Implement some more checks in ptr_guaranteed_cmp.

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -289,13 +289,91 @@ impl<'tcx> CompileTimeInterpCx<'tcx> {
             }
             // Other ways of comparing integers and pointers can never be known for sure.
             (Scalar::Int { .. }, Scalar::Ptr(..)) | (Scalar::Ptr(..), Scalar::Int { .. }) => 2,
-            // FIXME: return a `1` for when both sides are the same pointer, *except* that
-            // some things (like functions and vtables) do not have stable addresses
-            // so we need to be careful around them (see e.g. #73722).
-            // FIXME: return `0` for at least some comparisons where we can reliably
-            // determine the result of runtime inequality tests at compile-time.
-            // Examples include comparison of addresses in different static items.
-            (Scalar::Ptr(..), Scalar::Ptr(..)) => 2,
+            (Scalar::Ptr(a, _), Scalar::Ptr(b, _)) => {
+                let (a_prov, a_offset) = a.prov_and_relative_offset();
+                let (b_prov, b_offset) = b.prov_and_relative_offset();
+                let a_allocid = a_prov.alloc_id();
+                let b_allocid = b_prov.alloc_id();
+                let a_info = self.get_alloc_info(a_allocid);
+                let b_info = self.get_alloc_info(b_allocid);
+
+                // Check if the pointers cannot be equal due to alignment
+                if a_info.align > Align::ONE && b_info.align > Align::ONE {
+                    let min_align = Ord::min(a_info.align.bytes(), b_info.align.bytes());
+                    let a_residue = a_offset.bytes() % min_align;
+                    let b_residue = b_offset.bytes() % min_align;
+                    if a_residue != b_residue {
+                        // If the two pointers have a different residue from their
+                        // common alignment, they cannot be equal.
+                        return interp_ok(0);
+                    }
+                    // The pointers have the same residue modulo their common alignment,
+                    // so they could be equal. Try the other checks.
+                }
+
+                if a_allocid == b_allocid {
+                    match self.tcx.try_get_global_alloc(a_allocid) {
+                        None => 2,
+                        // A static cannot be duplicated, so if two pointers are into the same
+                        // static, they are equal if and only if their offsets into the static
+                        // are equal
+                        Some(GlobalAlloc::Static(_)) => (a_offset == b_offset) as u8,
+                        // Functions and vtables can be duplicated (and deduplicated), so we
+                        // cannot be sure of runtime equality of pointers to the same one, (or the
+                        // runtime inequality of pointers to different ones) (see e.g. #73722).
+                        Some(GlobalAlloc::Function { .. } | GlobalAlloc::VTable(..)) => 2,
+                        // FIXME: Can these be duplicated (or deduplicated)?
+                        Some(GlobalAlloc::Memory(..) | GlobalAlloc::TypeId { .. }) => 2,
+                    }
+                } else {
+                    if let (Some(GlobalAlloc::Static(a_did)), Some(GlobalAlloc::Static(b_did))) = (
+                        self.tcx.try_get_global_alloc(a_allocid),
+                        self.tcx.try_get_global_alloc(b_allocid),
+                    ) {
+                        debug_assert_ne!(
+                            a_did, b_did,
+                            "same static item DefId had two different AllocIds? {a_allocid:?} != {b_allocid:?}, {a_did:?} == {b_did:?}"
+                        );
+
+                        if a_info.size == Size::ZERO || b_info.size == Size::ZERO {
+                            // One or both allocations is zero-sized, so we can't know if the
+                            // pointers are (in)equal.
+                            // FIXME: Can zero-sized static be "within" non-zero-sized statics?
+                            // Conservatively we say yes, since that doesn't cause them to
+                            // "overlap" any bytes, but if not, then we could delete this branch
+                            // and have the other branches handle ZST allocations.
+                            2
+                        } else if a_offset > a_info.size || b_offset > b_info.size {
+                            // One or both pointers are out of bounds of their allocation,
+                            // so conservatively say we don't know.
+                            // FIXME: we could reason about how far out of bounds the pointers are,
+                            // e.g. two pointers cannot be equal if them being equal would require
+                            // their statics to overlap.
+                            2
+                        } else if (a_offset == Size::ZERO && b_offset == b_info.size)
+                            || (a_offset == a_info.size && b_offset == Size::ZERO)
+                        {
+                            // The pointers are on opposite ends of different allocations, we
+                            // cannot know if they are equal, since the allocations may end up
+                            // adjacent at runtime.
