[cfe-dev] Option -mtune

Sjoerd Meijer via cfe-dev cfe-dev at lists.llvm.org
Sun Apr 19 14:01:35 PDT 2020

Thanks for your reply, and I can see that this is the only benefit of this no-op: if your move to Clang and if you still have -mtune set in your build environment, you don't get an "unknown option" error. But for me personally, silently ignoring things and giving the impression it works is worse than a simple Makefile fix, but maybe I am wrong, which is why I checked here on the list. I was guessing that a diagnostic in this case would only be value if it's enabled by default as I'm afraid many users won't enable it? And if you need to add a flag to get this diagnostic, you might as well get rid of -mtune?


From: David Blaikie <dblaikie at gmail.com>
Sent: 19 April 2020 20:58
To: Sjoerd Meijer <Sjoerd.Meijer at arm.com>
Cc: cfe-dev at lists.llvm.org Developers <cfe-dev at lists.llvm.org>
Subject: Re: [cfe-dev] Option -mtune

On Sun, Apr 19, 2020 at 12:55 PM Sjoerd Meijer via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:

Quick question what you think we should be doing with option -mtune. Problem is that it looks like we support it because it is documented, it can be supplied on the command line, but it is silently ignored:

  // FIXME: Handle -mtune=.

giving the false impression to users it is doing something is probably the worst of options we have

Not /the worst/ as such, many options are added to Clang so it's command line compatible (in the sense that you'll get a running program that behaves correctly) with GCC - I imagine the commit history of this feature probably justifies the addition with that sort of reason.

Seems quite reasonable for --help and web documentation to mention that it's a no-op/supported-for-compatibility flag.

As for adding a warning for these sort of no-op flags, maybe? Probably opt-in, though.

(we get regularly questions about this).
We could simply remove it, or if this is too radical, issue a diagnostic that this is an unsupported option? Any thoughts/preferences?


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