Re: [PATCH] kbuild: Change fallthrough comments to attributes
From: Nick Desaulniers
Date: Mon Aug 12 2019 - 19:06:34 EST
On Mon, Aug 12, 2019 at 2:48 PM 'Nathan Huckleberry' via Clang Built
Linux <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
>
> Clang does not support the use of comments to label
> intentional fallthrough. This patch replaces some uses
> of comments to attributesto cut down a significant number
> of warnings on clang (from ~50000 to ~200). Only comments
> in commonly used header files have been replaced.
>
> Since there is still quite a bit of noise, this
> patch moves -Wimplicit-fallthrough to
> Makefile.extrawarn if you are compiling with
> clang.
>
> Signed-off-by: Nathan Huckleberry <nhuck@xxxxxxxxxx>
> ---
> Makefile | 4 +++
> include/linux/jhash.h | 60 ++++++++++++++++++++++++++++----------
> include/linux/mm.h | 9 ++++--
> include/linux/signal.h | 14 +++++----
> include/linux/skbuff.h | 12 ++++----
> lib/zstd/bitstream.h | 10 +++----
> scripts/Makefile.extrawarn | 3 ++
> 7 files changed, 77 insertions(+), 35 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index c391fbb07195..875930c19619 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -847,7 +847,11 @@ NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
> KBUILD_CFLAGS += -Wdeclaration-after-statement
>
> # Warn about unmarked fall-throughs in switch statement.
> +# If the compiler is clang, this warning is only enabled if W=1 in
> +# Makefile.extrawarn
> +ifndef CONFIG_CC_IS_CLANG
> KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough,)
This should go in the block higher up (see line 739) that already
exists for CONFIG_CC_IS_CLANG.
> +endif
>
> # Variable Length Arrays (VLAs) should not be used anywhere in the kernel
> KBUILD_CFLAGS += -Wvla
> diff --git a/include/linux/jhash.h b/include/linux/jhash.h
> index ba2f6a9776b6..6cb2381501d1 100644
> --- a/include/linux/jhash.h
> +++ b/include/linux/jhash.h
Probably should split this patch into:
1. Makefile and scripts/Makefile.extrawarn hunks
2. hunks for each other file that can be applied individually (as
makes sense for the maintainers' trees).
> @@ -86,19 +86,43 @@ static inline u32 jhash(const void *key, u32 length, u32 initval)
> }
> /* Last block: affect all 32 bits of (c) */
> switch (length) {
> - case 12: c += (u32)k[11]<<24; /* fall through */
> - case 11: c += (u32)k[10]<<16; /* fall through */
> - case 10: c += (u32)k[9]<<8; /* fall through */
> - case 9: c += k[8]; /* fall through */
> - case 8: b += (u32)k[7]<<24; /* fall through */
> - case 7: b += (u32)k[6]<<16; /* fall through */
> - case 6: b += (u32)k[5]<<8; /* fall through */
> - case 5: b += k[4]; /* fall through */
> - case 4: a += (u32)k[3]<<24; /* fall through */
> - case 3: a += (u32)k[2]<<16; /* fall through */
> - case 2: a += (u32)k[1]<<8; /* fall through */
> - case 1: a += k[0];
> + case 12:
> + c += (u32)k[11]<<24;
> + __attribute__((fallthrough));
> + case 11:
> + c += (u32)k[10]<<16;
> + __attribute__((fallthrough));
> + case 10:
> + c += (u32)k[9]<<8;
> + __attribute__((fallthrough));
> + case 9:
> + c += k[8];
> + __attribute__((fallthrough));
> + case 8:
> + b += (u32)k[7]<<24;
> + __attribute__((fallthrough));
> + case 7:
> + b += (u32)k[6]<<16;
> + __attribute__((fallthrough));