+                            2
+                        } else {
+                            // The pointers are within (or one past the end of) different
+                            // non-zero-sized static allocations, and they are not at oppotiste
+                            // ends, so we know they are not equal because statics cannot
+                            // overlap or be deduplicated.
+                            0
+                        }
+                    } else {
+                        // Even if one of them is a static, as per https://doc.rust-lang.org/nightly/reference/items/static-items.html#r-items.static.storage-disjointness
+                        // immutable statics can overlap with other kinds of allocations somtimes.
+                        // FIXME: We could be more decisive for mutable statics, which cannot
+                        // overlap with other kinds of allocations.
+                        // FIXME: Can we determine any other cases?
+                        2
+                    }
+                }
+            }
         })
     }
 }

tests/ui/const-ptr/guaranteed_cmp.rs

@@ -0,0 +1,171 @@
+//@ build-pass
+//@ edition: 2024
+#![feature(const_raw_ptr_comparison)]
+#![feature(fn_align)]
+// Generally:
+// For any `Some` return, `None` would also be valid, unless otherwise noted.
+// For any `None` return, only `None` is valid, unless otherwise noted.
+
+macro_rules! do_test {
+    ($a:expr, $b:expr, $expected:pat) => {
+        const _: () = {
+            let a: *const _ = $a;
+            let b: *const _ = $b;
+            assert!(matches!(<*const u8>::guaranteed_eq(a.cast(), b.cast()), $expected));
+        };
+    };
+}
+
+#[repr(align(2))]
+struct T(#[allow(unused)] u16);
+
+#[repr(align(2))]
+struct AlignedZst;
+
+static A: T = T(42);
+static B: T = T(42);
+static mut MUT_STATIC: T = T(42);
+static ZST: () = ();
+static ALIGNED_ZST: AlignedZst = AlignedZst;
+static LARGE_WORD_ALIGNED: [usize; 2] = [0, 1];
+static mut MUT_LARGE_WORD_ALIGNED: [usize; 2] = [0, 1];
+
+const FN_PTR: *const () = {
+    fn foo() {}
+    unsafe { std::mem::transmute(foo as fn()) }
+};
+
+const ALIGNED_FN_PTR: *const () = {
+    #[rustc_align(2)]
+    fn aligned_foo() {}
+    unsafe { std::mem::transmute(aligned_foo as fn()) }
+};
+
+// Only on armv5te-* and armv4t-*
+#[cfg(all(
+    target_arch = "arm",
+    not(target_feature = "v6"),
+))]
+const ALIGNED_THUMB_FN_PTR: *const () = {
+    #[rustc_align(2)]
+    #[instruction_set(arm::t32)]
+    fn aligned_thumb_foo() {}
+    unsafe { std::mem::transmute(aligned_thumb_foo as fn()) }
+};
+
+trait Trait {
+    #[allow(unused)]
+    fn method(&self) -> u8;
+}
+impl Trait for u32 {
+    fn method(&self) -> u8 { 1 }
+}
+impl Trait for i32 {
+    fn method(&self) -> u8 { 2 }
+}
+
+const VTABLE_PTR_1: *const () = {
+    let [_data, vtable] = unsafe {
+        std::mem::transmute::<&dyn Trait, [*const (); 2]>(&42_u32 as &dyn Trait)
+    };
+    vtable
+};
+const VTABLE_PTR_2: *const () = {
+    let [_data, vtable] = unsafe {
+        std::mem::transmute::<&dyn Trait, [*const (); 2]>(&42_i32 as &dyn Trait)
+    };
+    vtable
+};
+
+// Cannot be `None`: static's address, references, and `fn` pointers cannot be null,
+// and `is_null` is stable with strong guarantees, and `is_null` is implemented using
+// `guaranteed_cmp`.
+do_test!(&A, std::ptr::null::<()>(), Some(false));
+do_test!(&ZST, std::ptr::null::<()>(), Some(false));
+do_test!(&(), std::ptr::null::<()>(), Some(false));
+do_test!(const { &() }, std::ptr::null::<()>(), Some(false));
+do_test!(FN_PTR, std::ptr::null::<()>(), Some(false));
+
+// Statics cannot be duplicated
+do_test!(&A, &A, Some(true));
+
+// Two non-ZST statics cannot have the same address
+do_test!(&A, &B, Some(false));
+do_test!(&A, &raw const MUT_STATIC, Some(false));
+
+// One-past-the-end of one static can be equal to the address of another static.
+do_test!(&A, (&raw const B).wrapping_add(1), None);
+
+// Cannot know if ZST static is at the same address with anything non-null (if alignment allows).
