A recent Clang optimization introduced in version 17 regressed performance when compiling code containing large switch statements within inlined functions. This regression manifested as significantly increased compile times, sometimes by orders of magnitude, and occasionally resulted in internal compiler errors. The issue stems from Clang's attempt to optimize switch lowering by transforming it into a series of conditional moves based on jump tables. This optimization, while beneficial in some cases, interacts poorly with inlining, exploding the complexity of the generated intermediate representation (IR) when a function with a large switch is inlined multiple times. This ultimately overwhelms the compiler's later optimization passes. A workaround involves disabling the problematic optimization via a compiler flag (-mllvm -switch-to-lookup-table-threshold=0) until a proper fix is implemented in a future Clang release.
Some websites display boxes instead of flag emojis in Chrome on Windows due to a font substitution issue. Windows uses its own Segoe UI Emoji font for most emoji, but defaults to a lower-quality bitmap font called "Segoe UI Symbol" specifically for flag emojis. This bitmap font lacks the necessary glyphs for many flag combinations, resulting in the missing emoji. Websites can force Chrome to use the correct, vector-based Segoe UI Emoji font by explicitly specifying it in their CSS, ensuring flags render properly.
Commenters on Hacker News largely discuss the technical details behind the issue, focusing on the surprising interaction between Chrome, Windows, and the specific way flags are rendered using two combined code points. Several point out the complexity and unexpected behaviors that arise from combining characters, particularly when dealing with different systems and fonts. Some users express frustration with the inconsistency and lack of clear documentation around emoji rendering. A few commenters offer potential workarounds or solutions, including using a fallback font or pre-rendering the flags as images. Others delve into the history and evolution of emoji standards and the challenges of maintaining compatibility across platforms. A compelling comment thread explores the tradeoffs between using the combined code points for flags versus using dedicated single code points, highlighting the performance implications and rendering complexities. Another interesting discussion revolves around the role of fonts and the challenges of designing fonts that support a rapidly expanding set of emojis.
Pixel 4a owners who haven't updated their phones are now stuck with a buggy December 2022 battery update as Google has removed older firmware versions from its servers. This means users can no longer downgrade to escape the battery drain and random shutdown issues introduced by the update. While Google has acknowledged the problem and promised a fix, there's no ETA, leaving affected users with no immediate solution. Essentially, Pixel 4a owners are forced to endure the battery problems until Google releases the corrected update.
HN commenters generally express frustration and disappointment with Google's handling of the Pixel 4a battery issue. Several users report experiencing the battery drain problem after the update, with some claiming significantly reduced battery life. Some criticize Google's lack of communication and the removal of older firmware, making it impossible to revert to a working version. Others discuss potential workarounds, including custom ROMs like LineageOS, but acknowledge the risks and technical knowledge required. A few commenters mention the declining quality control of Pixel phones and question Google's commitment to supporting older devices. The overall sentiment is negative, with many expressing regret over purchasing a Pixel phone and a loss of trust in Google's hardware division.
A quirk in the Motorola 68030 processor inadvertently enabled the Mac Classic II to boot despite its ROM lacking proper 32-bit addressing support. The Classic II's ROM mistakenly used a "MOVEA" instruction with a 32-bit address, which should have caused a failure on the 24-bit address bus. However, the 68030, when configured for a 24-bit bus, ignores the upper byte of the 32-bit address in this specific instruction. This unintentional compatibility allowed the flawed ROM to function, making the Classic II's boot process seemingly normal despite the underlying programming error.
Hacker News commenters on the Mac Classic II boot anomaly generally express fascination with the technical details and the serendipitous nature of the discovery. Several commenters delve into the specifics of 680x0 instruction sets and how an invalid instruction could inadvertently lead to a successful boot, speculating about memory initialization and undocumented behavior. Some share anecdotes about similar unexpected behaviors encountered during their own retrocomputing explorations. A few commenters also highlight the importance of such stories in preserving computer history and understanding the quirks of older hardware. The overall sentiment reflects appreciation for the ingenuity and occasional happy accidents that shaped early computing.
Summary of Comments ( 2 )
https://news.ycombinator.com/item?id=43088797
The Hacker News comments discuss a performance regression in Clang involving large switch statements and inlining. Several commenters confirm experiencing similar issues, particularly when compiling large codebases. Some suggest the regression might be related to changes in the inlining heuristics or the way Clang handles jump tables. One commenter points out that using a
constexprhash table for large switches can be a faster alternative. Another suggests profiling and selective inlining as a workaround. The lack of clear identification of the root cause and the potential impact on compile times and performance are highlighted as concerning. Some users express frustration with the frequency of such regressions in Clang.The Hacker News post discussing the Clang regression related to switch statements and inlining sparked a conversation revolving primarily around compiler optimization, code generation, and debugging challenges. Several commenters delved into the technical intricacies of the issue.
One commenter highlighted the complexities involved in compiler optimization, specifically mentioning the difficulty in striking a balance between performance gains and potential code bloat. They pointed out that aggressive inlining, while often beneficial, can sometimes lead to larger binaries and potentially slower execution in certain scenarios, as was seemingly the case with the Clang regression described in the article. This commenter also touched upon the trade-offs compilers must make and how these decisions can sometimes have unforeseen consequences.
Another commenter focused on the debugging challenges introduced by such optimizations. They argued that overly aggressive inlining can obscure the relationship between the original source code and the generated assembly, making it harder to debug issues. This difficulty stems from the fact that the inlined code is effectively "merged" into the calling function, making it harder to trace back to the original source location when stepping through a debugger.
The discussion also touched upon the specifics of switch statement optimization. One commenter explained how compilers often transform switch statements into various forms, such as jump tables or binary search trees, depending on the density and distribution of the cases. They suggested that the Clang regression might be related to a suboptimal choice of switch implementation in specific scenarios.
Furthermore, a commenter mentioned the importance of profiling and benchmarking in identifying and addressing such performance regressions. They emphasized that relying solely on theoretical analysis of code transformations can be misleading and that empirical data is crucial for understanding the actual impact of compiler optimizations.
Finally, some commenters discussed potential workarounds and suggested exploring compiler flags to fine-tune inlining behavior or to disable specific optimizations. This highlighted the importance of having granular control over the compiler's optimization strategies to mitigate potential performance issues.
Overall, the comments on Hacker News provided valuable insights into the technical nuances of the Clang regression, focusing on the challenges related to compiler optimization, debugging, and the importance of profiling and benchmarking. The discussion demonstrated a deep understanding of compiler internals and offered practical suggestions for dealing with similar issues.