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Merged
merged 6 commits into from
Jul 27, 2024
Merged

Allow optimizing u32::from::<char>. #124905

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cad3e7b
Allow optimizing `u32::from::<char>`.
reitermarkus May 8, 2024
64fb903
Add test for cast optimization.
reitermarkus May 10, 2024
45b87fb
Simplify test.
reitermarkus May 17, 2024
832ccdc
Remove unneeded assumption.
reitermarkus May 17, 2024
b455e43
Clarify when `None` is returned in `cast_immediate` function.
reitermarkus May 18, 2024
2f2eb22
Use `leading_zeros` instead of `ilog2` in tests.
reitermarkus Jul 21, 2024
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136 changes: 79 additions & 57 deletions compiler/rustc_codegen_ssa/src/mir/rvalue.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,7 +8,6 @@ use crate::traits::*;
use crate::MemFlags;

use rustc_middle::mir;
use rustc_middle::ty::cast::{CastTy, IntTy};
use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf, TyAndLayout};
use rustc_middle::ty::{self, adjustment::PointerCoercion, Instance, Ty, TyCtxt};
use rustc_middle::{bug, span_bug};
Expand Down Expand Up @@ -238,21 +237,21 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
}
}
OperandValue::Immediate(imm) => {
let OperandValueKind::Immediate(in_scalar) = operand_kind else {
let OperandValueKind::Immediate(from_scalar) = operand_kind else {
bug!("Found {operand_kind:?} for operand {operand:?}");
};
if let OperandValueKind::Immediate(out_scalar) = cast_kind
&& in_scalar.size(self.cx) == out_scalar.size(self.cx)
if let OperandValueKind::Immediate(to_scalar) = cast_kind
&& from_scalar.size(self.cx) == to_scalar.size(self.cx)
{
let operand_bty = bx.backend_type(operand.layout);
let cast_bty = bx.backend_type(cast);
let from_backend_ty = bx.backend_type(operand.layout);
let to_backend_ty = bx.backend_type(cast);
Some(OperandValue::Immediate(self.transmute_immediate(
bx,
imm,
in_scalar,
operand_bty,
out_scalar,
cast_bty,
from_scalar,
from_backend_ty,
to_scalar,
to_backend_ty,
)))
} else {
None
Expand Down Expand Up @@ -281,6 +280,58 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
}
}

/// Cast one of the immediates from an [`OperandValue::Immediate`]
/// or an [`OperandValue::Pair`] to an immediate of the target type.
///
/// Returns `None` if the cast is not possible.
fn cast_immediate(
&self,
bx: &mut Bx,
mut imm: Bx::Value,
from_scalar: abi::Scalar,
from_backend_ty: Bx::Type,
to_scalar: abi::Scalar,
to_backend_ty: Bx::Type,
) -> Option<Bx::Value> {
use abi::Primitive::*;

// When scalars are passed by value, there's no metadata recording their
// valid ranges. For example, `char`s are passed as just `i32`, with no
// way for LLVM to know that they're 0x10FFFF at most. Thus we assume
// the range of the input value too, not just the output range.
self.assume_scalar_range(bx, imm, from_scalar, from_backend_ty);

imm = match (from_scalar.primitive(), to_scalar.primitive()) {
(Int(_, is_signed), Int(..)) => bx.intcast(imm, to_backend_ty, is_signed),
(Float(_), Float(_)) => {
let srcsz = bx.cx().float_width(from_backend_ty);
let dstsz = bx.cx().float_width(to_backend_ty);
if dstsz > srcsz {
bx.fpext(imm, to_backend_ty)
} else if srcsz > dstsz {
bx.fptrunc(imm, to_backend_ty)
} else {
imm
}
}
(Int(_, is_signed), Float(_)) => {
if is_signed {
bx.sitofp(imm, to_backend_ty)
} else {
bx.uitofp(imm, to_backend_ty)
}
}
(Pointer(..), Pointer(..)) => bx.pointercast(imm, to_backend_ty),
(Int(_, is_signed), Pointer(..)) => {
let usize_imm = bx.intcast(imm, bx.cx().type_isize(), is_signed);
bx.inttoptr(usize_imm, to_backend_ty)
}
(Float(_), Int(_, is_signed)) => bx.cast_float_to_int(is_signed, imm, to_backend_ty),
_ => return None,
};
Some(imm)
}

/// Transmutes one of the immediates from an [`OperandValue::Immediate`]
/// or an [`OperandValue::Pair`] to an immediate of the target type.
///
Expand Down Expand Up @@ -487,62 +538,33 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
| mir::CastKind::IntToFloat
| mir::CastKind::PtrToPtr
| mir::CastKind::FnPtrToPtr