+do_test!(&A, &ZST, None);
+do_test!(&A, &ALIGNED_ZST, None);
+
+// Unclear if ZST statics can be placed "in the middle of" non-ZST statics.
+// For now, we conservatively say they could, and return None here.
+do_test!(&ZST, (&raw const A).wrapping_byte_add(1), None);
+
+// As per https://doc.rust-lang.org/nightly/reference/items/static-items.html#r-items.static.storage-disjointness
+// immutable statics are allowed to overlap with const items and promoteds.
+do_test!(&A, &T(42), None);
+do_test!(&A, const { &T(42) }, None);
+do_test!(&A, { const X: T = T(42); &X }, None);
+
+// These could return Some(false), since only immutable statics can overlap with const items
+// and promoteds.
+do_test!(&raw const MUT_STATIC, &T(42), None);
+do_test!(&raw const MUT_STATIC, const { &T(42) }, None);
+do_test!(&raw const MUT_STATIC, { const X: T = T(42); &X }, None);
+
+// An odd offset from a 2-aligned allocation can never be equal to an even offset from a
+// 2-aligned allocation, even if the offsets are out-of-bounds.
+do_test!(&A, (&raw const B).wrapping_byte_add(1), Some(false));
+do_test!(&A, (&raw const B).wrapping_byte_add(5), Some(false));
+do_test!(&A, (&raw const ALIGNED_ZST).wrapping_byte_add(1), Some(false));
+do_test!(&ALIGNED_ZST, (&raw const A).wrapping_byte_add(1), Some(false));
+do_test!(&A, (&T(42) as *const T).wrapping_byte_add(1), Some(false));
+do_test!(&A, (const { &T(42) } as *const T).wrapping_byte_add(1), Some(false));
+do_test!(&A, ({ const X: T = T(42); &X } as *const T).wrapping_byte_add(1), Some(false));
+
+// Pointers into the same static are equal if and only if their offset is the same,
+// even if either is out-of-bounds.
+do_test!(&A, &A, Some(true));
+do_test!(&A, &A.0, Some(true));
+do_test!(&A, (&raw const A).wrapping_byte_add(1), Some(false));
+do_test!(&A, (&raw const A).wrapping_byte_add(2), Some(false));
+do_test!(&A, (&raw const A).wrapping_byte_add(51), Some(false));
+do_test!((&raw const A).wrapping_byte_add(51), (&raw const A).wrapping_byte_add(51), Some(true));
+
+// Pointers to the same fn may be unequal, since `fn`s can be duplicated.
+do_test!(FN_PTR, FN_PTR, None);
+do_test!(ALIGNED_FN_PTR, ALIGNED_FN_PTR, None);
+
+// Pointers to different fns may be equal, since `fn`s can be deduplicated.
+do_test!(FN_PTR, ALIGNED_FN_PTR, None);
+
+// Pointers to the same vtable may be unequal, since vtables can be duplicated.
+do_test!(VTABLE_PTR_1, VTABLE_PTR_1, None);
+
+// Pointers to different vtables may be equal, since vtables can be deduplicated.
+do_test!(VTABLE_PTR_1, VTABLE_PTR_2, None);
+
+// Function pointers to aligned function allocations are not necessarily actually aligned,
+// due to platform-specific semantics.
+// See https://github.com/rust-lang/rust/issues/144661
+// FIXME: This could return `Some` on platforms where function pointers' addresses actually
+// correspond to function addresses including alignment, or on ARM if t32 function pointers
+// have their low bit set for consteval.
+do_test!(ALIGNED_FN_PTR, ALIGNED_FN_PTR.wrapping_byte_offset(1), None);
+#[cfg(all(
+    target_arch = "arm",
+    not(target_feature = "v6"),
+))]
+do_test!(ALIGNED_THUMB_FN_PTR, ALIGNED_THUMB_FN_PTR.wrapping_byte_offset(1), None);
+
+// Conservatively say we don't know.
+do_test!(FN_PTR, VTABLE_PTR_1, None);
+do_test!((&raw const LARGE_WORD_ALIGNED).cast::<usize>().wrapping_add(1), VTABLE_PTR_1, None);
+do_test!((&raw const MUT_LARGE_WORD_ALIGNED).cast::<usize>().wrapping_add(1), VTABLE_PTR_1, None);
+do_test!((&raw const LARGE_WORD_ALIGNED).cast::<usize>().wrapping_add(1), FN_PTR, None);
+do_test!((&raw const MUT_LARGE_WORD_ALIGNED).cast::<usize>().wrapping_add(1), FN_PTR, None);
+
+fn main() {}