Re: manywarnings and -f options

[Eric Blake]

On 12/03/2011 09:00 AM, Simon Josefsson wrote:
> What does -funit-at-a-time really do?  My gcc 4.4 manual says:
> 
> `-funit-at-a-time'
>      This option is left for compatibility reasons. `-funit-at-a-time'
>      has no effect, while `-fno-unit-at-a-time' implies
>      `-fno-toplevel-reorder' and `-fno-section-anchors'.
> 
>      Enabled by default.

That's the case for 4.4 and later.  But in gcc 4.3, it was not
unconditionally enabled, and as I said earlier, at least coreutils ran
into a situation where gcc 4.3. failed to compile at -Werror because
-Wdisabled-optimization warned that -fno-unit-at-a-time was required,
which warning turned into an error.

At this point, gcc 4.3 is slowly phasing out; most Linux distros and
Cygwin have moved on to newer compilers, where the problem is less
likely to happen.

> 
> The manual for -ipa-pure-const says:
> 
> `-fipa-pure-const'
>      Discover which functions are pure or constant.  Enabled by default
>      at `-O' and higher.
> 
> That seems harmless and as far I understand would not modify code
> generation.

It doesn't modify code generation, but it DOES modify warning
generation, and in a way that negatively interacts with libtool:
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=10197

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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