> + case 6:
> + b += (u32)k[5]<<8;
> + __attribute__((fallthrough));
> + case 5:
> + b += k[4];
> + __attribute__((fallthrough));
> + case 4:
> + a += (u32)k[3]<<24;
> + __attribute__((fallthrough));
> + case 3:
> + a += (u32)k[2]<<16;
> + __attribute__((fallthrough));
> + case 2:
> + a += (u32)k[1]<<8;
> + __attribute__((fallthrough));
> + case 1:
> + a += k[0];
> __jhash_final(a, b, c);
> + break;
> case 0: /* Nothing left to add */
> break;
> }
> @@ -132,10 +156,16 @@ static inline u32 jhash2(const u32 *k, u32 length, u32 initval)
>
> /* Handle the last 3 u32's */
> switch (length) {
> - case 3: c += k[2]; /* fall through */
> - case 2: b += k[1]; /* fall through */
> - case 1: a += k[0];
> + case 3:
> + c += k[2];
> + __attribute__((fallthrough));
> + case 2:
> + b += k[1];
> + __attribute__((fallthrough));
> + case 1:
> + a += k[0];
> __jhash_final(a, b, c);
> + break;
> case 0: /* Nothing left to add */
> break;
> }
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 0334ca97c584..52d99f263ca3 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -158,11 +158,14 @@ static inline void __mm_zero_struct_page(struct page *page)
>
> switch (sizeof(struct page)) {
> case 80:
> - _pp[9] = 0; /* fallthrough */
> + _pp[9] = 0;
> + __attribute__((fallthrough));
> case 72:
> - _pp[8] = 0; /* fallthrough */
> + _pp[8] = 0;
> + __attribute__((fallthrough));
> case 64:
> - _pp[7] = 0; /* fallthrough */
> + _pp[7] = 0;
> + __attribute__((fallthrough));
> case 56:
> _pp[6] = 0;
> _pp[5] = 0;
> diff --git a/include/linux/signal.h b/include/linux/signal.h
> index b5d99482d3fe..4fb0a0041a37 100644
> --- a/include/linux/signal.h
> +++ b/include/linux/signal.h
> @@ -129,11 +129,11 @@ static inline void name(sigset_t *r, const sigset_t *a, const sigset_t *b) \
> b3 = b->sig[3]; b2 = b->sig[2]; \
> r->sig[3] = op(a3, b3); \
> r->sig[2] = op(a2, b2); \
> - /* fall through */ \
> + __attribute__((fallthrough)); \
> case 2: \
> a1 = a->sig[1]; b1 = b->sig[1]; \
> r->sig[1] = op(a1, b1); \
> - /* fall through */ \
> + __attribute__((fallthrough)); \
> case 1: \
> a0 = a->sig[0]; b0 = b->sig[0]; \
> r->sig[0] = op(a0, b0); \
> @@ -163,9 +163,9 @@ static inline void name(sigset_t *set) \
> switch (_NSIG_WORDS) { \
> case 4: set->sig[3] = op(set->sig[3]); \
> set->sig[2] = op(set->sig[2]); \
> - /* fall through */ \
> + __attribute__((fallthrough)); \
> case 2: set->sig[1] = op(set->sig[1]); \
> - /* fall through */ \
> + __attribute__((fallthrough)); \
> case 1: set->sig[0] = op(set->sig[0]); \
> break; \
> default: \
> @@ -186,7 +186,7 @@ static inline void sigemptyset(sigset_t *set)
> memset(set, 0, sizeof(sigset_t));
> break;
> case 2: set->sig[1] = 0;
> - /* fall through */
> + __attribute__((fallthrough));
> case 1: set->sig[0] = 0;
> break;
> }
> @@ -199,7 +199,7 @@ static inline void sigfillset(sigset_t *set)
> memset(set, -1, sizeof(sigset_t));
> break;
> case 2: set->sig[1] = -1;
> - /* fall through */
> + __attribute__((fallthrough));
> case 1: set->sig[0] = -1;
> break;
> }
> @@ -230,6 +230,7 @@ static inline void siginitset(sigset_t *set, unsigned long mask)
> memset(&set->sig[1], 0, sizeof(long)*(_NSIG_WORDS-1));
> break;
> case 2: set->sig[1] = 0;
> + __attribute__((fallthrough));
> case 1: ;
> }
> }
