Auto merge of rust-lang#116042 - Nadrieril:linear-pass-take-2, r=<try>

[Experiment] Rewrite exhaustiveness in one pass

Arm reachability checking does a quadratic amount of work: for each arm we check if it is reachable given the arms above it. This feels wasteful since we often end up re-exploring the same cases when we check for exhaustiveness.

This PR is an attempt to check reachability at the same time as exhaustiveness. This opens the door to a bunch of code simplifications I'm very excited about. The main question is whether I can get actual performance gains out of this.

I had started the experiment in rust-lang#111720 but I can't reopen it.

r? `@ghost`

Author: bors

compiler/rustc_middle/src/thir.rs

@@ -16,16 +16,18 @@ use rustc_hir::RangeEnd;
 use rustc_index::newtype_index;
 use rustc_index::IndexVec;
 use rustc_middle::middle::region;
-use rustc_middle::mir::interpret::AllocId;
+use rustc_middle::mir::interpret::{AllocId, Scalar};
 use rustc_middle::mir::{self, BinOp, BorrowKind, FakeReadCause, Mutability, UnOp};
 use rustc_middle::ty::adjustment::PointerCoercion;
+use rustc_middle::ty::layout::IntegerExt;
 use rustc_middle::ty::GenericArgsRef;
-use rustc_middle::ty::{self, AdtDef, FnSig, List, Ty, UpvarArgs};
+use rustc_middle::ty::{self, AdtDef, FnSig, List, Ty, TyCtxt, UpvarArgs};
 use rustc_middle::ty::{CanonicalUserType, CanonicalUserTypeAnnotation};
 use rustc_span::def_id::LocalDefId;
 use rustc_span::{sym, Span, Symbol, DUMMY_SP};
-use rustc_target::abi::{FieldIdx, VariantIdx};
+use rustc_target::abi::{FieldIdx, Integer, Size, VariantIdx};
 use rustc_target::asm::InlineAsmRegOrRegClass;
+use std::cmp::Ordering;
 use std::fmt;
 use std::ops::Index;
 
