Auto merge of #114720 - scottmcm:better-sub, r=workingjubilee

Tell LLVM that the negation in `<*const T>::sub` cannot overflow

Today it's just `sub` <https://rust.godbolt.org/z/8EzEPnMr5>; with this PR it's `sub nsw`.

1 files changed, 34 insertions, 0 deletions

diff --git a/tests/codegen/ptr-arithmetic.rs b/tests/codegen/ptr-arithmetic.rs
new file mode 100644
index 00000000000..292bfdaf357
--- /dev/null
+++ b/tests/codegen/ptr-arithmetic.rs
@@ -0,0 +1,34 @@
+// compile-flags: -O -Z merge-functions=disabled
+// ignore-debug (the extra assertions get in the way)
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: ptr @i32_add(
+// CHECK-SAME: [[WORD:i[0-9]+]] noundef %n)
+#[no_mangle]
+pub unsafe fn i32_add(p: *const i32, n: usize) -> *const i32 {
+    // CHECK: %[[TEMP:.+]] = getelementptr inbounds i32, ptr %p, [[WORD]] %n
+    // CHECK: ret ptr %[[TEMP]]
+    p.add(n)
+}
+
+// Ensure we tell LLVM that the negation in `sub` can't overflow.
+
+// CHECK-LABEL: ptr @i32_sub(
+// CHECK-SAME: [[WORD:i[0-9]+]] noundef %n)
+#[no_mangle]
+pub unsafe fn i32_sub(p: *const i32, n: usize) -> *const i32 {
+    // CHECK: %[[DELTA:.+]] = sub nsw [[WORD]] 0, %n
+    // CHECK: %[[TEMP:.+]] = getelementptr inbounds i32, ptr %p, [[WORD]] %[[DELTA]]
+    // CHECK: ret ptr %[[TEMP]]
+    p.sub(n)
+}
+
+// CHECK-LABEL: ptr @i32_offset(
+// CHECK-SAME: [[WORD:i[0-9]+]] noundef %d)
+#[no_mangle]
+pub unsafe fn i32_offset(p: *const i32, d: isize) -> *const i32 {
+    // CHECK: %[[TEMP:.+]] = getelementptr inbounds i32, ptr %p, [[WORD]] %d
+    // CHECK: ret ptr %[[TEMP]]
+    p.offset(d)
+}