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Made the logical shift / artihmetic shift not rely on signs of intigers #749

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93ba988
Made the logical shift / artihmetic shift not relly on signs of intigers
FractalFir Aug 12, 2025
f2c00cf
Update src/builder.rs
FractalFir Aug 14, 2025
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10 changes: 8 additions & 2 deletions src/builder.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -41,6 +41,12 @@ use crate::type_of::LayoutGccExt;
// TODO(antoyo)
type Funclet = ();

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ShiftKind {
Logical,
Arithmetic,
}

enum ExtremumOperation {
Max,
Min,
Expand Down Expand Up @@ -827,13 +833,13 @@ impl<'a, 'gcc, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'gcc, 'tcx> {
}

fn lshr(&mut self, a: RValue<'gcc>, b: RValue<'gcc>) -> RValue<'gcc> {
self.gcc_lshr(a, b)
self.gcc_shr(a, b, ShiftKind::Logical)
}

fn ashr(&mut self, a: RValue<'gcc>, b: RValue<'gcc>) -> RValue<'gcc> {
// TODO(antoyo): check whether behavior is an arithmetic shift for >> .
// It seems to be if the value is signed.
self.gcc_lshr(a, b)
self.gcc_shr(a, b, ShiftKind::Arithmetic)
}

fn and(&mut self, a: RValue<'gcc>, b: RValue<'gcc>) -> RValue<'gcc> {
Expand Down
70 changes: 48 additions & 22 deletions src/int.rs
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Expand Up @@ -14,7 +14,7 @@ use rustc_middle::ty::{self, Ty};
use rustc_target::callconv::{ArgAbi, ArgAttributes, FnAbi, PassMode};
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Would it be easy to add a test for this fix?

use rustc_type_ir::{Interner, TyKind};

use crate::builder::{Builder, ToGccComp};
use crate::builder::{Builder, ShiftKind, ToGccComp};
use crate::common::{SignType, TypeReflection};
use crate::context::CodegenCx;

Expand Down Expand Up @@ -68,36 +68,59 @@ impl<'a, 'gcc, 'tcx> Builder<'a, 'gcc, 'tcx> {
self.cx.bitwise_operation(BinaryOp::BitwiseAnd, a, b, self.location)
}

pub fn gcc_lshr(&mut self, a: RValue<'gcc>, b: RValue<'gcc>) -> RValue<'gcc> {
pub fn gcc_shr(
&mut self,
a: RValue<'gcc>,
b: RValue<'gcc>,
shift_kind: ShiftKind,
) -> RValue<'gcc> {
let a_type = a.get_type();
let b_type = b.get_type();
let a_native = self.is_native_int_type(a_type);
let b_native = self.is_native_int_type(b_type);
if a_native && b_native {
// FIXME(antoyo): remove the casts when libgccjit can shift an unsigned number by a signed number.
// TODO(antoyo): cast to unsigned to do a logical shift if that does not work.
if a_type.is_signed(self) != b_type.is_signed(self) {
let b = self.context.new_cast(self.location, b, a_type);
a >> b
} else {
let a_size = a_type.get_size();
let b_size = b_type.get_size();
match a_size.cmp(&b_size) {
std::cmp::Ordering::Less => {
let a = self.context.new_cast(self.location, a, b_type);
a >> b
}
std::cmp::Ordering::Equal => a >> b,
std::cmp::Ordering::Greater => {
let b = self.context.new_cast(self.location, b, a_type);
a >> b
}
let (a, a_type) = match (shift_kind, a_type.is_signed(self.cx)) {
(ShiftKind::Logical, true) => {
let a_type = a_type.to_unsigned(self.cx);
(self.gcc_int_cast(a, a_type), a_type)
}
(ShiftKind::Logical, false) => (a, a_type),
(ShiftKind::Arithmetic, true) => (a, a_type),
(ShiftKind::Arithmetic, false) => {
let a_type = a_type.to_signed(self.cx);
(self.gcc_int_cast(a, a_type), a_type)
}
};
let (b, b_type) = match (shift_kind, b_type.is_signed(self.cx)) {
(ShiftKind::Logical, true) => {
let b_type = b_type.to_unsigned(self.cx);
(self.gcc_int_cast(b, b_type), b_type)
}
(ShiftKind::Logical, false) => (b, b_type),
(ShiftKind::Arithmetic, true) => (b, b_type),
(ShiftKind::Arithmetic, false) => {
let b_type = b_type.to_signed(self.cx);
(self.gcc_int_cast(b, b_type), b_type)
}
};
Comment on lines -86 to +105
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Could you please add a comment to explain that this casts to unsigned if needed to do a logical shift and to signed if needed to do an arithmetic shift?


let a_size = a_type.get_size();
let b_size = b_type.get_size();
match a_size.cmp(&b_size) {
std::cmp::Ordering::Less => {
let a = self.context.new_cast(self.location, a, b_type);
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Could you add a debug_assert!() that checks that the sign of a and b is the same here?

