Subj : Python tail-call speedup based on LLVM regression
To   : All
From : LWN.net
Date : Mon Mar 10 2025 15:00:07

Python tail-call speedup based on LLVM regression

Date:
Mon, 10 Mar 2025 14:56:13 +0000

Description:
The Python project's recent switch to a tail-calling interpreter may not 
provide as large a speed advantage as initially thought. A blog post from 
Nelson Elhage gives the details. In short, switching to a tail-call-based 
interpreter accidentally works around an unfixed regression in LLVM 19. On 
other compilers, the performance benefit (while still present) is more 
moderate. When the tail-call interpreter was announced, I was surprised and 
impressed by the performance improvements, but also confused: I'm not an 
expert, but I'm passingly-familiar with modern CPU hardware, compilers, and 
interpreter design, and I couldn't explain why this change would be so 
effective. I became curious  and perhaps slightly obsessed  and the reports 
in this post are the result of a few weeks of off-and-on compiling and 
benchmarking and disassembly of dozens of different Python binaries, in an 
attempt to understand what I was seeing.

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Link to news story:
https://lwn.net/Articles/1013581/


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