Subject: Re: selftests/powerpc: Remove -flto from common CFLAGS

From: Cyril Bur <[email-masked]>
Date: Tue, 1 Mar 2016 10:08:25 +1100

On Mon, 29 Feb 2016 22:10:13 +1100 (AEDT)
Michael Ellerman <[email-masked]> wrote:

> Hi Suraj,
> 
> On Mon, 2016-29-02 at 06:29:55 UTC, Suraj Jitindar Singh wrote:
> > LTO can cause GCC to inline some functions which have attributes set. The  
> 
> You should define what LTO is the first time you use it.
> 
> > act of inlining the functions can lead to GCC forgetting about the
> > attributes which leads to incorrect tests.
> > Notable example being: __attribute__((__target__("no-vsx")))  
> 
> That is probably a GCC bug, but we still need to work around it for now.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70010

> 
> > LTO can also interact strangely with custom assembly functions and cause
> > tests to intermittently fail.  
> 
> That's probably Cyril writing bad asm :)

No doubt.

> 
> > Both these cases are hard to detect and require manual inspection of
> > binaries which is unlikely to happen for all tests. Furthermore, LTO
> > optimisations are not necessary for selftests and correctness is paramount
> > and as such it is best to disable LTO.
> > 
> > LTO can be enabled on a per test basis.
> > 
> > A pseries_le_defconfig kernel on a POWER8 was used to determine that the
> > same subset of selftests pass and fail with and without -flto in the
> > common Makefile.
> > 
> > These tests always fail:
> > selftests: per_event_excludes [FAIL]
> > selftests: event_attributes_test [FAIL]
> > selftests: ebb_vs_cpu_event_test [FAIL]
> > selftests: cpu_event_vs_ebb_test [FAIL]
> > selftests: cpu_event_pinned_vs_ebb_test [FAIL]  
> 
> They shouldn't :)
> 
> Are you running as root? Bare metal or guest?
> 

Interesting. I believe this was run baremetal as root. I'm going to test the
patch in qemu at root. Is there a list of expected failures in certain
situations?

> > selftests: ipc_unmuxed [FAIL]  
> 
> That one is expected.
> 
> cheers
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