https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83377
Bug ID: 83377
Summary: Missed optimization (x86): Bit operations should be
converted to arithmetic
Product: gcc
Version: 7.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: matthew at wil dot cx
Target Milestone: ---
GCC fails to optimise
if (x & 2) y = (x &~ 2)
into
if (x & 2) y = (x - 2)
in cases where that would be advantageous.
Here's one example, which I'm sure will upset the C pedants, but is relatively
common in codebases which use typed pointers based on the lower bits.
int a(void *);
struct s { struct s *s; };
int b(void *x) {
void *p = x;
if ((unsigned long)x & 2)
p = ((struct s *)((unsigned long)x & ~2UL))->s;
return a(p);
}
int c(void *x) {
void *p = x;
if ((unsigned long)x & 2)
p = ((struct s *)((unsigned long)x - 2))->s;
return a(p);
}
On x86, the difference between the assembly output is clear; function c is
smaller than function b:
0000000000000000 <b>:
0: 40 f6 c7 02 test $0x2,%dil
4: 74 07 je d <b+0xd>
6: 48 83 e7 fd and $0xfffffffffffffffd,%rdi
a: 48 8b 3f mov (%rdi),%rdi
d: e9 00 00 00 00 jmpq 12 <b+0x12>
12: 0f 1f 40 00 nopl 0x0(%rax)
16: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
1d: 00 00 00
0000000000000020 <c>:
20: 40 f6 c7 02 test $0x2,%dil
24: 74 04 je 2a <c+0xa>
26: 48 8b 7f fe mov -0x2(%rdi),%rdi
2a: e9 00 00 00 00 jmpq 2f <c+0xf>
This is true with both -O3 and -O2. I have other functions where the savings
are greater (two insns and seven bytes), but the root cause is the same; a
failure to optimise an AND into a SUB (which can then be fused with a load)
I'm filing this one under tree-optimisation rather than RTL, because I think
it's a common feature in CPUs to have load (reg + offset), and relatively
uncommon (in fact I don't know of one) to have load (reg & mask). I'm sure
some CPUs don't even have (reg + offset) addressing modes, but they wouldn't be
harmed by such an optimisation.
