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[naga hlsl-out msl-out spv-out] Avoid undefined behaviour when casting floats to integers #7422

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Merged
merged 1 commit into from
Mar 27, 2025
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[naga hlsl-out msl-out spv-out] Avoid undefined behaviour when casting floats to integers #7422

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105 changes: 103 additions & 2 deletions naga/src/back/hlsl/help.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -32,8 +32,8 @@ use core::fmt::Write;
use super::{
super::FunctionCtx,
writer::{
ABS_FUNCTION, DIV_FUNCTION, EXTRACT_BITS_FUNCTION, INSERT_BITS_FUNCTION, MOD_FUNCTION,
NEG_FUNCTION,
ABS_FUNCTION, DIV_FUNCTION, EXTRACT_BITS_FUNCTION, F2I32_FUNCTION, F2I64_FUNCTION,
F2U32_FUNCTION, F2U64_FUNCTION, INSERT_BITS_FUNCTION, MOD_FUNCTION, NEG_FUNCTION,
},
BackendResult, WrappedType,
};
Expand Down Expand Up @@ -97,6 +97,15 @@ pub(super) struct WrappedBinaryOp {
pub(super) right_ty: (Option<crate::VectorSize>, crate::Scalar),
}

#[derive(Clone, Copy, Debug, Hash, Eq, Ord, PartialEq, PartialOrd)]
pub(super) struct WrappedCast {
// This can only represent scalar or vector types. If we ever need to wrap
// casts with other types, we'll need a better representation.
pub(super) vector_size: Option<crate::VectorSize>,
pub(super) src_scalar: crate::Scalar,
pub(super) dst_scalar: crate::Scalar,
}

/// HLSL backend requires its own `ImageQuery` enum.
///
/// It is used inside `WrappedImageQuery` and should be unique per ImageQuery function.
Expand Down Expand Up @@ -1355,6 +1364,97 @@ impl<W: Write> super::Writer<'_, W> {
Ok(())
}

fn write_wrapped_cast_functions(
&mut self,
module: &crate::Module,
func_ctx: &FunctionCtx,
) -> BackendResult {
for (_, expression) in func_ctx.expressions.iter() {
if let crate::Expression::As {
expr,
kind,
convert: Some(width),
} = *expression
{
// Avoid undefined behaviour when casting from a float to integer
// when the value is out of range for the target type. Additionally
// ensure we clamp to the correct value as per the WGSL spec.
//
// https://www.w3.org/TR/WGSL/#floating-point-conversion:
// * If X is exactly representable in the target type T, then the
// result is that value.
// * Otherwise, the result is the value in T closest to
// truncate(X) and also exactly representable in the original
// floating point type.
let src_ty = func_ctx.resolve_type(expr, &module.types);
let Some((vector_size, src_scalar)) = src_ty.vector_size_and_scalar() else {
continue;
};
let dst_scalar = crate::Scalar { kind, width };
if src_scalar.kind != ScalarKind::Float
|| (dst_scalar.kind != ScalarKind::Sint && dst_scalar.kind != ScalarKind::Uint)
{
continue;
}

let wrapped = WrappedCast {
src_scalar,
vector_size,
dst_scalar,
};
if !self.wrapped.insert(WrappedType::Cast(wrapped)) {
continue;
}

let (src_ty, dst_ty) = match vector_size {
None => (
crate::TypeInner::Scalar(src_scalar),
crate::TypeInner::Scalar(dst_scalar),
),
Some(vector_size) => (
crate::TypeInner::Vector {
scalar: src_scalar,
size: vector_size,
},
crate::TypeInner::Vector {
scalar: dst_scalar,
size: vector_size,
},
),
};
let (min, max) =
crate::proc::min_max_float_representable_by(src_scalar, dst_scalar);
let cast_str = format!(
"{}{}",
dst_scalar.to_hlsl_str()?,
vector_size
.map(crate::common::vector_size_str)
.unwrap_or(""),
);
let fun_name = match dst_scalar {
crate::Scalar::I32 => F2I32_FUNCTION,
crate::Scalar::U32 => F2U32_FUNCTION,
crate::Scalar::I64 => F2I64_FUNCTION,
crate::Scalar::U64 => F2U64_FUNCTION,
_ => unreachable!(),
};
self.write_value_type(module, &dst_ty)?;
write!(self.out, " {fun_name}(")?;
self.write_value_type(module, &src_ty)?;
writeln!(self.out, " value) {{")?;
let level = crate::back::Level(1);
write!(self.out, "{level}return {cast_str}(clamp(value, ")?;
self.write_literal(min)?;
write!(self.out, ", ")?;
self.write_literal(max)?;
writeln!(self.out, "));",)?;
writeln!(self.out, "}}")?;
writeln!(self.out)?;
}
}
Ok(())
}