@@ -773,12 +775,217 @@ pub enum PatKind<'tcx> {
     },
 }
 
+/// A range pattern.
+/// The boundaries must be of the same type and that type must be numeric.
 #[derive(Clone, Debug, PartialEq, HashStable, TypeVisitable)]
 pub struct PatRange<'tcx> {
-    pub lo: mir::Const<'tcx>,
-    pub hi: mir::Const<'tcx>,
+    pub lo: PatRangeBoundary<'tcx>,
+    pub hi: PatRangeBoundary<'tcx>,
     #[type_visitable(ignore)]
     pub end: RangeEnd,
+    pub ty: Ty<'tcx>,
+}
+
+impl<'tcx> PatRange<'tcx> {
+    /// Whether this range covers the full extent of possible values (best-effort, we ignore floats).
+    #[inline]
+    pub fn is_full_range(&self, tcx: TyCtxt<'tcx>) -> Option<bool> {
+        let (min, max, size, bias) = match *self.ty.kind() {
+            ty::Char => (0, std::char::MAX as u128, Size::from_bits(32), 0),
+            ty::Int(ity) => {
+                let size = Integer::from_int_ty(&tcx, ity).size();
+                let max = size.truncate(u128::MAX);
+                let bias = 1u128 << (size.bits() - 1);
+                (0, max, size, bias)
+            }
+            ty::Uint(uty) => {
+                let size = Integer::from_uint_ty(&tcx, uty).size();
+                let max = size.unsigned_int_max();
+                (0, max, size, 0)
+            }
+            _ => return None,
+        };
+
+        // We want to compare ranges numerically, but the order of the bitwise representation of
+        // signed integers does not match their numeric order. Thus, to correct the ordering, we
+        // need to shift the range of signed integers to correct the comparison. This is achieved by
+        // XORing with a bias (see pattern/deconstruct_pat.rs for another pertinent example of this
+        // pattern).
+        //
+        // Also, for performance, it's important to only do the second `try_to_bits` if necessary.
+        let lo_is_min = match self.lo {
+            PatRangeBoundary::Finite(value) => {
+                let lo = value.try_to_bits(size).unwrap() ^ bias;
+                lo <= min
+            }
+        };
+        if lo_is_min {
+            let hi_is_max = match self.hi {
+                PatRangeBoundary::Finite(value) => {
+                    let hi = value.try_to_bits(size).unwrap() ^ bias;
+                    hi > max || hi == max && self.end == RangeEnd::Included
+                }
+            };
+            if hi_is_max {
+                return Some(true);
+            }
+        }
+        Some(false)
+    }
+
+    #[inline]
+    pub fn contains(
+        &self,
+        value: mir::Const<'tcx>,
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> Option<bool> {
+        use Ordering::*;
+        debug_assert_eq!(self.ty, value.ty());
+        let ty = self.ty;
+        let value = PatRangeBoundary::Finite(value);
+        // For performance, it's important to only do the second comparison if necessary.
+        Some(
+            match self.lo.compare_with(value, ty, tcx, param_env)? {
+                Less | Equal => true,
+                Greater => false,
+            } && match value.compare_with(self.hi, ty, tcx, param_env)? {
+                Less => true,
+                Equal => self.end == RangeEnd::Included,
+                Greater => false,
+            },
+        )
+    }
+
+    #[inline]
+    pub fn overlaps(
+        &self,
+        other: &Self,
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> Option<bool> {
+        use Ordering::*;
+        debug_assert_eq!(self.ty, other.ty);
+        // For performance, it's important to only do the second comparison if necessary.
+        Some(
+            match other.lo.compare_with(self.hi, self.ty, tcx, param_env)? {
+                Less => true,
+                Equal => self.end == RangeEnd::Included,
+                Greater => false,
+            } && match self.lo.compare_with(other.hi, self.ty, tcx, param_env)? {
+                Less => true,
+                Equal => other.end == RangeEnd::Included,
+                Greater => false,
+            },
+        )
+    }
+}
+
+impl<'tcx> fmt::Display for PatRange<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let PatRangeBoundary::Finite(value) = &self.lo;
+        write!(f, "{value}")?;
+        write!(f, "{}", self.end)?;
+        let PatRangeBoundary::Finite(value) = &self.hi;
+        write!(f, "{value}")?;
+        Ok(())
+    }
+}
+
+/// A (possibly open) boundary of a range pattern.
+/// If present, the const must be of a numeric type.
+#[derive(Copy, Clone, Debug, PartialEq, HashStable, TypeVisitable)]
+pub enum PatRangeBoundary<'tcx> {
+    Finite(mir::Const<'tcx>),
+}
+
+impl<'tcx> PatRangeBoundary<'tcx> {
+    #[inline]
+    pub fn lower_bound(ty: Ty<'tcx>, tcx: TyCtxt<'tcx>) -> Self {
+        // Unwrap is ok because the type is known to be numeric.
+        let c = ty.numeric_min_val(tcx).unwrap();
+        let value = mir::Const::from_ty_const(c, tcx);
+        Self::Finite(value)
+    }
+    #[inline]
+    pub fn upper_bound(ty: Ty<'tcx>, tcx: TyCtxt<'tcx>) -> Self {
+        // Unwrap is ok because the type is known to be numeric.
+        let c = ty.numeric_max_val(tcx).unwrap();
+        let value = mir::Const::from_ty_const(c, tcx);
+        Self::Finite(value)
+    }
+
+    #[inline]
+    pub fn to_const(self, _ty: Ty<'tcx>, _tcx: TyCtxt<'tcx>) -> mir::Const<'tcx> {
+        match self {
+            Self::Finite(value) => value,
+        }
+    }
+    pub fn eval_bits(
+        self,
+        _ty: Ty<'tcx>,
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> u128 {
+        match self {
+            Self::Finite(value) => value.eval_bits(tcx, param_env),
+        }
+    }
+
+    #[instrument(skip(tcx, param_env), level = "debug", ret)]
+    pub fn compare_with(
+        self,
+        other: Self,
+        ty: Ty<'tcx>,
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> Option<Ordering> {
+        use PatRangeBoundary::*;
+        match (self, other) {
+            // This code is hot when compiling matches with many ranges. So we
+            // special-case extraction of evaluated scalars for speed, for types where
+            // raw data comparisons are appropriate. E.g. `unicode-normalization` has
+            // many ranges such as '\u{037A}'..='\u{037F}', and chars can be compared
+            // in this way.
+            (Finite(mir::Const::Ty(a)), Finite(mir::Const::Ty(b)))
+                if matches!(ty.kind(), ty::Uint(_) | ty::Char) =>
+            {
+                return Some(a.kind().cmp(&b.kind()));
+            }
+            (
+                Finite(mir::Const::Val(mir::ConstValue::Scalar(Scalar::Int(a)), _)),
+                Finite(mir::Const::Val(mir::ConstValue::Scalar(Scalar::Int(b)), _)),
+            ) if matches!(ty.kind(), ty::Uint(_) | ty::Char) => return Some(a.cmp(&b)),
+            _ => {}
+        }
+
+        let a = self.eval_bits(ty, tcx, param_env);
+        let b = other.eval_bits(ty, tcx, param_env);
+
+        match ty.kind() {
+            ty::Float(ty::FloatTy::F32) => {
+                use rustc_apfloat::Float;
+                let a = rustc_apfloat::ieee::Single::from_bits(a);
+                let b = rustc_apfloat::ieee::Single::from_bits(b);
+                a.partial_cmp(&b)
+            }
+            ty::Float(ty::FloatTy::F64) => {
+                use rustc_apfloat::Float;
+                let a = rustc_apfloat::ieee::Double::from_bits(a);
+                let b = rustc_apfloat::ieee::Double::from_bits(b);
+                a.partial_cmp(&b)
+            }
+            ty::Int(ity) => {
+                use rustc_middle::ty::layout::IntegerExt;
+                let size = rustc_target::abi::Integer::from_int_ty(&tcx, *ity).size();
+                let a = size.sign_extend(a) as i128;
+                let b = size.sign_extend(b) as i128;
+                Some(a.cmp(&b))
+            }
+            ty::Uint(_) | ty::Char => Some(a.cmp(&b)),
+            _ => bug!(),
+        }
+    }
 }
 