// Since int2ptr can have arbitrary integer types as input (so we have to do
// sign extension and all that), it is currently best handled in the same code
// path as the other integer-to-X casts.
| mir::CastKind::PointerWithExposedProvenance => {
let imm = operand.immediate();
let operand_kind = self.value_kind(operand.layout);
let OperandValueKind::Immediate(from_scalar) = operand_kind else {
bug!("Found {operand_kind:?} for operand {operand:?}");
};
let from_backend_ty = bx.cx().immediate_backend_type(operand.layout);

assert!(bx.cx().is_backend_immediate(cast));
let ll_t_out = bx.cx().immediate_backend_type(cast);
let to_backend_ty = bx.cx().immediate_backend_type(cast);
if operand.layout.abi.is_uninhabited() {
let val = OperandValue::Immediate(bx.cx().const_poison(ll_t_out));
let val = OperandValue::Immediate(bx.cx().const_poison(to_backend_ty));
return OperandRef { val, layout: cast };
}
let r_t_in =
CastTy::from_ty(operand.layout.ty).expect("bad input type for cast");
let r_t_out = CastTy::from_ty(cast.ty).expect("bad output type for cast");
let ll_t_in = bx.cx().immediate_backend_type(operand.layout);
let llval = operand.immediate();

let newval = match (r_t_in, r_t_out) {
(CastTy::Int(i), CastTy::Int(_)) => {
bx.intcast(llval, ll_t_out, i.is_signed())
}
(CastTy::Float, CastTy::Float) => {
let srcsz = bx.cx().float_width(ll_t_in);
let dstsz = bx.cx().float_width(ll_t_out);
if dstsz > srcsz {
bx.fpext(llval, ll_t_out)
} else if srcsz > dstsz {
bx.fptrunc(llval, ll_t_out)
} else {
llval
}
}
(CastTy::Int(i), CastTy::Float) => {
if i.is_signed() {
bx.sitofp(llval, ll_t_out)
} else {
bx.uitofp(llval, ll_t_out)
}
}
(CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Ptr(_)) => {
bx.pointercast(llval, ll_t_out)
}
(CastTy::Int(i), CastTy::Ptr(_)) => {
let usize_llval =
bx.intcast(llval, bx.cx().type_isize(), i.is_signed());
bx.inttoptr(usize_llval, ll_t_out)
}
(CastTy::Float, CastTy::Int(IntTy::I)) => {
bx.cast_float_to_int(true, llval, ll_t_out)
}
(CastTy::Float, CastTy::Int(_)) => {
bx.cast_float_to_int(false, llval, ll_t_out)
}
_ => bug!("unsupported cast: {:?} to {:?}", operand.layout.ty, cast.ty),
let cast_kind = self.value_kind(cast);
let OperandValueKind::Immediate(to_scalar) = cast_kind else {
bug!("Found {cast_kind:?} for operand {cast:?}");
};
OperandValue::Immediate(newval)

self.cast_immediate(bx, imm, from_scalar, from_backend_ty, to_scalar, to_backend_ty)
.map(OperandValue::Immediate)
.unwrap_or_else(|| {
bug!("Unsupported cast of {operand:?} to {cast:?}");
})
}
mir::CastKind::Transmute => {
self.codegen_transmute_operand(bx, operand, cast).unwrap_or_else(|| {
Expand Down
35 changes: 35 additions & 0 deletions tests/codegen/cast-optimized.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,35 @@
//@ compile-flags: -O -Z merge-functions=disabled
#![crate_type = "lib"]

// This tests that LLVM can optimize based on the niches in the source or
// destination types for casts.

// CHECK-LABEL: @u32_index
#[no_mangle]
pub fn u32_index(c: u32) -> [bool; 22] {
let mut array = [false; 22];

let index = 32 - c.leading_zeros();

// CHECK: call core::panicking::panic
array[index as usize] = true;

array
}

// CHECK-LABEL: @char_as_u32_index
#[no_mangle]
pub fn char_as_u32_index(c: char) -> [bool; 22] {
// CHECK: %[[B:.+]] = icmp ult i32 %c, 1114112
// CHECK: call void @llvm.assume(i1 %[[B]])
let c = c as u32;

let mut array = [false; 22];

let index = 32 - c.leading_zeros();

// CHECK-NOT: call core::panicking::panic
array[index as usize] = true;

array
}
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