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codegen: tweak/extend shift comments

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This commit is contained in:
Ralf Jung 2024-05-15 17:33:26 +02:00
parent ade234d574
commit 17bd43cb25
2 changed files with 13 additions and 4 deletions
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9
compiler/rustc_codegen_ssa/src/base.rs
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@ -293,12 +293,13 @@ pub fn coerce_unsized_into<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
}
}
/// Returns `rhs` sufficiently masked, truncated, and/or extended so that
/// it can be used to shift `lhs`.
/// Returns `rhs` sufficiently masked, truncated, and/or extended so that it can be used to shift
/// `lhs`: it has the same size as `lhs`, and the value, when interpreted unsigned (no matter its
/// type), will not exceed the size of `lhs`.
///
/// Shifts in MIR are all allowed to have mismatched LHS & RHS types.
/// Shifts in MIR are all allowed to have mismatched LHS & RHS types, and signed RHS.
/// The shift methods in `BuilderMethods`, however, are fully homogeneous
/// (both parameters and the return type are all the same type).
/// (both parameters and the return type are all the same size) and assume an unsigned RHS.
///
/// If `is_unchecked` is false, this masks the RHS to ensure it stays in-bounds,
/// as the `BuilderMethods` shifts are UB for out-of-bounds shift amounts.

8
compiler/rustc_codegen_ssa/src/traits/builder.rs
View File

@ -110,8 +110,16 @@ pub trait BuilderMethods<'a, 'tcx>:
fn frem(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
fn frem_fast(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
fn frem_algebraic(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
/// Generate a left-shift. Both operands must have the same size. The right operand must be
/// interpreted as unsigned and can be assumed to be less than the size of the left operand.
fn shl(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
/// Generate a logical right-shift. Both operands must have the same size. The right operand
/// must be interpreted as unsigned and can be assumed to be less than the size of the left
/// operand.
fn lshr(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
/// Generate an arithmetic right-shift. Both operands must have the same size. The right operand
/// must be interpreted as unsigned and can be assumed to be less than the size of the left
/// operand.
fn ashr(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
fn unchecked_sadd(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
fn unchecked_uadd(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;