 impl<'tcx> fmt::Display for Pat<'tcx> {
@@ -904,11 +1111,7 @@ impl<'tcx> fmt::Display for Pat<'tcx> {
                 write!(f, "{subpattern}")
             }
             PatKind::Constant { value } => write!(f, "{value}"),
-            PatKind::Range(box PatRange { lo, hi, end }) => {
-                write!(f, "{lo}")?;
-                write!(f, "{end}")?;
-                write!(f, "{hi}")
-            }
+            PatKind::Range(ref range) => write!(f, "{range}"),
             PatKind::Slice { ref prefix, ref slice, ref suffix }
             | PatKind::Array { ref prefix, ref slice, ref suffix } => {
                 write!(f, "[")?;

compiler/rustc_middle/src/ty/util.rs

@@ -19,7 +19,7 @@ use rustc_index::bit_set::GrowableBitSet;
 use rustc_macros::HashStable;
 use rustc_session::Limit;
 use rustc_span::sym;
-use rustc_target::abi::{Integer, IntegerType, Size};
+use rustc_target::abi::{Integer, IntegerType, Primitive, Size};
 use rustc_target::spec::abi::Abi;
 use smallvec::SmallVec;
 use std::{fmt, iter};
@@ -917,54 +917,62 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for OpaqueTypeExpander<'tcx> {
 }
 
 impl<'tcx> Ty<'tcx> {
+    /// Returns the `Size` for primitive types (bool, uint, int, char, float).
+    pub fn primitive_size(self, tcx: TyCtxt<'tcx>) -> Size {
+        match *self.kind() {
+            ty::Bool => Size::from_bytes(1),
+            ty::Char => Size::from_bytes(4),
+            ty::Int(ity) => Integer::from_int_ty(&tcx, ity).size(),
+            ty::Uint(uty) => Integer::from_uint_ty(&tcx, uty).size(),
+            ty::Float(ty::FloatTy::F32) => Primitive::F32.size(&tcx),
+            ty::Float(ty::FloatTy::F64) => Primitive::F64.size(&tcx),
+            _ => bug!("non primitive type"),
+        }
+    }
+
     pub fn int_size_and_signed(self, tcx: TyCtxt<'tcx>) -> (Size, bool) {
-        let (int, signed) = match *self.kind() {
-            ty::Int(ity) => (Integer::from_int_ty(&tcx, ity), true),
-            ty::Uint(uty) => (Integer::from_uint_ty(&tcx, uty), false),
+        match *self.kind() {
+            ty::Int(ity) => (Integer::from_int_ty(&tcx, ity).size(), true),
+            ty::Uint(uty) => (Integer::from_uint_ty(&tcx, uty).size(), false),
             _ => bug!("non integer discriminant"),
-        };
-        (int.size(), signed)
+        }
     }
 
