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Re: [PATCH] Two level comdat priorities in gold

On Thu, Jul 23, 2015 at 10:09 AM, Sriraman Tallam <> wrote:
> On Thu, Jul 23, 2015 at 8:49 AM, Ian Lance Taylor <> wrote:
>> On Thu, Jul 23, 2015 at 12:05 AM, Sriraman Tallam <> wrote:
>>> 2) Cary's idea of simply not applying the extra -mxxx flags on
>>> COMDATS, the out of line inlines and the instantiated template bodies.
>>> Unfortunately this idea does not work too for  code like this:
>>> #ifdef __AVX__
>>> inline foo () {
>>>   avx intrnsics
>>> }
>>> ...
>>> #else
>>> inline foo() {
>>>   non-avx intrinsics.
>>> }
>>> ...
>>> #endif
>>> This means the AVX comdats are already there past the pre-processor
>>> once we use -mavx and we cannot undo this in the compiler.  Example
>>> header :
>> If you compile this code both with and without -mavx and link the
>> results together, you have a simple, no excuses, ODR violation, so I
>> don't find this example convincing.
> Agreed.  It is clear there is a ODR violation but this usage is also
> not uncommon.  Unless multiple binaries, one for each target variant,
> are built there is going to be a ODR violation with using such
> headers.  I was hoping for a mitigation mechanism.

I think it's one thing to discuss some way to handle CPU-specific
variants of code where there is no ODR violation.  That's a real
problem and I agree that some solution seems necessary.

It is an entirely different thing to discuss ways to permit actual ODR
violations.  This example makes clear that your proposal is in fact a
powerful mechanism to let people mix and match multiple copies of
functions with the same name in the same binary, as long as those
functions are inline.  The language forbids this.  It seems that the
result can be very confusing.  It's like an overload where the
resolution of the overload depends not on any language rule but rather
than the specific set of compilation options.  Why should we permit


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