Auto merge of rust-lang#126844 - scottmcm:more-ptr-cast-gvn, r=saethlin

Remove more `PtrToPtr` casts in GVN

This addresses two things I noticed in MIR:

1. `NonNull::<T>::eq` does `(a as *mut T) == (b as *mut T)`, but it could just compare the `*const T`s, so this removes `PtrToPtr` casts that are on both sides of a pointer comparison, so long as they're not fat-to-thin casts.

2. `NonNull::<T>::addr` does `transmute::<_, usize>(p as *const ())`, but so long as `T: Thin` that cast doesn't do anything, and thus we can directly transmute the `*const T` instead.

r? mir-opt

Author: bors

compiler/rustc_middle/src/mir/tcx.rs

@@ -289,19 +289,7 @@ impl<'tcx> UnOp {
     pub fn ty(&self, tcx: TyCtxt<'tcx>, arg_ty: Ty<'tcx>) -> Ty<'tcx> {
         match self {
             UnOp::Not | UnOp::Neg => arg_ty,
-            UnOp::PtrMetadata => {
-                let pointee_ty = arg_ty
-                    .builtin_deref(true)
-                    .unwrap_or_else(|| bug!("PtrMetadata of non-dereferenceable ty {arg_ty:?}"));
-                if pointee_ty.is_trivially_sized(tcx) {
-                    tcx.types.unit
-                } else {
-                    let Some(metadata_def_id) = tcx.lang_items().metadata_type() else {
-                        bug!("No metadata_type lang item while looking at {arg_ty:?}")
-                    };
-                    Ty::new_projection(tcx, metadata_def_id, [pointee_ty])
-                }
-            }
+            UnOp::PtrMetadata => arg_ty.pointee_metadata_ty_or_projection(tcx),
         }
     }
 }

compiler/rustc_middle/src/ty/sty.rs

@@ -1647,6 +1647,34 @@ impl<'tcx> Ty<'tcx> {
         }
     }
 
+    /// Given a pointer or reference type, returns the type of the *pointee*'s
+    /// metadata. If it can't be determined exactly (perhaps due to still
+    /// being generic) then a projection through `ptr::Pointee` will be returned.
+    ///
+    /// This is particularly useful for getting the type of the result of
+    /// [`UnOp::PtrMetadata`](crate::mir::UnOp::PtrMetadata).
+    ///
+    /// Panics if `self` is not dereferencable.
+    #[track_caller]
+    pub fn pointee_metadata_ty_or_projection(self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
+        let Some(pointee_ty) = self.builtin_deref(true) else {
+            bug!("Type {self:?} is not a pointer or reference type")
+        };
+        if pointee_ty.is_trivially_sized(tcx) {
+            tcx.types.unit
+        } else {
+            match pointee_ty.ptr_metadata_ty_or_tail(tcx, |x| x) {
+                Ok(metadata_ty) => metadata_ty,
+                Err(tail_ty) => {
+                    let Some(metadata_def_id) = tcx.lang_items().metadata_type() else {
+                        bug!("No metadata_type lang item while looking at {self:?}")
+                    };
+                    Ty::new_projection(tcx, metadata_def_id, [tail_ty])
+                }
+            }
+        }
+    }
+
     /// When we create a closure, we record its kind (i.e., what trait
     /// it implements, constrained by how it uses its borrows) into its
     /// [`ty::ClosureArgs`] or [`ty::CoroutineClosureArgs`] using a type

