i386: switch_to(): misplaced parentheses

arch/i386/kernel/process.c struct task_struct fastcall * __switch_t · line 697

Unlikely's with or's are kind of ambiguous. An 'and' makes sense but or's don't. Because a branch is going to happen anyway. Just to test this out, I made a little function and tried out different types of parenthesis placements.

Here: (the original placement)

	#define unlikely(x) __builtin_expect(!!(x), 0)

	int switch_to(int x, int y)
	{
		int a = 0;
		if (unlikely(x==24) || (y==83))
			a=10;
		return a;
	}

and now yours:

	#define unlikely(x) __builtin_expect(!!(x), 0)

	int switch_to(int x, int y)
	{
		int a = 0;
		if (unlikely(x==24 || y==83))
			a=10;
		return a;
	}

The original gave me this: gcc -O2 -c switch.c

objdump -Dr switch.o

00000000 <switch_to>:

   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   83 7d 08 18             cmpl   $0x18,0x8(%ebp)
   7:   74 0a                   je     13 <switch_to+0x13>
   9:   31 c0                   xor    %eax,%eax
   b:   83 7d 0c 53             cmpl   $0x53,0xc(%ebp)
   f:   74 02                   je     13 <switch_to+0x13>
  11:   5d                      pop    %ebp
  12:   c3                      ret
  13:   5d                      pop    %ebp
  14:   b8 0a 00 00 00          mov    $0xa,%eax
  19:   c3                      ret

and then yours: gcc -O2 -c switch_alt.c

objdump -Dr switch_alt.o

00000000 <switch_to>:

   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   83 7d 08 18             cmpl   $0x18,0x8(%ebp)
   7:   74 0a                   je     13 <switch_to+0x13>
   9:   31 c0                   xor    %eax,%eax
   b:   83 7d 0c 53             cmpl   $0x53,0xc(%ebp)
   f:   74 02                   je     13 <switch_to+0x13>
  11:   5d                      pop    %ebp
  12:   c3                      ret
  13:   5d                      pop    %ebp
  14:   b8 0a 00 00 00          mov    $0xa,%eax
  19:   c3                      ret

They are identical!!!

This is not surprising since you still need to test the other OR case if it fails, which you are saying it will. So saying it is unlikely is meaningless. Likely's are good for 'or' and unlikely's are good for 'and'. But not the other way around.

|| and && short circuit when they can.

So the statement of A || B is:

TEST A
JUMP_IF_TRUE true
TEST B
JUMP_IF_FALSE false
true:
do true statement
JUMP out:
false:
do else statement
out:

Saying TEST A is likely to fail really doesn't help the situation. Saying it will likely pass will. Then we could do this:

TEST A
JUMP_IF_FALSE test2
true:
do true statement
JUMP out:
false:
do false statement
jump out:
test2:
TEST B
JUMP_IF_TRUE true
JUMP false:
out:

Where here we can move the true after the first compare and prevent the conditional branch.

So what I'm saying is that unlikely(a || b) has really no good meaning.

Out of curiosity, I removed the unlikely altogether and here's what I got.

gcc -O2 -c switch_none.c

00000000 <switch_to>:

   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   83 7d 08 18             cmpl   $0x18,0x8(%ebp)
   7:   74 0a                   je     13 <switch_to+0x13>
   9:   31 c0                   xor    %eax,%eax
   b:   83 7d 0c 53             cmpl   $0x53,0xc(%ebp)
   f:   74 02                   je     13 <switch_to+0x13>
  11:   5d                      pop    %ebp
  12:   c3                      ret
  13:   5d                      pop    %ebp
  14:   b8 0a 00 00 00          mov    $0xa,%eax
  19:   c3                      ret

Ha! still the same.

Is that unlikely there really do any good?

-- Steve