[RFC] minimum gcc version for kernel: raise to gcc-4.3 or 4.6?

[Arnd Bergmann]

I had some fun doing build testing with older gcc versions, building
every release from 4.0 through 7.0 and running that on my randconfig
setup to see what comes out.

First of all, gcc-4.9 and higher is basically warning-free everywhere,
although gcc-7 introduces some interesting new warnings (I have started
doing patches for those as well). gcc-4.8 is probably good, too, and
gcc-4.6 and 4.7 at least don't produce build failures in general, though
the level of false-positive warnings increases (we could decide to turn
those off for older compilers for build test purposes).

In gcc-4.5 and below, dead code elimination is not as good as later,
causing a couple of link errors, and some of them have no good workaround
(see patch 1). It would be nice to declare that version too old, but
several older distros that are still in wide use ship with compilers
earlier than 4.6:

 RHEL6:		gcc-4.4
 Debian 6:	gcc-4.4
 Ubuntu 10.04:	gcc-4.4
 SLES11:	gcc-4.3

With gcc-4.3, we need a couple of workaround patches beyond the problem
mentioned above, more configuration options are unavailable and we get
a significant number of false-positive warnings, but it's not much worse
than gcc-4.5 otherwise.

These are the options I had to disable to get gcc-4.3 randconfig builds
working:

 CONFIG_HAVE_GCC_PLUGINS
 CONFIG_CC_STACKPROTECTOR_STRONG
 CONFIG_ARM_SINGLE_ARMV7M
 CONFIG_THUMB2_KERNEL
 CONFIG_KERNEL_MODE_NEON
 CONFIG_VDSO
 CONFIG_FUNCTION_TRACER (with CONFIG_FRAME_POINTER=n)

I have not checked in detail which version is required for
each of the above.

Specifically on ARM, going further makes things rather useless especially
for build testing: with gcc-4.2, we lose support for ARMv7, EABI, and
effectively ARMv6 (as it relies on EABI for building reliably). Also,
the number of false-positive build warnings is so high that it is useless
for finding actual bugs from the warnings.

See the replies to this mail for 13 patches I needed to work around
issues for each of the releases before 4.6. I have also submitted
some separate patches for issues that I considered actual bugs
uncovered by the older compilers and that should be applied regardless.

The original gcc-4.3 release was in early 2008. If we decide to still
support that, we probably want the first 10 quirks in this series,
while gcc-4.6 (released in 2011) requires none of them.

	Arnd

Arnd Bergmann (13):
  [HACK] gcc-4.5: avoid  link errors for unused function pointers
  KVM: arm: fix gcc-4.5 build
  ARM: div64: fix building with gcc-4.5 and lower
  vfio-pci: use 32-bit comparisons for register address for gcc-4.5
  clk: pxa: fix gcc-4.4 build
  ARM: atomic: fix gcc-4.4 build
  watchdog: kempld: fix gcc-4.3 build
  arm/arm64: xen: avoid gcc-4.4 warning
  ARM: mark cmpxchg and xchg __always_inline for gcc-4.3
  asm-generic: mark cmpxchg as __always_inline for gcc-4.3
  fs: fix unsigned enum warning with gcc-4.2
  KVM: arm: avoid binary number literals for gcc-4.2
  ARM: avoid 'Q' asm constraint for gcc-4.1 and earlier

 arch/arm/include/asm/atomic.h        | 10 ++++++++--
 arch/arm/include/asm/cmpxchg.h       | 12 ++++++------
 arch/arm/include/asm/div64.h         | 17 +++--------------
 arch/arm/include/asm/io.h            |  8 ++++++++
 arch/arm/include/asm/kvm_mmu.h       |  2 +-
 arch/arm/include/asm/percpu.h        |  5 ++++-
 arch/arm/mach-imx/pm-imx5.c          | 20 ++++++++++++++++----
 arch/arm/mach-sa1100/pm.c            |  2 ++
 arch/arm/plat-samsung/pm.c           |  4 ++++
 drivers/clk/pxa/clk-pxa.c            |  3 +--
 drivers/dma/ti-dma-crossbar.c        |  4 ++++
 drivers/firmware/psci_checker.c      |  3 +++
 drivers/iio/adc/exynos_adc.c         |  3 +++
 drivers/net/ethernet/via/via-rhine.c |  6 ++++++
 drivers/vfio/pci/vfio_pci_rdwr.c     |  5 ++++-
 drivers/watchdog/kempld_wdt.c        |  9 ++++++++-
 include/asm-generic/cmpxchg-local.h  |  7 ++++---
 include/linux/fs.h                   |  2 +-
 include/xen/arm/page.h               |  1 +
 virt/kvm/arm/vgic/vgic-its.c         |  4 ++--
 virt/kvm/arm/vgic/vgic-mmio-v3.c     |  8 ++++----
 virt/kvm/arm/vgic/vgic-mmio.c        | 16 ++++++++--------
 virt/kvm/arm/vgic/vgic-mmio.h        | 12 ++++++------
 23 files changed, 107 insertions(+), 56 deletions(-)

-- 
2.9.0