One case is the `allocPgoInstrumentationBySchema()` API which returns a pointer to a PGO data buffer. An address within this buffer is constructed via a sequence such as: ``` mov x0, dotnet#63408 movk x0, dotnet#23602, lsl dotnet#16 movk x0, dotnet#606, lsl dotnet#32 ``` When SuperPMI replays this API, it constructs a new buffer and returns that pointer, which is used to construct various actual addresses that are generated as "constructed" constants, shown above. This change "de-constructs" the constants and looks them up in the replay address map. If base and diff match the mapped constants, there is no asm diff.
BruceForstall
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Oct 5, 2022
* Support Arm64 "constructed" constants in SuperPMI asm diffs SuperPMI asm diffs tries to ignore constants that can change between multiple replays, such as addresses that the replay engine must generate and not simply hand back from the collected data. Often, addresses have associated relocations generated during replay. SuperPMI can use these relocations to adjust the constants to allow two replays to match. However, there are cases on Arm64 where an address both doesn't report a relocation and is "constructed" using multiple `mov`/`movk` instructions. One case is the `allocPgoInstrumentationBySchema()` API which returns a pointer to a PGO data buffer. An address within this buffer is constructed via a sequence such as: ``` mov x0, #63408 movk x0, #23602, lsl #16 movk x0, #606, lsl #32 ``` When SuperPMI replays this API, it constructs a new buffer and returns that pointer, which is used to construct various actual addresses that are generated as "constructed" constants, shown above. This change "de-constructs" the constants and looks them up in the replay address map. If base and diff match the mapped constants, there is no asm diff. * Fix 32-bit build I don't think we fully support 64-bit replay on 32-bit host, but this fix at least makes it possible for this case. * Support more general mov/movk sequence Allow JIT1 and JIT2 to have a different sequence of mov/movk[/movk[/movk]] that map to the same address in the address map. That is, the replay constant might require a different set of instructions (e.g., if a `movk` is missing because its constant is zero).
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@cshung, this has been a WIP for almost a year. Are you still working on it? |
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closing since this is no longer as important with regions enabled. |
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Fixes #61428
@L2
WIP