compiler/rustc_mir_transform/src/cost_checker.rs

@@ -60,7 +60,15 @@ impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
 
     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, _location: Location) {
         match rvalue {
-            Rvalue::NullaryOp(NullOp::UbChecks, ..) if !self.tcx.sess.ub_checks() => {
+            Rvalue::NullaryOp(NullOp::UbChecks, ..)
+                if !self
+                    .tcx
+                    .sess
+                    .opts
+                    .unstable_opts
+                    .inline_mir_preserve_debug
+                    .unwrap_or(self.tcx.sess.ub_checks()) =>
+            {
                 // If this is in optimized MIR it's because it's used later,
                 // so if we don't need UB checks this session, give a bonus
                 // here to offset the cost of the call later.
@@ -111,12 +119,19 @@ impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
                 }
             }
             TerminatorKind::Assert { unwind, msg, .. } => {
-                self.penalty +=
-                    if msg.is_optional_overflow_check() && !self.tcx.sess.overflow_checks() {
-                        INSTR_COST
-                    } else {
-                        CALL_PENALTY
-                    };
+                self.penalty += if msg.is_optional_overflow_check()
+                    && !self
+                        .tcx
+                        .sess
+                        .opts
+                        .unstable_opts
+                        .inline_mir_preserve_debug
+                        .unwrap_or(self.tcx.sess.overflow_checks())
+                {
+                    INSTR_COST
+                } else {
+                    CALL_PENALTY
+                };
                 if let UnwindAction::Cleanup(_) = unwind {
                     self.penalty += LANDINGPAD_PENALTY;
                 }

compiler/rustc_mir_transform/src/gvn.rs

@@ -823,18 +823,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                 return self.simplify_cast(kind, value, to, location);
             }
             Rvalue::BinaryOp(op, box (ref mut lhs, ref mut rhs)) => {
-                let ty = lhs.ty(self.local_decls, self.tcx);
-                let lhs = self.simplify_operand(lhs, location);
-                let rhs = self.simplify_operand(rhs, location);
-                // Only short-circuit options after we called `simplify_operand`
-                // on both operands for side effect.
-                let lhs = lhs?;
-                let rhs = rhs?;
-
-                if let Some(value) = self.simplify_binary(op, ty, lhs, rhs) {
-                    return Some(value);
-                }
-                Value::BinaryOp(op, lhs, rhs)
+                return self.simplify_binary(op, lhs, rhs, location);
             }
             Rvalue::UnaryOp(op, ref mut arg_op) => {
                 return self.simplify_unary(op, arg_op, location);
@@ -987,23 +976,10 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                     // `*const [T]` -> `*const T` which remove metadata.
                     // We run on potentially-generic MIR, though, so unlike codegen
                     // we can't always know exactly what the metadata are.
-                    // Thankfully, equality on `ptr_metadata_ty_or_tail` gives us
-                    // what we need: `Ok(meta_ty)` if the metadata is known, or
-                    // `Err(tail_ty)` if not. Matching metadata is ok, but if
-                    // that's not known, then matching tail types is also ok,
-                    // allowing things like `*mut (?A, ?T)` <-> `*mut (?B, ?T)`.
-                    // FIXME: Would it be worth trying to normalize, rather than
-                    // passing the identity closure?  Or are the types in the
-                    // Cast realistically about as normalized as we can get anyway?
+                    // To allow things like `*mut (?A, ?T)` <-> `*mut (?B, ?T)`,
+                    // it's fine to get a projection as the type.
                     Value::Cast { kind: CastKind::PtrToPtr, value: inner, from, to }
-                        if from
-                            .builtin_deref(true)
-                            .unwrap()
-                            .ptr_metadata_ty_or_tail(self.tcx, |t| t)
-                            == to
-                                .builtin_deref(true)
-                                .unwrap()
-                                .ptr_metadata_ty_or_tail(self.tcx, |t| t) =>
+                        if self.pointers_have_same_metadata(*from, *to) =>
                     {
                         arg_index = *inner;
                         was_updated = true;
@@ -1068,6 +1044,52 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
 
