Save another BB by using SubUnchecked instead of a call to arith_offset

Probably reasonable anyway since it more obviously drops provenance.

Author: scottmcm

library/core/src/slice/iter.rs

@@ -11,7 +11,7 @@ use crate::iter::{
 use crate::marker::PhantomData;
 use crate::mem::{self, SizedTypeProperties};
 use crate::num::NonZero;
-use crate::ptr::{NonNull, null, without_provenance, without_provenance_mut};
+use crate::ptr::{NonNull, without_provenance, without_provenance_mut};
 use crate::{cmp, fmt};
 
 #[stable(feature = "boxed_slice_into_iter", since = "1.80.0")]

library/core/src/slice/iter/macros.rs

@@ -162,13 +162,13 @@ macro_rules! iterator {
                 // SAFETY: Type invariants.
                 unsafe {
                     if T::IS_ZST {
-                        let byte_end = end_or_len as *const u8;
-                        if byte_end == null() {
+                        let len = end_or_len.addr();
+                        if len == 0 {
                             return None;
                         }
-                        self.end_or_len = byte_end.wrapping_sub(1) as _;
+                        self.end_or_len = without_provenance_mut(len.unchecked_sub(1));
                     } else {
-                        if ptr == crate::intrinsics::transmute::<*const T, NonNull<T>>(end_or_len) {
+                        if ptr == crate::intrinsics::transmute::<$ptr, NonNull<T>>(end_or_len) {
                             return None;
                         }
                         self.ptr = ptr.add(1);

tests/mir-opt/pre-codegen/slice_iter.enumerated_loop.PreCodegen.after.panic-abort.mir

@@ -7,27 +7,27 @@ fn enumerated_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
     let mut _11: std::ptr::NonNull<T>;
     let mut _12: *const T;
     let mut _13: usize;
-    let mut _33: std::option::Option<(usize, &T)>;
-    let mut _36: &impl Fn(usize, &T);
-    let mut _37: (usize, &T);
-    let _38: ();
+    let mut _31: std::option::Option<(usize, &T)>;
+    let mut _34: &impl Fn(usize, &T);
+    let mut _35: (usize, &T);
+    let _36: ();
     scope 1 {
         debug (((iter: Enumerate<std::slice::Iter<'_, T>>).0: std::slice::Iter<'_, T>).0: std::ptr::NonNull<T>) => _11;
         debug (((iter: Enumerate<std::slice::Iter<'_, T>>).0: std::slice::Iter<'_, T>).1: *const T) => _12;
         debug (((iter: Enumerate<std::slice::Iter<'_, T>>).0: std::slice::Iter<'_, T>).2: std::marker::PhantomData<&T>) => const ZeroSized: PhantomData<&T>;
         debug ((iter: Enumerate<std::slice::Iter<'_, T>>).1: usize) => _13;
-        let _34: usize;
-        let _35: &T;
+        let _32: usize;
+        let _33: &T;
         scope 2 {
-            debug i => _34;
-            debug x => _35;
+            debug i => _32;
+            debug x => _33;
         }
         scope 19 (inlined <Enumerate<std::slice::Iter<'_, T>> as Iterator>::next) {
-            let mut _27: std::option::Option<&T>;
-            let mut _31: (usize, bool);
-            let mut _32: (usize, &T);
+            let mut _25: std::option::Option<&T>;
+            let mut _29: (usize, bool);
+            let mut _30: (usize, &T);
             scope 20 {
-                let _30: usize;
+                let _28: usize;
                 scope 25 {
                 }
             }
@@ -42,8 +42,8 @@ fn enumerated_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
                 }
             }
             scope 26 (inlined <Option<&T> as Try>::branch) {
-                let mut _28: isize;
-                let _29: &T;
+                let mut _26: isize;
+                let _27: &T;
                 scope 27 {
                 }
             }
@@ -52,43 +52,37 @@ fn enumerated_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
                 let _16: std::ptr::NonNull<T>;
                 let mut _19: bool;
                 let mut _22: std::ptr::NonNull<T>;
-                let mut _24: bool;
-                let mut _25: *const u8;
-                let mut _26: *const T;
+                let mut _24: usize;
                 scope 30 {
                     let _15: *const T;
                     scope 31 {
-                        let _23: *const u8;
+                        let _23: usize;
                         scope 32 {
-                            scope 33 (inlined null::<u8>) {
-                                scope 34 (inlined without_provenance::<()>) {
-                                    scope 35 (inlined without_provenance_mut::<()>) {
+                            scope 35 (inlined core::num::<impl usize>::unchecked_sub) {
+                                scope 36 (inlined core::ub_checks::check_language_ub) {
+                                    scope 37 (inlined core::ub_checks::check_language_ub::runtime) {
                                     }
                                 }
-                                scope 36 (inlined std::ptr::from_raw_parts::<u8, ()>) {
-                                }
                             }
-                            scope 37 (inlined std::ptr::const_ptr::<impl *const u8>::wrapping_sub) {
-                                scope 38 (inlined core::num::<impl isize>::wrapping_neg) {
-                                    scope 39 (inlined core::num::<impl isize>::wrapping_sub) {
-                                    }
-                                }
-                                scope 40 (inlined std::ptr::const_ptr::<impl *const u8>::wrapping_offset) {
-                                }
+                            scope 38 (inlined without_provenance_mut::<T>) {
                             }
                         }
-                        scope 41 (inlined <NonNull<T> as PartialEq>::eq) {
+                        scope 33 (inlined std::ptr::const_ptr::<impl *const T>::addr) {
+                            scope 34 (inlined std::ptr::const_ptr::<impl *const T>::cast::<()>) {
+                            }
+                        }
+                        scope 39 (inlined <NonNull<T> as PartialEq>::eq) {
                             let mut _17: *mut T;
                             let mut _18: *mut T;
-                            scope 42 (inlined NonNull::<T>::as_ptr) {
+                            scope 40 (inlined NonNull::<T>::as_ptr) {
                             }
-                            scope 43 (inlined NonNull::<T>::as_ptr) {
+                            scope 41 (inlined NonNull::<T>::as_ptr) {
                             }
                         }
-                        scope 44 (inlined NonNull::<T>::add) {
+                        scope 42 (inlined NonNull::<T>::add) {
                             let mut _20: *const T;
                             let mut _21: *const T;
-                            scope 45 (inlined NonNull::<T>::as_ptr) {
+                            scope 43 (inlined NonNull::<T>::as_ptr) {
                             }
                         }
                     }
@@ -181,13 +175,14 @@ fn enumerated_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
     }
 