/// Helper function that writes various wrapped functions
pub(super) fn write_wrapped_functions(
&mut self,
Expand All @@ -1366,6 +1466,7 @@ impl<W: Write> super::Writer<'_, W> {
self.write_wrapped_binary_ops(module, func_ctx)?;
self.write_wrapped_expression_functions(module, func_ctx.expressions, Some(func_ctx))?;
self.write_wrapped_zero_value_functions(module, func_ctx.expressions)?;
self.write_wrapped_cast_functions(module, func_ctx)?;

for (handle, _) in func_ctx.expressions.iter() {
match func_ctx.expressions[handle] {
Expand Down
4 changes: 4 additions & 0 deletions naga/src/back/hlsl/keywords.rs
View file
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Original file line number Diff line number Diff line change
Expand Up @@ -830,6 +830,10 @@ pub const RESERVED: &[&str] = &[
super::writer::DIV_FUNCTION,
super::writer::MOD_FUNCTION,
super::writer::NEG_FUNCTION,
super::writer::F2I32_FUNCTION,
super::writer::F2U32_FUNCTION,
super::writer::F2I64_FUNCTION,
super::writer::F2U64_FUNCTION,
];

// DXC scalar types, from https://github.com/microsoft/DirectXShaderCompiler/blob/18c9e114f9c314f93e68fbc72ce207d4ed2e65ae/tools/clang/lib/AST/ASTContextHLSL.cpp#L48-L254
Expand Down
1 change: 1 addition & 0 deletions naga/src/back/hlsl/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -462,6 +462,7 @@ enum WrappedType {
Math(help::WrappedMath),
UnaryOp(help::WrappedUnaryOp),
BinaryOp(help::WrappedBinaryOp),
Cast(help::WrappedCast),
}

#[derive(Default)]
Expand Down
156 changes: 92 additions & 64 deletions naga/src/back/hlsl/writer.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -38,6 +38,10 @@ pub(crate) const ABS_FUNCTION: &str = "naga_abs";
pub(crate) const DIV_FUNCTION: &str = "naga_div";
pub(crate) const MOD_FUNCTION: &str = "naga_mod";
pub(crate) const NEG_FUNCTION: &str = "naga_neg";
pub(crate) const F2I32_FUNCTION: &str = "naga_f2i32";
pub(crate) const F2U32_FUNCTION: &str = "naga_f2u32";
pub(crate) const F2I64_FUNCTION: &str = "naga_f2i64";
pub(crate) const F2U64_FUNCTION: &str = "naga_f2u64";

struct EpStructMember {
name: String,
Expand Down Expand Up @@ -2612,6 +2616,28 @@ impl<'a, W: fmt::Write> super::Writer<'a, W> {
})
}

pub(super) fn write_literal(&mut self, literal: crate::Literal) -> BackendResult {
match literal {
crate::Literal::F64(value) => write!(self.out, "{value:?}L")?,
crate::Literal::F32(value) => write!(self.out, "{value:?}")?,
crate::Literal::F16(value) => write!(self.out, "{value:?}h")?,
crate::Literal::U32(value) => write!(self.out, "{value}u")?,
// HLSL has no suffix for explicit i32 literals, but not using any suffix
// makes the type ambiguous which prevents overload resolution from
// working. So we explicitly use the int() constructor syntax.
crate::Literal::I32(value) => write!(self.out, "int({value})")?,
crate::Literal::U64(value) => write!(self.out, "{value}uL")?,
crate::Literal::I64(value) => write!(self.out, "{value}L")?,
crate::Literal::Bool(value) => write!(self.out, "{value}")?,
crate::Literal::AbstractInt(_) | crate::Literal::AbstractFloat(_) => {
return Err(Error::Custom(
"Abstract types should not appear in IR presented to backends".into(),
));
}
}
Ok(())
}

fn write_possibly_const_expression<E>(
&mut self,
module: &Module,
Expand All @@ -2625,26 +2651,9 @@ impl<'a, W: fmt::Write> super::Writer<'a, W> {
use crate::Expression;