> @@ -242,6 +243,7 @@ static inline void siginitsetinv(sigset_t *set, unsigned long mask)
> memset(&set->sig[1], -1, sizeof(long)*(_NSIG_WORDS-1));
> break;
> case 2: set->sig[1] = -1;
> + __attribute__((fallthrough));
> case 1: ;
> }
> }
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index d8af86d995d6..4a1df6714dfe 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -3639,19 +3639,19 @@ static inline bool __skb_metadata_differs(const struct sk_buff *skb_a,
> #define __it(x, op) (x -= sizeof(u##op))
> #define __it_diff(a, b, op) (*(u##op *)__it(a, op)) ^ (*(u##op *)__it(b, op))
> case 32: diffs |= __it_diff(a, b, 64);
> - /* fall through */
> + __attribute__((fallthrough));
> case 24: diffs |= __it_diff(a, b, 64);
> - /* fall through */
> + __attribute__((fallthrough));
> case 16: diffs |= __it_diff(a, b, 64);
> - /* fall through */
> + __attribute__((fallthrough));
> case 8: diffs |= __it_diff(a, b, 64);
> break;
> case 28: diffs |= __it_diff(a, b, 64);
> - /* fall through */
> + __attribute__((fallthrough));
> case 20: diffs |= __it_diff(a, b, 64);
> - /* fall through */
> + __attribute__((fallthrough));
> case 12: diffs |= __it_diff(a, b, 64);
> - /* fall through */
> + __attribute__((fallthrough));
> case 4: diffs |= __it_diff(a, b, 32);
> break;
> }
> diff --git a/lib/zstd/bitstream.h b/lib/zstd/bitstream.h
> index 3a49784d5c61..cc311bae44da 100644
> --- a/lib/zstd/bitstream.h
> +++ b/lib/zstd/bitstream.h
> @@ -259,15 +259,15 @@ ZSTD_STATIC size_t BIT_initDStream(BIT_DStream_t *bitD, const void *srcBuffer, s
> bitD->bitContainer = *(const BYTE *)(bitD->start);
> switch (srcSize) {
> case 7: bitD->bitContainer += (size_t)(((const BYTE *)(srcBuffer))[6]) << (sizeof(bitD->bitContainer) * 8 - 16);
> - /* fall through */
> + __attribute__((fallthrough));
> case 6: bitD->bitContainer += (size_t)(((const BYTE *)(srcBuffer))[5]) << (sizeof(bitD->bitContainer) * 8 - 24);
> - /* fall through */
> + __attribute__((fallthrough));
> case 5: bitD->bitContainer += (size_t)(((const BYTE *)(srcBuffer))[4]) << (sizeof(bitD->bitContainer) * 8 - 32);
> - /* fall through */
> + __attribute__((fallthrough));
> case 4: bitD->bitContainer += (size_t)(((const BYTE *)(srcBuffer))[3]) << 24;
> - /* fall through */
> + __attribute__((fallthrough));
> case 3: bitD->bitContainer += (size_t)(((const BYTE *)(srcBuffer))[2]) << 16;
> - /* fall through */
> + __attribute__((fallthrough));
> case 2: bitD->bitContainer += (size_t)(((const BYTE *)(srcBuffer))[1]) << 8;
> default:;
> }
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index a74ce2e3c33e..e12359d69bb7 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -30,6 +30,9 @@ warning-1 += $(call cc-option, -Wunused-but-set-variable)
> warning-1 += $(call cc-option, -Wunused-const-variable)
> warning-1 += $(call cc-option, -Wpacked-not-aligned)
> warning-1 += $(call cc-option, -Wstringop-truncation)
> +ifdef CONFIG_CC_IS_CLANG
> +KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough,)
copy+pasta? Should this be:
warning-1 += ...
further, there's a block at the end of this file already for
CONFIG_CC_IS_CLANG that this should go in. Unlike the rest of the
items there, you SHOULD keep the call to cc-option since older
versions of Clang won't support the warning.
Thanks for the patch!
--
Thanks,
~Nick Desaulniers