     bb4: {
-        StorageLive(_33);
-        StorageLive(_30);
         StorageLive(_31);
-        StorageLive(_27);
+        StorageLive(_28);
+        StorageLive(_29);
+        StorageLive(_25);
         StorageLive(_14);
         StorageLive(_15);
         StorageLive(_23);
+        StorageLive(_24);
         StorageLive(_16);
         _14 = copy _11;
         _15 = copy _12;
@@ -225,104 +220,97 @@ fn enumerated_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
     bb7: {
         StorageDead(_19);
         StorageDead(_16);
+        StorageDead(_24);
         StorageDead(_23);
         StorageDead(_15);
         StorageDead(_14);
-        StorageLive(_28);
-        goto -> bb17;
+        StorageLive(_26);
+        goto -> bb12;
     }
 
     bb8: {
-        _23 = copy _15 as *const u8 (PtrToPtr);
-        StorageLive(_24);
-        _24 = Eq(copy _23, const {0x0 as *const u8});
-        switchInt(move _24) -> [0: bb9, otherwise: bb16];
+        _23 = copy _15 as usize (Transmute);
+        switchInt(copy _23) -> [0: bb9, otherwise: bb10];
     }
 
     bb9: {
+        StorageDead(_16);
         StorageDead(_24);
-        StorageLive(_25);
-        _25 = arith_offset::<u8>(move _23, const -1_isize) -> [return: bb10, unwind unreachable];
+        StorageDead(_23);
+        StorageDead(_15);
+        StorageDead(_14);
+        StorageLive(_26);
+        goto -> bb12;
     }
 
     bb10: {
-        _26 = move _25 as *const T (PtrToPtr);
-        StorageDead(_25);
-        _12 = copy _26;
+        _24 = SubUnchecked(copy _23, const 1_usize);
+        _12 = copy _24 as *const T (Transmute);
         goto -> bb11;
     }
 
