Re: [RFC] clang: 'unused-function' warning on static inline functions

[Arnd Bergmann <arnd@arndb.de>]

On Thu, Jun 1, 2017 at 1:55 AM, Matthias Kaehlcke <mka@chromium.org> wrote:
>> ASoC: Intel: sst: Delete sst_shim_regs64; saved regs are never used
>> https://patchwork.kernel.org/patch/9754923/
>>
>> The goal of this thread is to arrive to a conclusion on how to deal
>> with the warning. If we want clang to help to detect unused static
>> inline functions I think we should suppress the warning on false
>> positives (the number is limited, at least for x86 and arm64
>> defconfig, patches for most instances have already been sent out). And
>> if maintainers consider that having the extra ability to spot dead
>> code doesn't justify the clutter of marking some functions as
>> __maybe_used (or using #ifdef if preferred) we should probably apply
>> Davids patch (https://patchwork.kernel.org/patch/9746913/) to make
>> clang behave like gcc for unused static inline functions.
>
> As suggested in one of the other threads, a list of the false
> positives (from x86 and arm64 defconfig) that would need attention in
> case of leaving the warning enabled:
>
> arch/x86/kernel/cpu/common.c:267:19: warning: unused function 'flag_is_changeable_p' [-Wunused-function]
> arch/x86/platform/efi/efi_64.c:240:1: warning: unused function 'virt_to_phys_or_null_size' [-Wunused-function]
> block/cfq-iosched.c:451:1: warning: unused function 'cfq_clear_cfqq_sync' [-Wunused-function]
> block/cfq-iosched.c:590:20: warning: unused function 'cfqg_stats_set_start_group_wait_time' [-Wunused-function]
> block/cfq-iosched.c:591:20: warning: unused function 'cfqg_stats_end_empty_time' [-Wunused-function]
> drivers/gpu/drm/drm_mm.c:165:1: warning: unused function 'drm_mm_interval_tree_insert' [-Wunused-function]
> drivers/gpu/drm/drm_mm.c:165:1: warning: unused function 'drm_mm_interval_tree_iter_next' [-Wunused-function]
> drivers/gpu/drm/i915/i915_sw_fence.c:97:20: warning: unused function 'debug_fence_free' [-Wunused-function]
> drivers/hid/hid-sony.c:2105:20: warning: unused function 'sony_send_output_report' [-Wunused-function]
> drivers/media/media-entity.c:41:27: warning: unused function 'intf_type' [-Wunused-function]
> drivers/watchdog/s3c2410_wdt.c:212:35: warning: unused function 'freq_to_wdt' [-Wunused-function]
> drivers/watchdog/s3c2410_wdt.c:308:19: warning: unused function 's3c2410wdt_is_running' [-Wunused-function]
> kernel/events/core.c:928:19: warning: unused function 'perf_cgroup_event_cgrp_time' [-Wunused-function]
> kernel/locking/osq_lock.c:24:19: warning: unused function 'node_cpu' [-Wunused-function]
> kernel/power/snapshot.c:1256:21: warning: unused function 'saveable_highmem_page' [-Wunused-function]
> kernel/sched/cputime.c:258:19: warning: unused function 'account_other_time' [-Wunused-function]
> lib/zlib_inflate/inffast.c:31:1: warning: unused function 'get_unaligned16' [-Wunused-function]
> mm/page_alloc.c:1312:30: warning: unused function 'meminit_pfn_in_nid' [-Wunused-function]
> mm/slub.c:1328:21: warning: unused function 'slab_free_hook' [-Wunused-function]
> mm/slub.c:1945:28: warning: unused function 'tid_to_cpu' [-Wunused-function]
> mm/slub.c:1950:29: warning: unused function 'tid_to_event' [-Wunused-function]
> net/netfilter/nf_conntrack_netlink.c:534:22: warning: unused function 'ctnetlink_proto_size' [-Wunused-function]
> net/netfilter/nf_conntrack_netlink.c:551:22: warning: unused function 'ctnetlink_acct_size' [-Wunused-function]
> net/netfilter/nf_conntrack_netlink.c:561:19: warning: unused function 'ctnetlink_secctx_size' [-Wunused-function]
> net/netfilter/nf_conntrack_netlink.c:577:22: warning: unused function 'ctnetlink_timestamp_size' [-Wunused-function]
>
> The list does not include instances that have already been addressed
> in maintainer trees or warnings about actual dead code. Obviously
> there will be more instances for other architectures and
> configurations.

Those should all be fairly easy to address, I'd vote for leaving the
warning enabled
in clang, and possibly asking the gcc maintainers to add a similar feature for
warning about it.

On a more general note, I'd suggest classifying clang warning options
like this as 'W=1'
until we have addressed the known output, i.e. move all the warnings
we currently avoid
with clang into scripts/Makefile.extrawarn so that they are active
when building with
'make W=1' but disabled by default, leading to a clean build with a
plain 'make -s'.

I have started a patch set to eventually move all warning options into a new
include/linux/compiler-warnings.h that would allow a more convenient way of
controlling the warning options depending on the compiler version, e.g. some
warnings are useful with recent gcc versions but show lots of clutter
when enabled
on older versions.

        Arnd