Re: [RFC PATCH] fs: micro-optimization remove branches by adjusting flag values

[Matthew Wilcox]

On Mon, Sep 14, 2020 at 07:43:38PM +0200, mateusznosek0@xxxxxxxxx wrote:
> From: Mateusz Nosek <mateusznosek0@xxxxxxxxx>
> 
> When flags A and B have equal values than the following code
> 
> if(flags1 & A)
> 	flags2 |= B;
> 
> is equivalent to
> 
> flags2 |= (flags1 & A);
> 
> The latter code should generate less instructions and be faster as one
> branch is omitted in it.

[citation needed]

$ cat test.c 
int a(int x)
{
	int y = 0;

	if (x & 1)
		y |= 1;
	if (x & 2)
		y |= 2;

	return y;
}

$ objdump -d test.o
0000000000000000 <a>:
   0:   89 f8                   mov    %edi,%eax
   2:   83 e0 03                and    $0x3,%eax
   5:   c3                      retq   

Please stop submitting uglifying patches without checking they actually
improve anything.  GCC is smarter than you think it is.