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From: Alexander Monakov <amonakov@ispras.ru>
To: "Alex Rønne Petersen" <alex@alexrp.com>
Cc: musl@lists.openwall.com
Subject: Re: [musl] [PATCH] configure: prevent compilers from turning a * b + c into fma(a, b, c)
Date: Thu, 29 Aug 2024 18:09:16 +0300 (MSK)	[thread overview]
Message-ID: <78a7c979-19a0-b9bb-95d1-3da2f75322f0@ispras.ru> (raw)
In-Reply-To: <CAH9TF6M7Xey=COOVGDvQovTyyqum_8k783RJjVUXqUKYsSxa=Q@mail.gmail.com>

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On Thu, 29 Aug 2024, Alex Rønne Petersen wrote:

> That aside, while the motivating issue doesn't (easily) reproduce with
> vanilla Clang, it's nonetheless still the case that Clang folds
> multiple expressions in `fma()` into `llvm.fmuladd.*` intrinsic calls.
> While this might work out in some cases, we've still basically lost at
> the LLVM IR level; we're at the mercy of the target backend in regards
> to whether it gets lowered to an actual FMA instruction or split back
> to the ~original FMUL + FADD. And this isn't even considering what
> other nonsense the optimizer pipeline might get up to before that.

Thank you for uncovering what was happening in LLVM! I agree there's
a backend bug, but the point is moot since disabling FMA contraction
globally is the way to go, as discussed in the longer sub-thread.

Alexander

      reply	other threads:[~2024-08-29 15:09 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-08-28 15:28 Alex Rønne Petersen
2024-08-28 15:53 ` Alexander Monakov
2024-08-28 16:31   ` Alex Rønne Petersen
2024-08-28 20:15   ` Rich Felker
2024-08-28 20:32     ` Alexander Monakov
2024-08-28 20:47       ` Rich Felker
2024-08-28 21:11         ` Alexander Monakov
2024-08-29 13:37           ` Alex Rønne Petersen
2024-08-28 19:56 ` Alexander Monakov
2024-08-29 13:36   ` Alex Rønne Petersen
2024-08-29 15:09     ` Alexander Monakov [this message]

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