     #[instrument(level = "trace", skip(self), ret)]
     fn simplify_binary(
+        &mut self,
+        op: BinOp,
+        lhs_operand: &mut Operand<'tcx>,
+        rhs_operand: &mut Operand<'tcx>,
+        location: Location,
+    ) -> Option<VnIndex> {
+        let lhs = self.simplify_operand(lhs_operand, location);
+        let rhs = self.simplify_operand(rhs_operand, location);
+        // Only short-circuit options after we called `simplify_operand`
+        // on both operands for side effect.
+        let mut lhs = lhs?;
+        let mut rhs = rhs?;
+
+        let lhs_ty = lhs_operand.ty(self.local_decls, self.tcx);
+
+        // If we're comparing pointers, remove `PtrToPtr` casts if the from
+        // types of both casts and the metadata all match.
+        if let BinOp::Eq | BinOp::Ne | BinOp::Lt | BinOp::Le | BinOp::Gt | BinOp::Ge = op
+            && lhs_ty.is_any_ptr()
+            && let Value::Cast {
+                kind: CastKind::PtrToPtr, value: lhs_value, from: lhs_from, ..
+            } = self.get(lhs)
+            && let Value::Cast {
+                kind: CastKind::PtrToPtr, value: rhs_value, from: rhs_from, ..
+            } = self.get(rhs)
+            && lhs_from == rhs_from
+            && self.pointers_have_same_metadata(*lhs_from, lhs_ty)
+        {
+            lhs = *lhs_value;
+            rhs = *rhs_value;
+            if let Some(op) = self.try_as_operand(lhs, location) {
+                *lhs_operand = op;
+            }
+            if let Some(op) = self.try_as_operand(rhs, location) {
+                *rhs_operand = op;
+            }
+        }
+
+        if let Some(value) = self.simplify_binary_inner(op, lhs_ty, lhs, rhs) {
+            return Some(value);
+        }
+        let value = Value::BinaryOp(op, lhs, rhs);
+        Some(self.insert(value))
+    }
+
+    fn simplify_binary_inner(
         &mut self,
         op: BinOp,
         lhs_ty: Ty<'tcx>,
@@ -1228,14 +1250,33 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             }
         }
 
+        // PtrToPtr-then-PtrToPtr can skip the intermediate step
         if let PtrToPtr = kind
             && let Value::Cast { kind: inner_kind, value: inner_value, from: inner_from, to: _ } =
                 *self.get(value)
             && let PtrToPtr = inner_kind
         {
             from = inner_from;
             value = inner_value;
-            *kind = PtrToPtr;
+            was_updated = true;
+            if inner_from == to {
+                return Some(inner_value);
+            }
+        }
+
+        // PtrToPtr-then-Transmute can just transmute the original, so long as the
+        // PtrToPtr didn't change metadata (and thus the size of the pointer)
+        if let Transmute = kind
+            && let Value::Cast {
+                kind: PtrToPtr,
+                value: inner_value,
+                from: inner_from,
+                to: inner_to,
+            } = *self.get(value)
+            && self.pointers_have_same_metadata(inner_from, inner_to)
+        {
+            from = inner_from;
+            value = inner_value;
             was_updated = true;
             if inner_from == to {
                 return Some(inner_value);
@@ -1289,6 +1330,21 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
         // Fallback: a symbolic `Len`.
         Some(self.insert(Value::Len(inner)))
     }
+
+    fn pointers_have_same_metadata(&self, left_ptr_ty: Ty<'tcx>, right_ptr_ty: Ty<'tcx>) -> bool {
+        let left_meta_ty = left_ptr_ty.pointee_metadata_ty_or_projection(self.tcx);
+        let right_meta_ty = right_ptr_ty.pointee_metadata_ty_or_projection(self.tcx);
+        if left_meta_ty == right_meta_ty {
+            true
+        } else if let Ok(left) =
+            self.tcx.try_normalize_erasing_regions(self.param_env, left_meta_ty)
+            && let Ok(right) = self.tcx.try_normalize_erasing_regions(self.param_env, right_meta_ty)
+        {
+            left == right
+        } else {
+            false
+        }
+    }
 }
 
 fn op_to_prop_const<'tcx>(

tests/coverage/closure_macro.cov-map

@@ -22,19 +22,19 @@ Number of file 0 mappings: 5
 - Code(Counter(0)) at (prev + 3, 1) to (start + 0, 2)
 
