http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58073
--- Comment #1 from dhowells at redhat dot com <dhowells at redhat dot com> ---
Interestingly, the suboptimality shifts if the 'shift' value in the demo
program is changed to 0:
Going through the cases individually::
(1) return (mask(d) == (0x0 << shift));
This is rendered as a single TEST instruction in x86_64 asm:
0: f6 07 07 testb $0x7,(%rdi)
3: 0f 94 c0 sete %al
6: c3 retq
which is fine.
(2) return (mask(d) == (0x0 << shift) ||
mask(d) == (0x1 << shift));
is rendered as:
10: 8b 07 mov (%rdi),%eax
12: 83 e0 07 and $0x7,%eax
15: 83 f8 01 cmp $0x1,%eax
18: 0f 96 c0 setbe %al
1b: c3 retq
which is not what I'd expect. What happened to the single TEST instruction
that was produced for shift = 4?
And then:
(3) return (mask(d) == (0x0 << shift) ||
mask(d) == (0x1 << shift) ||
mask(d) == (0x2 << shift));
which is rendered as:
20: 8b 07 mov (%rdi),%eax
22: 83 e0 07 and $0x7,%eax
25: 83 f8 02 cmp $0x2,%eax
28: 0f 96 c0 setbe %al
2b: c3 retq
which is good (and which is similar to what I'd've expected case 3 to generate
when shift = 4).