a >> b
}
std::cmp::Ordering::Equal => a >> b,
std::cmp::Ordering::Greater => {
let b = self.context.new_cast(self.location, b, a_type);
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And here as well.

a >> b
}
}
} else if a_type.is_vector() && b_type.is_vector() {
a >> b
} else if a_native && !b_native {
self.gcc_lshr(a, self.gcc_int_cast(b, a_type))
self.gcc_shr(a, self.gcc_int_cast(b, a_type), shift_kind)
} else {
// NOTE: we cannot use the lshr builtin because it's calling hi() (to get the most
// significant half of the number) which uses lshr.
Expand All @@ -122,7 +145,10 @@ impl<'a, 'gcc, 'tcx> Builder<'a, 'gcc, 'tcx> {

let shift_value = self.gcc_sub(b, sixty_four);
let high = self.high(a);
let sign = if a_type.is_signed(self) { high >> sixty_three } else { zero };
let sign = match shift_kind {
ShiftKind::Logical => zero,
ShiftKind::Arithmetic => high >> sixty_three,
};
let array_value = self.concat_low_high_rvalues(a_type, high >> shift_value, sign);
then_block.add_assignment(self.location, result, array_value);
then_block.end_with_jump(self.location, after_block);
Expand Down
14 changes: 7 additions & 7 deletions src/intrinsic/mod.rs
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Expand Up @@ -865,7 +865,7 @@ impl<'a, 'gcc, 'tcx> Builder<'a, 'gcc, 'tcx> {
128 => {
// TODO(antoyo): find a more efficient implementation?
let sixty_four = self.gcc_int(typ, 64);
let right_shift = self.gcc_lshr(value, sixty_four);
let right_shift = self.lshr(value, sixty_four);
let high = self.gcc_int_cast(right_shift, self.u64_type);
let low = self.gcc_int_cast(value, self.u64_type);

Expand Down Expand Up @@ -998,7 +998,7 @@ impl<'a, 'gcc, 'tcx> Builder<'a, 'gcc, 'tcx> {
.new_local(None, array_type, "count_loading_zeroes_results");

let sixty_four = self.gcc_int(arg_type, 64);
let shift = self.gcc_lshr(arg, sixty_four);
let shift = self.lshr(arg, sixty_four);
let high = self.gcc_int_cast(shift, self.u64_type);
let low = self.gcc_int_cast(arg, self.u64_type);

Expand Down Expand Up @@ -1073,7 +1073,7 @@ impl<'a, 'gcc, 'tcx> Builder<'a, 'gcc, 'tcx> {
// TODO(antoyo): remove this if/when native 128-bit integers land in libgccjit
if value_type.is_u128(self.cx) && !self.cx.supports_128bit_integers {
let sixty_four = self.gcc_int(value_type, 64);
let right_shift = self.gcc_lshr(value, sixty_four);
let right_shift = self.lshr(value, sixty_four);
let high = self.gcc_int_cast(right_shift, self.cx.ulonglong_type);
let high = self.pop_count(high);
let low = self.gcc_int_cast(value, self.cx.ulonglong_type);
Expand Down Expand Up @@ -1196,12 +1196,12 @@ impl<'a, 'gcc, 'tcx> Builder<'a, 'gcc, 'tcx> {
// Return `result_type`'s maximum or minimum value on overflow
// NOTE: convert the type to unsigned to have an unsigned shift.
let unsigned_type = result_type.to_unsigned(self.cx);
let shifted = self.gcc_lshr(
let shifted = self.lshr(
self.gcc_int_cast(lhs, unsigned_type),
self.gcc_int(unsigned_type, width as i64 - 1),
);
let uint_max = self.gcc_not(self.gcc_int(unsigned_type, 0));
let int_max = self.gcc_lshr(uint_max, self.gcc_int(unsigned_type, 1));
let int_max = self.lshr(uint_max, self.gcc_int(unsigned_type, 1));
then_block.add_assignment(
self.location,
res,
Expand Down Expand Up @@ -1267,12 +1267,12 @@ impl<'a, 'gcc, 'tcx> Builder<'a, 'gcc, 'tcx> {
// Return `result_type`'s maximum or minimum value on overflow
// NOTE: convert the type to unsigned to have an unsigned shift.
let unsigned_type = result_type.to_unsigned(self.cx);
let shifted = self.gcc_lshr(
let shifted = self.lshr(
self.gcc_int_cast(lhs, unsigned_type),
self.gcc_int(unsigned_type, width as i64 - 1),
);
let uint_max = self.gcc_not(self.gcc_int(unsigned_type, 0));
let int_max = self.gcc_lshr(uint_max, self.gcc_int(unsigned_type, 1));
let int_max = self.lshr(uint_max, self.gcc_int(unsigned_type, 1));
then_block.add_assignment(
self.location,
res,
Expand Down