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 -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
