A bug in the gcc-6.0 prerelease version caused at least one driver (lpfc) to have excessive stack usage when dealing with wwn data, on the ARM architecture. lpfc_scsi.c: In function 'lpfc_find_next_oas_lun': lpfc_scsi.c:117:1: warning: the frame size of 1152 bytes is larger than 1024 bytes [-Wframe-larger-than=] I have reported this as a gcc regression in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70232 However, using a better implementation of wwn_to_u64() not only helps with the particular gcc problem but also leads to better object code for any version or architecture. The kernel already provides get_unaligned_be64() and put_unaligned_be64() helper functions that provide an optimized implementation with the desired semantics. The lpfc_find_next_oas_lun() function in the example that grew from 1146 bytes to 5144 bytes when moving from gcc-5.3 to gcc-6.0 is now 804 bytes, as the optimized get_unaligned_be64() load can be done in three instructions. The stack usage is now down to 28 bytes from 128 bytes with gcc-5.3 before. Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx> --- include/scsi/scsi_transport_fc.h | 15 +++------------ 1 file changed, 3 insertions(+), 12 deletions(-) diff --git a/include/scsi/scsi_transport_fc.h b/include/scsi/scsi_transport_fc.h index 784bc2c0929f..bf66ea6bed2b 100644 --- a/include/scsi/scsi_transport_fc.h +++ b/include/scsi/scsi_transport_fc.h @@ -28,6 +28,7 @@ #define SCSI_TRANSPORT_FC_H #include <linux/sched.h> +#include <asm/unaligned.h> #include <scsi/scsi.h> #include <scsi/scsi_netlink.h> @@ -797,22 +798,12 @@ fc_remote_port_chkready(struct fc_rport *rport) static inline u64 wwn_to_u64(u8 *wwn) { - return (u64)wwn[0] << 56 | (u64)wwn[1] << 48 | - (u64)wwn[2] << 40 | (u64)wwn[3] << 32 | - (u64)wwn[4] << 24 | (u64)wwn[5] << 16 | - (u64)wwn[6] << 8 | (u64)wwn[7]; + return get_unaligned_be64(wwn); } static inline void u64_to_wwn(u64 inm, u8 *wwn) { - wwn[0] = (inm >> 56) & 0xff; - wwn[1] = (inm >> 48) & 0xff; - wwn[2] = (inm >> 40) & 0xff; - wwn[3] = (inm >> 32) & 0xff; - wwn[4] = (inm >> 24) & 0xff; - wwn[5] = (inm >> 16) & 0xff; - wwn[6] = (inm >> 8) & 0xff; - wwn[7] = inm & 0xff; + put_unaligned_be64(inm, wwn); } /** -- 2.7.0 -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
