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Re: [RFC][PATCH] Change default to -fcommon

From: Sandra Loosemore <sandra at codesourcery dot com>
To: Michael Matz <matz at suse dot de>, Wilco Dijkstra <Wilco dot Dijkstra at arm dot com>
Cc: Richard Biener <richard dot guenther at gmail dot com>, "fortran at gcc dot gnu dot org" <fortran at gcc dot gnu dot org>, "ebotcazou at adacore dot com" <ebotcazou at adacore dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>, nd <nd at arm dot com>
Date: Mon, 20 Nov 2017 13:02:55 -0700
Subject: Re: [RFC][PATCH] Change default to -fcommon
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References: <DB6PR0801MB20537CD32063175699A3E50F832F0@DB6PR0801MB2053.eurprd08.prod.outlook.com> <839cf67b-dee2-7ee7-e59e-3315cbc6cece@codesourcery.com> <CAFiYyc1nHMX73evvnCcF1x0SeP7=p72VOinfw3oyaak-d2TanA@mail.gmail.com> <DB6PR0801MB2053E05E8489867CFC86DF2E83220@DB6PR0801MB2053.eurprd08.prod.outlook.com> <alpine.LSU.2.21.1711201827190.25295@wotan.suse.de>

On 11/20/2017 10:34 AM, Michael Matz wrote:

What's your rationale for changing this?  In your initial mail you said:

"On many targets this means global variable accesses having an unnecessary
codesize and performance penalty in C code (the same source generates
better code when built as C++)."

I have a hard time imaging that, so can you give details?  FWIW I've
personally always considered using common symbols nicer.

The optimization case I'm familiar with is for targets that support -Gand GP-relative addressing modes to address small data. Generally youcan only do that if the variable is allocated in the same compilationunit as the reference, so the compiler knows definitely where it'splaced (unless the backend also supports some other mechanism forasserting that a variable is small data). Putting tentatively-definedvariables in common defeats that optimization.


-Sandra

Follow-Ups:
- Re: [RFC][PATCH] Change default to -fcommon
  - From: Richard Biener
- Re: [RFC][PATCH] Change default to -fcommon
  - From: Michael Matz

References:
- [RFC][PATCH] Change default to -fcommon
  - From: Wilco Dijkstra
- Re: [RFC][PATCH] Change default to -fcommon
  - From: Sandra Loosemore
- Re: [RFC][PATCH] Change default to -fcommon
  - From: Richard Biener
- Re: [RFC][PATCH] Change default to -fcommon
  - From: Wilco Dijkstra
- Re: [RFC][PATCH] Change default to -fcommon
  - From: Michael Matz

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