The performance difference does not appear to just be in aarch64-specific code. Instead, the difference seem to be spread over lots of different functions. When using the x86_64 PGO training data, the arm64 build logs contain 16231 warnings of the type `function control flow change detected (hash mismatch) `: ``` [task 2024-01-15T21:03:07.139Z] 21:03:07 INFO - warning: Unified_cpp_parser_html1.cpp: function control flow change detected (hash mismatch) _ZN16nsHtml5Tokenizer9stateLoopI20nsHtml5FastestPolicyEEiiDsiPDsbii Hash = 1006606701270676095 up to 423331622 count discarded [-Wbackend-plugin] ``` With the aarch64 PGO training data, the arm64 build log only contains 1651 of those warnings (10x less).
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The performance difference does not appear to just be in aarch64-specific code. Instead, the difference seem to be spread over lots of different functions. When using the x86_64 PGO training data, [the arm64 build log](https://firefoxci.taskcluster-artifacts.net/YWVa88h_QmqLePXIH_oVHw/0/public/logs/live_backing.log)s contain 16231 warnings of the type `function control flow change detected (hash mismatch) `: ``` [task 2024-01-15T21:03:07.139Z] 21:03:07 INFO - warning: Unified_cpp_parser_html1.cpp: function control flow change detected (hash mismatch) _ZN16nsHtml5Tokenizer9stateLoopI20nsHtml5FastestPolicyEEiiDsiPDsbii Hash = 1006606701270676095 up to 423331622 count discarded [-Wbackend-plugin] ``` With the aarch64 PGO training data, [the arm64 build log](https://firefoxci.taskcluster-artifacts.net/QwL3FGggQqiHkD5f1FLyUw/0/public/logs/live_backing.log) only contains 1651 of those warnings (10x less).