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Subject: bug#20899: Bug in libtool: -fno-strict-alias not passed to linker stage when -flto is used
Resent-From: Davin McCall <davmac@HIDDEN>
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From: Davin McCall <davmac@HIDDEN>
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Hi,

I have discovered what I believe is a bug in libtool: It does not pass 
the "-fno-strict-aliasing" compilation flag to the link stage. This is 
not normally a problem, but is certainly a problem is "-flto" (link time 
optimization) is used. In this case code that works correct only without 
strict aliasing will incorrectly after being linked.

If you need any further information, please don't hesitate to ask.

Davin

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From: help-debbugs@HIDDEN (GNU bug Tracking System)
To: Davin McCall <davmac@HIDDEN>
Subject: bug#20899: Acknowledgement (Bug in libtool: -fno-strict-alias not
 passed to linker stage when -flto is used)
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20899: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=3D20899
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Message sent to bug-libtool@HIDDEN:

Subject: bug#20899: Bug in libtool: -fno-strict-alias not passed to linker stage when -flto is used
Resent-From: Bob Friesenhahn <bfriesen@HIDDEN>
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To: Davin McCall <davmac@HIDDEN>
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Date: Thu, 25 Jun 2015 21:31:30 -0500 (CDT)
From: Bob Friesenhahn <bfriesen@HIDDEN>
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On Thu, 25 Jun 2015, Davin McCall wrote:

> Hi,
>
> I have discovered what I believe is a bug in libtool: It does not pass the 
> "-fno-strict-aliasing" compilation flag to the link stage. This is not 
> normally a problem, but is certainly a problem is "-flto" (link time 
> optimization) is used. In this case code that works correct only without 
> strict aliasing will incorrectly after being linked.

Does

   -Wl,--fno-strict-aliasing

work for you?  It might be necessary to also specify 
-fno-strict-aliasing if the compiler also uses this.

Bob
-- 
Bob Friesenhahn
bfriesen@HIDDEN, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

Message sent to bug-libtool@HIDDEN:

Subject: bug#20899: Bug in libtool: -fno-strict-alias not passed to linker stage when -flto is used
Resent-From: Bob Friesenhahn <bfriesen@HIDDEN>
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Date: Thu, 25 Jun 2015 21:31:30 -0500 (CDT)
From: Bob Friesenhahn <bfriesen@HIDDEN>
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On Thu, 25 Jun 2015, Davin McCall wrote:

> Hi,
>
> I have discovered what I believe is a bug in libtool: It does not pass the 
> "-fno-strict-aliasing" compilation flag to the link stage. This is not 
> normally a problem, but is certainly a problem is "-flto" (link time 
> optimization) is used. In this case code that works correct only without 
> strict aliasing will incorrectly after being linked.

Does

   -Wl,--fno-strict-aliasing

work for you?  It might be necessary to also specify 
-fno-strict-aliasing if the compiler also uses this.

Bob
-- 
Bob Friesenhahn
bfriesen@HIDDEN, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

Message sent to bug-libtool@HIDDEN:

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From: Davin McCall <davmac@HIDDEN>
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Hi Bob,

On 26/06/15 03:31, Bob Friesenhahn wrote:
> On Thu, 25 Jun 2015, Davin McCall wrote:
>
>> Hi,
>>
>> I have discovered what I believe is a bug in libtool: It does not 
>> pass the "-fno-strict-aliasing" compilation flag to the link stage. 
>> This is not normally a problem, but is certainly a problem is "-flto" 
>> (link time optimization) is used. In this case code that works 
>> correct only without strict aliasing will incorrectly after being 
>> linked.
>
> Does
>
>   -Wl,--fno-strict-aliasing
>
> work for you?  It might be necessary to also specify 
> -fno-strict-aliasing if the compiler also uses this.
>
> Bob

No. -Wl passes an option to the linker, but the linker does not accept 
the -fno-strict-aliasing option, and so using -Wl,-fno-strict-aliasing 
results in a link failure. It's my understanding that in LTO mode, gcc 
acts as a somewhat more complex frontend to the linker than usual, but 
it still passes -Wl,XXX options directly to the underlying linker. GNU 
ld then interprets '-fno-strict-aliasing' as its own '-f' option, used 
for a different purpose.

FWIW the libtool script contains a small bit of code with a comment 
header reading "Flags to be passed through unchanged, with rationale:". 
It includes -flto, but not -fno-strict-aliasing. If I add 
-fno-strict-aliasing to the list here, it does seem to work as I want, 
that is, -fno-strict-aliasing is visible on the link stage command line.

Davin

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Hi Bob,

On 26/06/15 03:31, Bob Friesenhahn wrote:
> On Thu, 25 Jun 2015, Davin McCall wrote:
>
>> Hi,
>>
>> I have discovered what I believe is a bug in libtool: It does not 
>> pass the "-fno-strict-aliasing" compilation flag to the link stage. 
>> This is not normally a problem, but is certainly a problem is "-flto" 
>> (link time optimization) is used. In this case code that works 
>> correct only without strict aliasing will incorrectly after being 
>> linked.
>
> Does
>
>   -Wl,--fno-strict-aliasing
>
> work for you?  It might be necessary to also specify 
> -fno-strict-aliasing if the compiler also uses this.
>
> Bob

No. -Wl passes an option to the linker, but the linker does not accept 
the -fno-strict-aliasing option, and so using -Wl,-fno-strict-aliasing 
results in a link failure. It's my understanding that in LTO mode, gcc 
acts as a somewhat more complex frontend to the linker than usual, but 
it still passes -Wl,XXX options directly to the underlying linker. GNU 
ld then interprets '-fno-strict-aliasing' as its own '-f' option, used 
for a different purpose.

FWIW the libtool script contains a small bit of code with a comment 
header reading "Flags to be passed through unchanged, with rationale:". 
It includes -flto, but not -fno-strict-aliasing. If I add 
-fno-strict-aliasing to the list here, it does seem to work as I want, 
that is, -fno-strict-aliasing is visible on the link stage command line.

Davin

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