Re: [AARCH64] Disable pc relative literal load irrespective of TARGET_FIX_ERR_A53_84341

[James Greenhalgh <james dot greenhalgh at arm dot com>]

On Wed, Mar 07, 2018 at 01:58:40AM +0000, Kugan Vivekanandarajah wrote:
> Hi James,
> 
> This patch has to be backported to gcc-7 branch as the build error for
> 521.wrf  with LTO is there too (for the same reason). This patch has
> been on trunk for some time now. So, is this  OK to backport this
> patch gcc-7 branch?

OK.

Thanks,
James

> On 30 August 2017 at 15:19, Kugan Vivekanandarajah
> <kugan.vivekanandarajah@linaro.org> wrote:
> > Hi James,
> >
> > On 29 August 2017 at 21:31, James Greenhalgh <james.greenhalgh@arm.com> wrote:
> >> On Tue, Jun 27, 2017 at 11:20:02AM +1000, Kugan Vivekanandarajah wrote:
> >>> https://gcc.gnu.org/ml/gcc-patches/2016-03/msg00614.html  added this
> >>> workaround to get kernel building with when TARGET_FIX_ERR_A53_843419
> >>> is enabled.
> >>>
> >>> This was added to support building kernel loadable modules. In kernel,
> >>> when CONFIG_ARM64_ERRATUM_843419 is selected, the relocation needed
> >>> for ADRP/LDR (R_AARCH64_ADR_PREL_PG_HI21 and
> >>> R_AARCH64_ADR_PREL_PG_HI21_NC are removed from the kernel to avoid
> >>> loading objects with possibly offending sequence). Thus, it could only
> >>> support pc relative literal loads.
> >>>
> >>> However, the following patch was posted to kernel to add
> >>> -mpc-relative-literal-loads
> >>> http://www.spinics.net/lists/arm-kernel/msg476149.html
> >>>
> >>> -mpc-relative-literal-loads is unconditionally added to the kernel
> >>> build as can be seen from:
> >>> https://github.com/torvalds/linux/blob/master/arch/arm64/Makefile
> >>>
> >>> Therefore this patch removes the hunk so that applications like
> >>> SPECcpu2017's 521/621.wrf can be built (with LTO in this case) without
> >>> -mno-pc-relative-literal-loads
> >>>
> >>> Bootstrapped and regression tested on aarch64-linux-gnu with no new regressions.
> >>>
> >>> Is this OK for trunk?
> >>
> >> Hi Kugan,
> >>
> >> I'm struggling a little to convince myself that this is correct. I think
> >> the argument is that this was a workaround for one very particular issue
> >> with the linux kernel module loader, which hard faults by refusing to
> >> handle these relocations when in a workaround mode for Erratum 843419.
> >
> > Yes.
> >
> >> Most of these relocations won't occur because the kernel builds with
> >> -mcmodel=large, but literals would always use a small model style
> >> adrp/load, unless we were using -mpc-relative-literal-loads . So, this
> >> workaround enabled -mpc-relative-literal-loads always when we were in
> >> a workaround mode, thus allowing the kernel loader to continue.
> >>
> >> The argument for removing it then, is that with the kernel now always using
> >> -mpc-relative-literal-loads there is no reason for this workaround to stay
> >> in place. The linkers which we will use will apply the workaround if needed.
> >
> > Yes.
> >
> >> Testcases and a detailed problem report of the build failures you had seen in
> >> 521.wrf would have helped me get closer to understanding this, and made
> >> review substantially easier.
> >
> > Sorry for not being clear with this. Unfortunately 521.wrf  was
> > showing this error with LTO so I could not reproduce with a small
> > enough test case.
> >
> >> Am I on the right track?
> >>
> >> If so, this is OK for trunk. If not, please can you expand on what is going
> >> on in this patch.
> >
> > Thanks for the review.
> > Kugan
> >
> >>
> >> Thanks,
> >> James
> >>
> >>
> >>>
> >>> Thanks,
> >>> Kugan
> >>>
> >>> gcc/testsuite/ChangeLog:
> >>>
> >>> 2017-06-27  Kugan Vivekanandarajah  <kuganv@linaro.org>
> >>>
> >>>     * gcc.target/aarch64/pr63304_1.c: Remove-mno-fix-cortex-a53-843419.
> >>>
> >>> gcc/ChangeLog:
> >>>
> >>> 2017-06-27  Kugan Vivekanandarajah  <kuganv@linaro.org>
> >>>
> >>>     * config/aarch64/aarch64.c (aarch64_override_options_after_change_1):
> >>>     Disable pc relative literal load irrespective of TARGET_FIX_ERR_A53_84341
> >>>     for default.
> >>
> >>