[llvm] [NVPTX] Reland `mad.wide` combine under (default off) CLI option (PR #160214)

Wed Sep 24 16:52:44 PDT 2025

justinfargnoli wrote:

> Do we have a good idea what exactly triggered the performance regression?

No, I haven't had the bandwidth to diagnose the issue properly. 

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> It appears to be "ptxas can't optimize loads that use mad.wide result as the source address".

Apologies if I'm being too pedantic, but I'd *guess* that `ptxas` *can* do this, but for whatever reason, it doesn't do it right now. 

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> we should make it a prominent FIXME/TODO next to the knob controlling it now.

Added a FIXME with 204513fb583b4778081e768bbaf0ea6872303821.

https://github.com/llvm/llvm-project/pull/160214