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Message-ID: <78a7c979-19a0-b9bb-95d1-3da2f75322f0@ispras.ru>
Date: Thu, 29 Aug 2024 18:09:16 +0300 (MSK)
From: Alexander Monakov <amonakov@...ras.ru>
To: Alex Rønne Petersen <alex@...xrp.com>
cc: musl@...ts.openwall.com
Subject: Re: [PATCH] configure: prevent compilers from turning a * b
+ c into fma(a, b, c)
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
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