-    /// Returns the maximum value for the given numeric type (including `char`s)
-    /// or returns `None` if the type is not numeric.
-    pub fn numeric_max_val(self, tcx: TyCtxt<'tcx>) -> Option<ty::Const<'tcx>> {
-        let val = match self.kind() {
+    /// Returns the minimum and maximum values for the given numeric type (including `char`s) or
+    /// returns `None` if the type is not numeric.
+    pub fn numeric_min_and_max_as_bits(self, tcx: TyCtxt<'tcx>) -> Option<(u128, u128)> {
+        use rustc_apfloat::ieee::{Double, Single};
+        Some(match self.kind() {
             ty::Int(_) | ty::Uint(_) => {
                 let (size, signed) = self.int_size_and_signed(tcx);
-                let val =
+                let min = if signed { size.truncate(size.signed_int_min() as u128) } else { 0 };
+                let max =
                     if signed { size.signed_int_max() as u128 } else { size.unsigned_int_max() };
-                Some(val)
+                (min, max)
             }
-            ty::Char => Some(std::char::MAX as u128),
-            ty::Float(fty) => Some(match fty {
-                ty::FloatTy::F32 => rustc_apfloat::ieee::Single::INFINITY.to_bits(),
-                ty::FloatTy::F64 => rustc_apfloat::ieee::Double::INFINITY.to_bits(),
-            }),
-            _ => None,
-        };
+            ty::Char => (0, std::char::MAX as u128),
+            ty::Float(ty::FloatTy::F32) => {
+                ((-Single::INFINITY).to_bits(), Single::INFINITY.to_bits())
+            }
+            ty::Float(ty::FloatTy::F64) => {
+                ((-Double::INFINITY).to_bits(), Double::INFINITY.to_bits())
+            }
+            _ => return None,
+        })
+    }
 
-        val.map(|v| ty::Const::from_bits(tcx, v, ty::ParamEnv::empty().and(self)))
+    /// Returns the maximum value for the given numeric type (including `char`s)
+    /// or returns `None` if the type is not numeric.
+    pub fn numeric_max_val(self, tcx: TyCtxt<'tcx>) -> Option<ty::Const<'tcx>> {
+        self.numeric_min_and_max_as_bits(tcx)
+            .map(|(_, max)| ty::Const::from_bits(tcx, max, ty::ParamEnv::empty().and(self)))
     }
 
     /// Returns the minimum value for the given numeric type (including `char`s)
     /// or returns `None` if the type is not numeric.
     pub fn numeric_min_val(self, tcx: TyCtxt<'tcx>) -> Option<ty::Const<'tcx>> {
-        let val = match self.kind() {
-            ty::Int(_) | ty::Uint(_) => {
-                let (size, signed) = self.int_size_and_signed(tcx);
-                let val = if signed { size.truncate(size.signed_int_min() as u128) } else { 0 };
-                Some(val)
-            }
-            ty::Char => Some(0),
-            ty::Float(fty) => Some(match fty {
-                ty::FloatTy::F32 => (-::rustc_apfloat::ieee::Single::INFINITY).to_bits(),
-                ty::FloatTy::F64 => (-::rustc_apfloat::ieee::Double::INFINITY).to_bits(),
-            }),
-            _ => None,
-        };
-
-        val.map(|v| ty::Const::from_bits(tcx, v, ty::ParamEnv::empty().and(self)))
+        self.numeric_min_and_max_as_bits(tcx)
+            .map(|(min, _)| ty::Const::from_bits(tcx, min, ty::ParamEnv::empty().and(self)))
     }
 
     /// Checks whether values of this type `T` are *moved* or *copied*

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -1018,7 +1018,7 @@ enum TestKind<'tcx> {
         ty: Ty<'tcx>,
     },
 
-    /// Test whether the value falls within an inclusive or exclusive range
+    /// Test whether the value falls within an inclusive or exclusive range.
     Range(Box<PatRange<'tcx>>),
 
     /// Test that the length of the slice is equal to `len`.