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optimize popcount implementation #348

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merged 1 commit into from
Oct 18, 2023
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optimize popcount implementation #348

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100 changes: 39 additions & 61 deletions src/intrinsic/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -4,7 +4,7 @@ mod simd;
#[cfg(feature="master")]
use std::iter;

use gccjit::{ComparisonOp, Function, RValue, ToRValue, Type, UnaryOp, FunctionType};
use gccjit::{BinaryOp, ComparisonOp, Function, RValue, ToRValue, Type, UnaryOp, FunctionType};
use rustc_codegen_ssa::MemFlags;
use rustc_codegen_ssa::base::wants_msvc_seh;
use rustc_codegen_ssa::common::IntPredicate;
Expand Down Expand Up @@ -820,74 +820,52 @@ impl<'a, 'gcc, 'tcx> Builder<'a, 'gcc, 'tcx> {
};

if value_type.is_u128(&self.cx) {
// TODO(antoyo): implement in the normal algorithm below to have a more efficient
// implementation (that does not require a call to __popcountdi2).
let popcount = self.context.get_builtin_function("__builtin_popcountll");
let sixty_four = self.gcc_int(value_type, 64);
let right_shift = self.gcc_lshr(value, sixty_four);
let high = self.gcc_int_cast(right_shift, self.cx.ulonglong_type);
let high = self.context.new_call(None, popcount, &[high]);
let high = self.pop_count(high);
let low = self.gcc_int_cast(value, self.cx.ulonglong_type);
let low = self.context.new_call(None, popcount, &[low]);
let low = self.pop_count(low);
let res = high + low;
return self.gcc_int_cast(res, result_type);
}

// First step.
let mask = self.context.new_rvalue_from_long(value_type, 0x5555555555555555);
let left = value & mask;
let shifted = value >> self.context.new_rvalue_from_int(value_type, 1);
let right = shifted & mask;
let value = left + right;

// Second step.
let mask = self.context.new_rvalue_from_long(value_type, 0x3333333333333333);
let left = value & mask;
let shifted = value >> self.context.new_rvalue_from_int(value_type, 2);
let right = shifted & mask;
let value = left + right;

// Third step.
let mask = self.context.new_rvalue_from_long(value_type, 0x0F0F0F0F0F0F0F0F);
let left = value & mask;
let shifted = value >> self.context.new_rvalue_from_int(value_type, 4);
let right = shifted & mask;
let value = left + right;

if value_type.is_u8(&self.cx) {
return self.context.new_cast(None, value, result_type);
}

// Fourth step.
let mask = self.context.new_rvalue_from_long(value_type, 0x00FF00FF00FF00FF);
let left = value & mask;
let shifted = value >> self.context.new_rvalue_from_int(value_type, 8);
let right = shifted & mask;
let value = left + right;

if value_type.is_u16(&self.cx) {
return self.context.new_cast(None, value, result_type);
}

// Fifth step.
let mask = self.context.new_rvalue_from_long(value_type, 0x0000FFFF0000FFFF);
let left = value & mask;
let shifted = value >> self.context.new_rvalue_from_int(value_type, 16);
let right = shifted & mask;
let value = left + right;

if value_type.is_u32(&self.cx) {
return self.context.new_cast(None, value, result_type);
}

// Sixth step.
let mask = self.context.new_rvalue_from_long(value_type, 0x00000000FFFFFFFF);
let left = value & mask;
let shifted = value >> self.context.new_rvalue_from_int(value_type, 32);
let right = shifted & mask;
let value = left + right;

self.context.new_cast(None, value, result_type)
// Use Wenger's algorithm for population count, gcc's seems to play better with it
// for (int counter = 0; value != 0; counter++) {
// value &= value - 1;
// }
let func = self.current_func.borrow().expect("func");
let loop_head = func.new_block("head");
let loop_body = func.new_block("body");
let loop_tail = func.new_block("tail");

let counter_type = self.int_type;
let counter = self.current_func().new_local(None, counter_type, "popcount_counter");
let val = self.current_func().new_local(None, value_type, "popcount_value");
let zero = self.context.new_rvalue_zero(counter_type);
self.llbb().add_assignment(None, counter, zero);
self.llbb().add_assignment(None, val, value);
self.br(loop_head);

// check if value isn't zero
self.switch_to_block(loop_head);
let zero = self.context.new_rvalue_zero(value_type);
let cond = self.context.new_comparison(None, ComparisonOp::NotEquals, val.to_rvalue(), zero);
self.cond_br(cond, loop_body, loop_tail);

// val &= val - 1;
self.switch_to_block(loop_body);
let sub = val.to_rvalue() - self.context.new_rvalue_one(value_type);
loop_body.add_assignment_op(None, val, BinaryOp::BitwiseAnd, sub);

// counter += 1
let one = self.context.new_rvalue_one(counter_type);
loop_body.add_assignment_op(None, counter, BinaryOp::Plus, one);
self.br(loop_head);

// end of loop
self.switch_to_block(loop_tail);
self.context.new_cast(None, counter.to_rvalue(), result_type)
}

// Algorithm from: https://blog.regehr.org/archives/1063
Expand Down