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John Regehrregehr@mastodon.social
Sep 28, 2026, 4:59 PM
LLVM's InstCombine has become a bit of a monster!
alt: graph showing lines of code starting with 6000 lines in 2006 to around 57,000 lines today
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Replies

Owen Andersonresistor@mastodon.online
Sep 28, 2026, 6:00 PM
@regehr Ablation study to identify potentially dead or counter-productive combines?
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John Regehrregehr@mastodon.social
Sep 28, 2026, 6:06 PM
@resistor yeah there's got to be a lot of stuff in there that hardly ever fires
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zwarichzwarich@hachyderm.io
Sep 28, 2026, 8:36 PM
@regehr @resistor unfortunately, some of them fire only once in Google's important library routine, exec_frequency_georg()
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Fabian Giesenrygorous@mastodon.gamedev.place
Sep 28, 2026, 8:39 PM
@zwarich @regehr @resistor it is an outlier and should not have been counted
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Ryan Huntryan@social.eqrion.net
Sep 28, 2026, 7:21 PM
@regehr Who’d of thought you could combine instructions in so many ways! It’s the revenge of combinatorics
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Billy O'Nealmalwareminigun@infosec.exchange
Sep 28, 2026, 7:46 PM
@regehr Some of those lines could be combined surely </s>
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Adrian Sampsonadrian
Sep 28, 2026, 7:50 PM
@regehr man, I wish there were some sort of DSL that could make these rewrites more succinct to express. then maybe you could even check their correctness with a solver. too bad no one has ever written any papers about this
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Samlenary@types.pl
Sep 28, 2026, 7:55 PM
@adrian @regehr LLVM only adds DSLs with really complex and partially undocumented semantics that only mean something when translated into the exact C++ that llvm-tblgen produces. How could we possibly do better than that?
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Samlenary@types.pl
Sep 28, 2026, 7:58 PM
@adrian @regehr I once made the mistake of buying an LLVM specific book and then trying to use it to understand how some very complex ISel pattern mechanism worked. The book didn't cover it, and in the end I reverse engineered it and wrote up a lot of docs. I wonder what I'll do that to next.
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Adrian Sampsonadrian
Sep 28, 2026, 8:01 PM
@lenary @regehr nice, let’s all get into this lucrative LLVM-specific book business
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Samlenary@types.pl
Sep 28, 2026, 8:02 PM
@adrian @regehr I almost threw it directly out the window.
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Samlenary@types.pl
Sep 28, 2026, 8:03 PM
@adrian @regehr but I guess I did buy it first…
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Guilherme EspadaGUIpsp@types.pl
Sep 28, 2026, 10:56 PM
@lenary @adrian @regehr and if you had thrown it out the window you'd have to buy it again!
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Joe Groffjoe@f.duriansoftware.com
Sep 28, 2026, 8:03 PM
@regehr is LLVM instcombine a key contributor to climate change? the correlation is undeniable
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Cole B.csb6@mastodon.social
Sep 28, 2026, 11:04 PM
@regehr They should have read your "Future Directions for Optimizing Compilers" (2018) paper! One of my favorite papers
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