{"slug": "gpt-5-6-found-an-intel-cpu-microcode-bug-causing-a-performance-regression", "title": "GPT-5.6 found an Intel CPU microcode bug causing a performance regression", "summary": "GPT-5.6 identified an Intel CPU microcode bug causing a performance regression, which was resolved by building with the -mbranches-within-32B-boundaries flag. The workaround slows conditional jumps straddling a 32-byte boundary, requiring specific program layout to restore speed.", "body_md": "Had a strange performance regression from a seemingly innocuous code change, and GPT-5.6 tracked it down to a microcode workaround for an Intel CPU bug.\nThe workaround can make conditional jumps that straddle a 32-byte boundary much slower, so programs have to be laid out around it.\nTried building with -mbranches-within-32B-boundaries and wouldn't you know it, we're fast again!\nThe future is so fucking cool!", "url": "https://wpnews.pro/news/gpt-5-6-found-an-intel-cpu-microcode-bug-causing-a-performance-regression", "canonical_source": "https://twitter.com/awesomekling/status/2079913512772522384", "published_at": "2026-07-22 12:59:59+00:00", "updated_at": "2026-07-22 13:23:13.772107+00:00", "lang": "en", "topics": ["artificial-intelligence", "large-language-models", "ai-tools", "ai-research"], "entities": ["GPT-5.6", "Intel"], "alternates": {"html": "https://wpnews.pro/news/gpt-5-6-found-an-intel-cpu-microcode-bug-causing-a-performance-regression", "markdown": "https://wpnews.pro/news/gpt-5-6-found-an-intel-cpu-microcode-bug-causing-a-performance-regression.md", "text": "https://wpnews.pro/news/gpt-5-6-found-an-intel-cpu-microcode-bug-causing-a-performance-regression.txt", "jsonld": "https://wpnews.pro/news/gpt-5-6-found-an-intel-cpu-microcode-bug-causing-a-performance-regression.jsonld"}}