match expressions[expr] {
Expression::Literal(literal) => match literal {
// Floats are written using `Debug` instead of `Display` because it always appends the
// decimal part even it's zero
crate::Literal::F64(value) => write!(self.out, "{value:?}L")?,
crate::Literal::F32(value) => write!(self.out, "{value:?}")?,
crate::Literal::F16(value) => write!(self.out, "{value:?}h")?,
crate::Literal::U32(value) => write!(self.out, "{value}u")?,
// HLSL has no suffix for explicit i32 literals, but not using any suffix
// makes the type ambiguous which prevents overload resolution from
// working. So we explicitly use the int() constructor syntax.
crate::Literal::I32(value) => write!(self.out, "int({value})")?,
crate::Literal::U64(value) => write!(self.out, "{value}uL")?,
crate::Literal::I64(value) => write!(self.out, "{value}L")?,
crate::Literal::Bool(value) => write!(self.out, "{value}")?,
crate::Literal::AbstractInt(_) | crate::Literal::AbstractFloat(_) => {
return Err(Error::Custom(
"Abstract types should not appear in IR presented to backends".into(),
));
}
},
Expression::Literal(literal) => {
self.write_literal(literal)?;
}
Expression::Constant(handle) => {
let constant = &module.constants[handle];
if constant.name.is_some() {
Expand Down Expand Up @@ -3320,53 +3329,72 @@ impl<'a, W: fmt::Write> super::Writer<'a, W> {
convert,
} => {
let inner = func_ctx.resolve_type(expr, &module.types);
let close_paren = match convert {
Some(dst_width) => {
let scalar = Scalar {
kind,
width: dst_width,
};
match *inner {
TypeInner::Vector { size, .. } => {
write!(
self.out,
"{}{}(",
scalar.to_hlsl_str()?,
common::vector_size_str(size)
)?;
}
TypeInner::Scalar(_) => {
write!(self.out, "{}(", scalar.to_hlsl_str()?,)?;
}
TypeInner::Matrix { columns, rows, .. } => {
write!(
self.out,
"{}{}x{}(",
scalar.to_hlsl_str()?,
common::vector_size_str(columns),
common::vector_size_str(rows)
)?;
}
_ => {
return Err(Error::Unimplemented(format!(
"write_expr expression::as {inner:?}"
)));
}
};
true
}
None => {
if inner.scalar_width() == Some(8) {
false
} else {
write!(self.out, "{}(", kind.to_hlsl_cast(),)?;
if inner.scalar_kind() == Some(ScalarKind::Float)
&& (kind == ScalarKind::Sint || kind == ScalarKind::Uint)
&& convert.is_some()
{
// Use helper functions for float to int casts in order to
// avoid undefined behaviour when value is out of range for
// the target type.
let fun_name = match (kind, convert) {
(ScalarKind::Sint, Some(4)) => F2I32_FUNCTION,
(ScalarKind::Uint, Some(4)) => F2U32_FUNCTION,
(ScalarKind::Sint, Some(8)) => F2I64_FUNCTION,
(ScalarKind::Uint, Some(8)) => F2U64_FUNCTION,
_ => unreachable!(),
};
write!(self.out, "{fun_name}(")?;
self.write_expr(module, expr, func_ctx)?;
write!(self.out, ")")?;
} else {
let close_paren = match convert {
Some(dst_width) => {
let scalar = Scalar {
kind,
width: dst_width,
};
match *inner {
TypeInner::Vector { size, .. } => {
write!(
self.out,
"{}{}(",
scalar.to_hlsl_str()?,
common::vector_size_str(size)
)?;
}
TypeInner::Scalar(_) => {
write!(self.out, "{}(", scalar.to_hlsl_str()?,)?;
}
TypeInner::Matrix { columns, rows, .. } => {
write!(
self.out,
"{}{}x{}(",
scalar.to_hlsl_str()?,
common::vector_size_str(columns),
common::vector_size_str(rows)
)?;
}
_ => {
return Err(Error::Unimplemented(format!(
"write_expr expression::as {inner:?}"
)));
}
};
true
}
None => {
if inner.scalar_width() == Some(8) {
false
} else {
write!(self.out, "{}(", kind.to_hlsl_cast(),)?;
true
}
}
};
self.write_expr(module, expr, func_ctx)?;
if close_paren {
write!(self.out, ")")?;
}
};
self.write_expr(module, expr, func_ctx)?;
if close_paren {
write!(self.out, ")")?;
}
}
Expression::Math {
Expand Down
4 changes: 4 additions & 0 deletions naga/src/back/msl/keywords.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -349,6 +349,10 @@ pub const RESERVED: &[&str] = &[
super::writer::DIV_FUNCTION,
super::writer::MOD_FUNCTION,
super::writer::NEG_FUNCTION,
super::writer::F2I32_FUNCTION,
super::writer::F2U32_FUNCTION,
super::writer::F2I64_FUNCTION,
super::writer::F2U64_FUNCTION,
super::writer::ARGUMENT_BUFFER_WRAPPER_STRUCT,
];

Expand Down
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