 Function name: closure_macro::main::{closure#0}
-Raw bytes (35): 0x[01, 01, 03, 01, 05, 05, 0b, 09, 0d, 05, 01, 10, 1c, 03, 21, 05, 04, 11, 01, 27, 02, 03, 11, 00, 16, 0d, 00, 17, 00, 1e, 07, 02, 09, 00, 0a]
+Raw bytes (35): 0x[01, 01, 03, 01, 05, 05, 0b, 09, 00, 05, 01, 10, 1c, 03, 21, 05, 04, 11, 01, 27, 02, 03, 11, 00, 16, 00, 00, 17, 00, 1e, 07, 02, 09, 00, 0a]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 3
 - expression 0 operands: lhs = Counter(0), rhs = Counter(1)
 - expression 1 operands: lhs = Counter(1), rhs = Expression(2, Add)
-- expression 2 operands: lhs = Counter(2), rhs = Counter(3)
+- expression 2 operands: lhs = Counter(2), rhs = Zero
 Number of file 0 mappings: 5
 - Code(Counter(0)) at (prev + 16, 28) to (start + 3, 33)
 - Code(Counter(1)) at (prev + 4, 17) to (start + 1, 39)
 - Code(Expression(0, Sub)) at (prev + 3, 17) to (start + 0, 22)
     = (c0 - c1)
-- Code(Counter(3)) at (prev + 0, 23) to (start + 0, 30)
+- Code(Zero) at (prev + 0, 23) to (start + 0, 30)
 - Code(Expression(1, Add)) at (prev + 2, 9) to (start + 0, 10)
-    = (c1 + (c2 + c3))
+    = (c1 + (c2 + Zero))
 

tests/coverage/closure_macro_async.cov-map

@@ -30,19 +30,19 @@ Number of file 0 mappings: 5
 - Code(Counter(0)) at (prev + 3, 1) to (start + 0, 2)
 
 Function name: closure_macro_async::test::{closure#0}::{closure#0}
-Raw bytes (35): 0x[01, 01, 03, 01, 05, 05, 0b, 09, 0d, 05, 01, 12, 1c, 03, 21, 05, 04, 11, 01, 27, 02, 03, 11, 00, 16, 0d, 00, 17, 00, 1e, 07, 02, 09, 00, 0a]
+Raw bytes (35): 0x[01, 01, 03, 01, 05, 05, 0b, 09, 00, 05, 01, 12, 1c, 03, 21, 05, 04, 11, 01, 27, 02, 03, 11, 00, 16, 00, 00, 17, 00, 1e, 07, 02, 09, 00, 0a]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 3
 - expression 0 operands: lhs = Counter(0), rhs = Counter(1)
 - expression 1 operands: lhs = Counter(1), rhs = Expression(2, Add)
-- expression 2 operands: lhs = Counter(2), rhs = Counter(3)
+- expression 2 operands: lhs = Counter(2), rhs = Zero
 Number of file 0 mappings: 5
 - Code(Counter(0)) at (prev + 18, 28) to (start + 3, 33)
 - Code(Counter(1)) at (prev + 4, 17) to (start + 1, 39)
 - Code(Expression(0, Sub)) at (prev + 3, 17) to (start + 0, 22)
     = (c0 - c1)
-- Code(Counter(3)) at (prev + 0, 23) to (start + 0, 30)
+- Code(Zero) at (prev + 0, 23) to (start + 0, 30)
 - Code(Expression(1, Add)) at (prev + 2, 9) to (start + 0, 10)
-    = (c1 + (c2 + c3))
+    = (c1 + (c2 + Zero))