     bb11: {
-        _27 = copy _14 as std::option::Option<&T> (Transmute);
+        _25 = copy _14 as std::option::Option<&T> (Transmute);
         StorageDead(_16);
+        StorageDead(_24);
         StorageDead(_23);
         StorageDead(_15);
         StorageDead(_14);
-        StorageLive(_28);
-        _28 = discriminant(_27);
-        switchInt(move _28) -> [0: bb17, 1: bb12, otherwise: bb15];
+        StorageLive(_26);
+        _26 = discriminant(_25);
+        switchInt(move _26) -> [0: bb12, 1: bb14, otherwise: bb17];
     }
 
     bb12: {
-        _29 = move ((_27 as Some).0: &T);
+        StorageDead(_26);
+        StorageDead(_25);
+        StorageDead(_29);
         StorageDead(_28);
-        StorageDead(_27);
-        _30 = copy _13;
-        _31 = AddWithOverflow(copy _13, const 1_usize);
-        assert(!move (_31.1: bool), "attempt to compute `{} + {}`, which would overflow", copy _13, const 1_usize) -> [success: bb13, unwind unreachable];
+        StorageDead(_31);
+        StorageDead(_11);
+        StorageDead(_12);
+        StorageDead(_13);
+        drop(_2) -> [return: bb13, unwind unreachable];
     }
 
     bb13: {
-        _13 = move (_31.0: usize);
-        StorageLive(_32);
-        _32 = (copy _30, copy _29);
-        _33 = Option::<(usize, &T)>::Some(move _32);
-        StorageDead(_32);
-        StorageDead(_31);
-        StorageDead(_30);
-        _34 = copy (((_33 as Some).0: (usize, &T)).0: usize);
-        _35 = copy (((_33 as Some).0: (usize, &T)).1: &T);
-        StorageLive(_36);
-        _36 = &_2;
-        StorageLive(_37);
-        _37 = (copy _34, copy _35);
-        _38 = <impl Fn(usize, &T) as Fn<(usize, &T)>>::call(move _36, move _37) -> [return: bb14, unwind unreachable];
+        return;
     }
 
     bb14: {
-        StorageDead(_37);
-        StorageDead(_36);
-        StorageDead(_33);
-        goto -> bb4;
+        _27 = move ((_25 as Some).0: &T);
+        StorageDead(_26);
+        StorageDead(_25);
+        _28 = copy _13;
+        _29 = AddWithOverflow(copy _13, const 1_usize);
+        assert(!move (_29.1: bool), "attempt to compute `{} + {}`, which would overflow", copy _13, const 1_usize) -> [success: bb15, unwind unreachable];
     }
 
     bb15: {
-        unreachable;
+        _13 = move (_29.0: usize);
+        StorageLive(_30);
+        _30 = (copy _28, copy _27);
+        _31 = Option::<(usize, &T)>::Some(move _30);
+        StorageDead(_30);
+        StorageDead(_29);
+        StorageDead(_28);
+        _32 = copy (((_31 as Some).0: (usize, &T)).0: usize);
+        _33 = copy (((_31 as Some).0: (usize, &T)).1: &T);
+        StorageLive(_34);
+        _34 = &_2;
+        StorageLive(_35);
+        _35 = (copy _32, copy _33);
+        _36 = <impl Fn(usize, &T) as Fn<(usize, &T)>>::call(move _34, move _35) -> [return: bb16, unwind unreachable];
     }
 
     bb16: {
-        StorageDead(_24);
-        StorageDead(_16);
-        StorageDead(_23);
-        StorageDead(_15);
-        StorageDead(_14);
-        StorageLive(_28);
-        goto -> bb17;
-    }
-
-    bb17: {
-        StorageDead(_28);
-        StorageDead(_27);
+        StorageDead(_35);
+        StorageDead(_34);
         StorageDead(_31);
-        StorageDead(_30);
-        StorageDead(_33);
-        StorageDead(_11);
-        StorageDead(_12);
-        StorageDead(_13);
-        drop(_2) -> [return: bb18, unwind unreachable];
+        goto -> bb4;
     }
 
-    bb18: {
-        return;
+    bb17: {
+        unreachable;
     }
 }