https://bugs.llvm.org/show_bug.cgi?id=47433
Bug ID: 47433
Summary: bad code for __builtin_parity on x86 and ARM
Product: new-bugs
Version: 10.0
Hardware: Other
OS: FreeBSD
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
I have found that __builtin_parity generates poor code on ARM (soft float).
extern int foo(int x) {
return __builtin_parity(x);
}
compiles to
mov r1, #85
and r1, r1, r0, lsr #1
sub r0, r0, r1
movw r1, #13107
movt r1, #13107
and r2, r1, r0, lsr #2
and r0, r0, r1
add r0, r0, r2
movw r1, #3855
movt r1, #271
add r0, r0, r0, lsr #4
and r0, r0, r1
movw r1, #257
movt r1, #257
mul r0, r0, r1
ubfx r0, r0, #24, #1
bx lr
which essentially performs a population count and then truncates to one bit.
This seems awfully suboptimal. Why not a series of shifts and xors?
eor r0, r0, r0, lsr #16
eor r0, r0, r0, lsr #8
eor r0, r0, r0, lsr #4
eor r0, r0, r0, lsr #2
eor r0, r0, r0, lsr #1
and r0, r0, #1
This seems a lot better. The builtin could also recognise if the input has a
width of less than 32 bit and perform less reductions if possible.
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