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Restore LLVM 3.7 MemoryDependenceAnalysis default - #9003
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This restores the BlockScanLimit in MemoryDependenceAnalysis to its LLVM 3.7 default (100), from what it was changed to in PR microsoft#2725. I'm not entirely sure wht PR microsoft#2725 was trying to accomplish, and the tests it added still pass even with the original limit. What I do know is that the larger limit can cause _massive_ compile-time regressions by allowing GVN to go off into the weeds for all eternity. The original PR also didn't seem to catch that the limit impacts not just DSE, but also GVN (and MemCpyOpt but DXC disables that). I've left the rest of PR microsoft#2725 in-tact, but I'm generally dubious of it. We certainly should be _a lot_ more careful about changing LLVM core IR passes since we don't have the ability to run any of the IR pass tests.
Damyan Pepper (damyanp)
approved these changes
Oct 5, 2026
Copilot stopped reviewing on behalf of
Chris B (llvm-beanz) due to an error
October 5, 2026 21:38
Sarah Spall (spall)
approved these changes
Oct 6, 2026
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This restores the BlockScanLimit in MemoryDependenceAnalysis to its LLVM 3.7 default (100), from what it was changed to in PR #2725.
I'm not entirely sure what PR #2725 was trying to accomplish, and the tests it added still pass even with the original limit. What I do know is that the larger limit can cause massive compile-time regressions by allowing GVN to go off into the weeds for all eternity.
The original PR also didn't seem to catch that the limit impacts not just DSE, but also GVN (and MemCpyOpt but DXC disables that).
I've left the rest of PR #2725 in-tact, but I'm generally dubious of it. We certainly should be a lot more careful about changing LLVM core IR passes since we don't have the ability to run any of the IR pass tests.