rustc_codegen_ssa/mir/constant.rs
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use rustc_abi::BackendRepr;
use rustc_middle::mir::interpret::ErrorHandled;
use rustc_middle::ty::layout::{HasTyCtxt, HasTypingEnv};
use rustc_middle::ty::{self, Ty};
use rustc_middle::{bug, mir, span_bug};
use super::FunctionCx;
use crate::errors;
use crate::mir::operand::OperandRef;
use crate::traits::*;
impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
pub(crate) fn eval_mir_constant_to_operand(
&self,
bx: &mut Bx,
constant: &mir::ConstOperand<'tcx>,
) -> OperandRef<'tcx, Bx::Value> {
let val = self.eval_mir_constant(constant);
let ty = self.monomorphize(constant.ty());
OperandRef::from_const(bx, val, ty)
}
pub fn eval_mir_constant(&self, constant: &mir::ConstOperand<'tcx>) -> mir::ConstValue<'tcx> {
// `MirUsedCollector` visited all required_consts before codegen began, so if we got here
// there can be no more constants that fail to evaluate.
self.monomorphize(constant.const_)
.eval(self.cx.tcx(), self.cx.typing_env(), constant.span)
.expect("erroneous constant missed by mono item collection")
}
/// This is a convenience helper for `immediate_const_vector`. It has the precondition
/// that the given `constant` is an `Const::Unevaluated` and must be convertible to
/// a `ValTree`. If you want a more general version of this, talk to `wg-const-eval` on zulip.
///
/// Note that this function is cursed, since usually MIR consts should not be evaluated to
/// valtrees!
fn eval_unevaluated_mir_constant_to_valtree(
&self,
constant: &mir::ConstOperand<'tcx>,
) -> Result<Result<ty::ValTree<'tcx>, Ty<'tcx>>, ErrorHandled> {
let uv = match self.monomorphize(constant.const_) {
mir::Const::Unevaluated(uv, _) => uv.shrink(),
mir::Const::Ty(_, c) => match c.kind() {
// A constant that came from a const generic but was then used as an argument to
// old-style simd_shuffle (passing as argument instead of as a generic param).
rustc_type_ir::ConstKind::Value(_, valtree) => return Ok(Ok(valtree)),
other => span_bug!(constant.span, "{other:#?}"),
},
// We should never encounter `Const::Val` unless MIR opts (like const prop) evaluate
// a constant and write that value back into `Operand`s. This could happen, but is
// unlikely. Also: all users of `simd_shuffle` are on unstable and already need to take
// a lot of care around intrinsics. For an issue to happen here, it would require a
// macro expanding to a `simd_shuffle` call without wrapping the constant argument in a
// `const {}` block, but the user pass through arbitrary expressions.
// FIXME(oli-obk): replace the magic const generic argument of `simd_shuffle` with a
// real const generic, and get rid of this entire function.
other => span_bug!(constant.span, "{other:#?}"),
};
let uv = self.monomorphize(uv);
self.cx.tcx().const_eval_resolve_for_typeck(self.cx.typing_env(), uv, constant.span)
}
/// process constant containing SIMD shuffle indices & constant vectors
pub fn immediate_const_vector(
&mut self,
bx: &Bx,
constant: &mir::ConstOperand<'tcx>,
) -> (Bx::Value, Ty<'tcx>) {
let ty = self.monomorphize(constant.ty());
assert!(ty.is_simd());
let field_ty = ty.simd_size_and_type(bx.tcx()).1;
let val = self
.eval_unevaluated_mir_constant_to_valtree(constant)
.ok()
.map(|x| x.ok())
.flatten()
.map(|val| {
// A SIMD type has a single field, which is an array.
let fields = val.unwrap_branch();
assert_eq!(fields.len(), 1);
let array = fields[0].unwrap_branch();
// Iterate over the array elements to obtain the values in the vector.
let values: Vec<_> = array
.iter()
.map(|field| {
if let Some(prim) = field.try_to_scalar() {
let layout = bx.layout_of(field_ty);
let BackendRepr::Scalar(scalar) = layout.backend_repr else {
bug!("from_const: invalid ByVal layout: {:#?}", layout);
};
bx.scalar_to_backend(prim, scalar, bx.immediate_backend_type(layout))
} else {
bug!("field is not a scalar {:?}", field)
}
})
.collect();
bx.const_vector(&values)
})
.unwrap_or_else(|| {
bx.tcx().dcx().emit_err(errors::ShuffleIndicesEvaluation { span: constant.span });
// We've errored, so we don't have to produce working code.
let llty = bx.backend_type(bx.layout_of(ty));
bx.const_undef(llty)
});
(val, ty)
}
}