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Fix Num.abs for ints

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This commit is contained in:
Richard Feldman 2020-07-06 22:27:53 -04:00
parent 8d2961c81e
commit 35b653636b
1 changed files with 36 additions and 2 deletions
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38
compiler/gen/src/llvm/build.rs
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@ -1904,7 +1904,7 @@ fn build_float_binop<'a, 'ctx, 'env>(
fn build_int_unary_op<'a, 'ctx, 'env>(
env: &Env<'a, 'ctx, 'env>,
arg: IntValue<'ctx>,
_arg_layout: &Layout<'a>,
arg_layout: &Layout<'a>,
op: LowLevel,
) -> BasicValueEnum<'ctx> {
use roc_module::low_level::LowLevel::*;
@ -1914,7 +1914,41 @@ fn build_int_unary_op<'a, 'ctx, 'env>(
match op {
NumNeg => bd.build_int_neg(arg, "negate_int").into(),
NumAbs => {
todo!("build_int_unary_op for integer absolute value. (possibly bitwise AND with 0b0111_1111_...)");
// This is how libc's abs() is implemented - it uses no branching!
//
// abs = \arg ->
// shifted = arg >>> 63
//
// (xor arg shifted) - shifted
let ctx = env.context;
let shifted_name = "abs_shift_right";
let shifted_alloca = {
let bits_to_shift = ((arg_layout.stack_size(env.ptr_bytes) as u64) * 8) - 1;
let shift_val = ctx.i64_type().const_int(bits_to_shift, false);
let shifted = bd.build_right_shift(arg, shift_val, true, shifted_name);
let alloca = bd.build_alloca(
basic_type_from_layout(env.arena, ctx, arg_layout, env.ptr_bytes),
"#int_abs_help",
);
// shifted = arg >>> 63
bd.build_store(alloca, shifted);
alloca
};
let xored_arg = bd.build_xor(
arg,
bd.build_load(shifted_alloca, shifted_name).into_int_value(),
"xor_arg_shifted",
);
BasicValueEnum::IntValue(bd.build_int_sub(
xored_arg.into(),
bd.build_load(shifted_alloca, shifted_name).into_int_value(),
"sub_xored_shifted",
))
}
NumToFloat => {
// TODO specialize this to be not just for i64!