Closed
Description
I had some code and wanted to wrap the return statement in parenthesis and do a bitwise negation to see if the emit is better. Here is the code after the aforementioned modification:
export fn swarControlCharMask1(v: u64) u64 {
const ones: u64 = 0x0101010101010101;
const mask = ones * 0x7F;
const low_7_bits = v & mask;
const del = low_7_bits + ones;
const non_control_or_space = low_7_bits + ones * (0x80 - 0x21);
return ~(~v & (del | ~non_control_or_space));
}What I got was:
swarControlCharMask1:
movabs rcx, 9187201950435737471
movabs rdx, 72340172838076673
movabs rax, -6872316419617283936
and rcx, rdi
add rdx, rcx
sub rax, rcx
or rax, rdx
not rax
or rax, rdi
retI had a hunch this was not the best the compiler could do, so I manually applied De Morgan's laws by inverting the bits of each operand and changing the bitwise OR's to AND's and the AND's to OR's:
export fn swarControlCharMask2(v: u64) u64 {
const ones: u64 = 0x0101010101010101;
const mask = ones * 0x7F;
const low_7_bits = v & mask;
const del = low_7_bits + ones;
const non_control_or_space = low_7_bits + ones * (0x80 - 0x21);
return (v | (~del & non_control_or_space));
}And, sure enough, we save an instruction:
swarControlCharMask2:
movabs rcx, 9187201950435737471
movabs rdx, 6872316419617283935
movabs rax, -72340172838076674
and rcx, rdi
add rdx, rcx
sub rax, rcx
and rax, rdx
or rax, rdi
ret