| Message ID | 348c2d3f19ffcff8abe50d52513f989c4581d000.1603375524.git.christophe.leroy@csgroup.eu (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | 1891ef21d92c4801ea082ee8ed478e304ddc6749 |
| Headers | show |
| Series | powerpc/bitops: Fix possible undefined behaviour with fls() and fls64() | expand |
| Context | Check | Description |
|---|---|---|
| snowpatch_ozlabs/apply_patch | success | Successfully applied on branch powerpc/merge (598b738031de83cadbcf745ad0ad2bd69a0ee012) |
| snowpatch_ozlabs/build-ppc64le | warning | Upstream build failed, couldn't test patch |
| snowpatch_ozlabs/build-ppc64be | warning | Upstream build failed, couldn't test patch |
| snowpatch_ozlabs/build-ppc64e | warning | Upstream build failed, couldn't test patch |
| snowpatch_ozlabs/build-pmac32 | warning | Upstream build failed, couldn't test patch |
| snowpatch_ozlabs/checkpatch | success | |
| snowpatch_ozlabs/needsstable | success | Patch is tagged for stable |
On Thu, Oct 22, 2020 at 02:05:46PM +0000, Christophe Leroy wrote: > fls() and fls64() are using __builtin_ctz() and _builtin_ctzll(). > On powerpc, those builtins trivially use ctlzw and ctlzd power > instructions. > > Allthough those instructions provide the expected result with > input argument 0, __builtin_ctz() and __builtin_ctzll() are > documented as undefined for value 0. > When the input of fls(x) is a constant, just check x for nullity and > return either 0 or __builtin_clz(x). Otherwise, use cntlzw instruction > directly. That looks good :-) Acked-by: Segher Boessenkool <segher@kernel.crashing.org> Segher
On Thu, 22 Oct 2020 14:05:46 +0000 (UTC), Christophe Leroy wrote: > fls() and fls64() are using __builtin_ctz() and _builtin_ctzll(). > On powerpc, those builtins trivially use ctlzw and ctlzd power > instructions. > > Allthough those instructions provide the expected result with > input argument 0, __builtin_ctz() and __builtin_ctzll() are > documented as undefined for value 0. > > [...] Applied to powerpc/next. [1/1] powerpc/bitops: Fix possible undefined behaviour with fls() and fls64() https://git.kernel.org/powerpc/c/1891ef21d92c4801ea082ee8ed478e304ddc6749 cheers
diff --git a/arch/powerpc/include/asm/bitops.h b/arch/powerpc/include/asm/bitops.h index 4a4d3afd5340..299ab33505a6 100644 --- a/arch/powerpc/include/asm/bitops.h +++ b/arch/powerpc/include/asm/bitops.h @@ -216,15 +216,34 @@ static inline void arch___clear_bit_unlock(int nr, volatile unsigned long *addr) */ static inline int fls(unsigned int x) { - return 32 - __builtin_clz(x); + int lz; + + if (__builtin_constant_p(x)) + return x ? 32 - __builtin_clz(x) : 0; + asm("cntlzw %0,%1" : "=r" (lz) : "r" (x)); + return 32 - lz; } #include <asm-generic/bitops/builtin-__fls.h> +/* + * 64-bit can do this using one cntlzd (count leading zeroes doubleword) + * instruction; for 32-bit we use the generic version, which does two + * 32-bit fls calls. + */ +#ifdef CONFIG_PPC64 static inline int fls64(__u64 x) { - return 64 - __builtin_clzll(x); + int lz; + + if (__builtin_constant_p(x)) + return x ? 64 - __builtin_clzll(x) : 0; + asm("cntlzd %0,%1" : "=r" (lz) : "r" (x)); + return 64 - lz; } +#else +#include <asm-generic/bitops/fls64.h> +#endif #ifdef CONFIG_PPC64 unsigned int __arch_hweight8(unsigned int w);
fls() and fls64() are using __builtin_ctz() and _builtin_ctzll(). On powerpc, those builtins trivially use ctlzw and ctlzd power instructions. Allthough those instructions provide the expected result with input argument 0, __builtin_ctz() and __builtin_ctzll() are documented as undefined for value 0. The easiest fix would be to use fls() and fls64() functions defined in include/asm-generic/bitops/builtin-fls.h and include/asm-generic/bitops/fls64.h, but GCC output is not optimal: 00000388 <testfls>: 388: 2c 03 00 00 cmpwi r3,0 38c: 41 82 00 10 beq 39c <testfls+0x14> 390: 7c 63 00 34 cntlzw r3,r3 394: 20 63 00 20 subfic r3,r3,32 398: 4e 80 00 20 blr 39c: 38 60 00 00 li r3,0 3a0: 4e 80 00 20 blr 000003b0 <testfls64>: 3b0: 2c 03 00 00 cmpwi r3,0 3b4: 40 82 00 1c bne 3d0 <testfls64+0x20> 3b8: 2f 84 00 00 cmpwi cr7,r4,0 3bc: 38 60 00 00 li r3,0 3c0: 4d 9e 00 20 beqlr cr7 3c4: 7c 83 00 34 cntlzw r3,r4 3c8: 20 63 00 20 subfic r3,r3,32 3cc: 4e 80 00 20 blr 3d0: 7c 63 00 34 cntlzw r3,r3 3d4: 20 63 00 40 subfic r3,r3,64 3d8: 4e 80 00 20 blr When the input of fls(x) is a constant, just check x for nullity and return either 0 or __builtin_clz(x). Otherwise, use cntlzw instruction directly. For fls64() on PPC64, do the same but with __builtin_clzll() and cntlzd instruction. On PPC32, lets take the generic fls64() which will use our fls(). The result is as expected: 00000388 <testfls>: 388: 7c 63 00 34 cntlzw r3,r3 38c: 20 63 00 20 subfic r3,r3,32 390: 4e 80 00 20 blr 000003a0 <testfls64>: 3a0: 2c 03 00 00 cmpwi r3,0 3a4: 40 82 00 10 bne 3b4 <testfls64+0x14> 3a8: 7c 83 00 34 cntlzw r3,r4 3ac: 20 63 00 20 subfic r3,r3,32 3b0: 4e 80 00 20 blr 3b4: 7c 63 00 34 cntlzw r3,r3 3b8: 20 63 00 40 subfic r3,r3,64 3bc: 4e 80 00 20 blr Fixes: 2fcff790dcb4 ("powerpc: Use builtin functions for fls()/__fls()/fls64()") Cc: stable@vger.kernel.org Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu> --- arch/powerpc/include/asm/bitops.h | 23 +++++++++++++++++++++-- 1 file changed, 21 insertions(+), 2 deletions(-)