Bug 1227361

== Comment: #0 - Tulio Magno Quites Machado Filho <tulioqm.com> - 2015-05-28 15:52:08 ==
Although GCC for ppc64le has ben configured using --with-cpu=power7, that doesn't ensure glibc will make usage of all the POWER7 optimizations during its build.  That only happens when glibc is configured with --with-cpu=power7.

As ppc64le supports POWER >=7, it's better to use this configure flag.

A possible solution for this case is to add ppc64le to the following block of code taken from glibc.spec:

%ifarch ppc64p7
		--with-cpu=power7 \
%endif

Sounds good to me. We'll put this into the backlog.

ppc64le requires POWER8 and later, so this no longer relevant.

------- Comment From tulioqm.com 2016-02-08 14:25 EDT-------
(In reply to comment #5)
> ppc64le requires POWER8 and later, so this no longer relevant.

This is not clear to me.
Are you saying that glibc is being configured with --with-cpu=power8 now?

When I opened this bug report, Fedora wanted to support POWER7 on ppc64le. If it has changed to support POWER8, it's necessary to configure glibc --with-cpu=power8 in order to correctly use the POWER8 files from glibc.

(In reply to IBM Bug Proxy from comment #4)

> When I opened this bug report, Fedora wanted to support POWER7 on ppc64le.

IBM does not want to support POWER7 on ppc64le, see the recent discussion on the Fedora secondary list.

> If it has changed to support POWER8, it's necessary to configure glibc
> --with-cpu=power8 in order to correctly use the POWER8 files from glibc.

I don't see any logic in the glibc build system which reacts to that.  As far as I can tell, just setting the GCC defaults appropriately is sufficient.

It might be theoretically possible to omit support code for older CPU variants, but I don't think glibc can do that for any architecture right now (there is nothing like __ASSUME_KERNEL for CPU features, as far as I know).

------- Comment From tulioqm.com 2016-02-08 15:02 EDT-------
(In reply to comment #9)
> > If it has changed to support POWER8, it's necessary to configure glibc
> > --with-cpu=power8 in order to correctly use the POWER8 files from glibc.
>
> I don't see any logic in the glibc build system which reacts to that.  As
> far as I can tell, just setting the GCC defaults appropriately is sufficient.

No it isn't.
If you don't set --with-cpu=power8, you won't be using POWER8-optimized files, i.e. those under the power8 directories.
That's why we have this patch: http://patchwork.sourceware.org/patch/10641/

IMO, you really want to use --with-cpu, unless you want to support POWER >= 4.

(In reply to IBM Bug Proxy from comment #6)
> ------- Comment From tulioqm.com 2016-02-08 15:02 EDT-------
> (In reply to comment #9)
> > > If it has changed to support POWER8, it's necessary to configure glibc
> > > --with-cpu=power8 in order to correctly use the POWER8 files from glibc.
> >
> > I don't see any logic in the glibc build system which reacts to that.  As
> > far as I can tell, just setting the GCC defaults appropriately is sufficient.
> 
> No it isn't.
> If you don't set --with-cpu=power8, you won't be using POWER8-optimized
> files, i.e. those under the power8 directories.
> That's why we have this patch: http://patchwork.sourceware.org/patch/10641/

Tulio is correct. However, perhaps you can answer a question I had. In general all of the functions in the power8/ sysdep directory appear to be routed to by IFUNCs anyway, so you will get their benefit in a generic build also? However, because it isn't guaranteed that all functions have IFUNC selectors you generally want to build with --with-cpu=powerx for POWERx (where x is an integer value).

Florian, if you look deeply the --with-cpu options feed into the sysdeps selection mechanism which makes it a required option to pickup the POWER8 optimized functions. 

This is fixed this in Rawhide.

e.g.
 889 %if %{buildpower8}
 890 (
 891   AddOns="$AddOns --with-cpu=power8"
 892   GCC="$GCC -mcpu=power8 -mtune=power8"
 893   GXX="$GXX -mcpu=power8 -mtune=power8"
 894   build power8
 895 )

I'm marking CLOSED/RAWHIDE since we aren't going to change the runtimes for F22.

Reopening because the default runtime for ppc64le doesn't use the %{buildpower8} path. We need this additional fix:

diff --git a/glibc.spec b/glibc.spec
index 5cb5262..da36986 100644
--- a/glibc.spec
+++ b/glibc.spec
@@ -818,6 +818,9 @@ build()
 %endif
                --enable-obsolete-rpc \
                --enable-systemtap \
+%ifarch ppc64le
+               --with-cpu=power8 \
+%endif
 %ifarch ppc64p7
                --with-cpu=power7 \
 %endif
---

Testing changes to fix this.

This is now fixed in rawhide.

The default ppc64le runtime now correctly configures using --with-cpu=power8.

Pre-tested using a ppc-koji scratch build and reviewed the final builds.

------- Comment From tulioqm.com 2016-02-09 07:47 EDT-------
> However, perhaps you can answer a question I had. In
> general all of the functions in the power8/ sysdep directory appear to be
> routed to by IFUNCs anyway, so you will get their benefit in a generic build
> also? However, because it isn't guaranteed that all functions have IFUNC
> selectors you generally want to build with --with-cpu=powerx for POWERx
> (where x is an integer value).

Not for all of the functions right now.  That's especially important for math functions which still lack IFUNC support.

Thanks Florian and Carlos!