On Thu, Nov 09 2017, Linus Torvalds torvalds@linux-foundation.org wrote:
The code disassembles to
0: 83 c9 08 or $0x8,%ecx 3: 40 f6 c6 04 test $0x4,%sil 7: 0f 45 d1 cmovne %ecx,%edx a: 89 d1 mov %edx,%ecx c: 80 cd 04 or $0x4,%ch f: 40 f6 c6 08 test $0x8,%sil 13: 0f 45 d1 cmovne %ecx,%edx 16: 89 d1 mov %edx,%ecx 18: 80 cd 08 or $0x8,%ch 1b: 40 f6 c6 10 test $0x10,%sil 1f: 0f 45 d1 cmovne %ecx,%edx 22: 89 d1 mov %edx,%ecx 24: 80 cd 10 or $0x10,%ch 27: 83 e6 20 and $0x20,%esi 2a:* 48 8b b7 30 02 00 00 mov 0x230(%rdi),%rsi <-- trapping instruction 31: 0f 45 d1 cmovne %ecx,%edx 34: 83 ca 20 or $0x20,%edx 37: 89 f1 mov %esi,%ecx 39: 83 e1 10 and $0x10,%ecx 3c: 89 cf mov %ecx,%edi
and all those odd cmovne and bit-ops are just the bit selection code in flags_by_mnt(), which is inlined through calculate_f_flags (which is _also_ inlined) into vfs_statfs().
Sadly, gcc makes a mess of it and actually generates code that looks like the original C. I would have hoped that gcc could have turned
if (x & BIT) y |= OTHER_BIT;
into
y |= (x & BIT) shifted-by-the-bit-difference-between BIT/OTHER_BIT;but that doesn't happen.
Actually, new enough gcc (7.1, I think) does contain a pattern that does this, but unfortunately only if one spells it
y |= (x & BIT) ? OTHER_BIT : 0;
which is half-way to doing it by hand, I suppose. Doing the
- if (mnt_flags & MNT_READONLY) - flags |= ST_RDONLY; + flags |= (mnt_flags & MNT_READONLY) ? ST_RDONLY : 0;
and pasting into godbolt.org, one can apparently get gcc to compile it to
flags_by_mnt(int): leal (%rdi,%rdi), %edx movl %edi, %eax sarl $6, %eax movl %edx, %ecx andl $1, %eax andl $12, %edx andl $2, %ecx orl %ecx, %eax orl %eax, %edx movl %edi, %eax sall $7, %eax andl $7168, %eax orl %edx, %eax ret
Rasmus