Rollup merge of #111282 - scottmcm:remove-unneeded-assumes, r=workingjubilee

Remove some `assume`s from slice iterators that don't do anything

Because the start pointer is iterators is already a `NonNull`, we emit the appropriate `!nonnull` metadata when loading the pointer to tell LLVM that it's non-null.

Probably the best way to see that it's the metadata that's important (and not the `assume`) is to observe that LLVM actually *removes* the `assume` from the optimized IR: <https://rust.godbolt.org/z/KhE6G963n>.

(I also checked that, yes, the if-not-ZST `assume` on `end` is still doing something: it's how there's a `!nonnull` metadata on its load, even though it's an ordinary raw pointer.  The codegen test added in this PR fails if the other `assume` is  removed.)

Author: matthiaskrgr

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

@@ -124,12 +124,10 @@ macro_rules! iterator {
             fn next(&mut self) -> Option<$elem> {
                 // could be implemented with slices, but this avoids bounds checks
 
-                // SAFETY: `assume` calls are safe since a slice's start pointer
-                // must be non-null, and slices over non-ZSTs must also have a
-                // non-null end pointer. The call to `next_unchecked!` is safe
-                // since we check if the iterator is empty first.
+                // SAFETY: `assume` call is safe because slices over non-ZSTs must
+                // have a non-null end pointer. The call to `next_unchecked!` is
+                // safe since we check if the iterator is empty first.
                 unsafe {
-                    assume(!self.ptr.as_ptr().is_null());
                     if !<T>::IS_ZST {
                         assume(!self.end.is_null());
                     }
@@ -339,12 +337,10 @@ macro_rules! iterator {
             fn next_back(&mut self) -> Option<$elem> {
                 // could be implemented with slices, but this avoids bounds checks
 
-                // SAFETY: `assume` calls are safe since a slice's start pointer must be non-null,
-                // and slices over non-ZSTs must also have a non-null end pointer.
-                // The call to `next_back_unchecked!` is safe since we check if the iterator is
-                // empty first.
+                // SAFETY: `assume` call is safe because slices over non-ZSTs must
+                // have a non-null end pointer. The call to `next_back_unchecked!`
+                // is safe since we check if the iterator is empty first.
                 unsafe {
-                    assume(!self.ptr.as_ptr().is_null());
                     if !<T>::IS_ZST {
                         assume(!self.end.is_null());
                     }

tests/codegen/slice-iter-nonnull.rs

@@ -0,0 +1,42 @@
+// no-system-llvm
+// compile-flags: -O
+// ignore-debug (these add extra checks that make it hard to verify)
+#![crate_type = "lib"]
+
+// The slice iterator used to `assume` that the `start` pointer was non-null.
+// That ought to be unneeded, though, since the type is `NonNull`, so this test
+// confirms that the appropriate metadata is included to denote that.
+
+// CHECK-LABEL: @slice_iter_next(
+#[no_mangle]
+pub fn slice_iter_next<'a>(it: &mut std::slice::Iter<'a, u32>) -> Option<&'a u32> {
+    // CHECK: %[[ENDP:.+]] = getelementptr{{.+}}ptr %it,{{.+}} 1
+    // CHECK: %[[END:.+]] = load ptr, ptr %[[ENDP]]
+    // CHECK-SAME: !nonnull
+    // CHECK-SAME: !noundef
+    // CHECK: %[[START:.+]] = load ptr, ptr %it,
+    // CHECK-SAME: !nonnull
+    // CHECK-SAME: !noundef
+    // CHECK: icmp eq ptr %[[START]], %[[END]]
+
+    // CHECK: store ptr{{.+}}, ptr %it,
+
+    it.next()
+}
+
+// CHECK-LABEL: @slice_iter_next_back(
+#[no_mangle]
+pub fn slice_iter_next_back<'a>(it: &mut std::slice::Iter<'a, u32>) -> Option<&'a u32> {
+    // CHECK: %[[ENDP:.+]] = getelementptr{{.+}}ptr %it,{{.+}} 1
+    // CHECK: %[[END:.+]] = load ptr, ptr %[[ENDP]]
+    // CHECK-SAME: !nonnull
+    // CHECK-SAME: !noundef
+    // CHECK: %[[START:.+]] = load ptr, ptr %it,
+    // CHECK-SAME: !nonnull
+    // CHECK-SAME: !noundef
+    // CHECK: icmp eq ptr %[[START]], %[[END]]
+
+    // CHECK: store ptr{{.+}}, ptr %[[ENDP]],
+
+    it.next_back()
+}