Auto merge of #121242 - joboet:ptr_invalid_codegen, r=scottmcm

Generate `getelementptr` instead of `inttoptr` for `ptr::invalid`

Currently, `ptr::invalid` [generates an `inttoptr`](https://godbolt.org/z/3cj15dEG1), which means LLVM doesn't know that the pointer shouldn't have provenance. This PR changes the implementation so that a `getelementptr` relative to the null pointer is generated, which LLVM knows not to have provenance.

library/core/src/ptr/mod.rs

@@ -590,11 +590,14 @@ pub const fn null_mut<T: ?Sized + Thin>() -> *mut T {
 #[unstable(feature = "strict_provenance", issue = "95228")]
 pub const fn without_provenance<T>(addr: usize) -> *const T {
     // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
-    // We use transmute rather than a cast so tools like Miri can tell that this
-    // is *not* the same as from_exposed_addr.
-    // SAFETY: every valid integer is also a valid pointer (as long as you don't dereference that
-    // pointer).
-    unsafe { mem::transmute(addr) }
+    // FIXME(maybe): simply transmute the address once codegen lowers that correctly.
+    // We offset the null pointer instead of using a cast so that this lowers to
+    // getelementptr instead of inttoptr.
+    // We use transmute instead of casting so that in miri the pointer truly
+    // has no provenance.
+    // SAFETY: on all current platforms, usize and pointers have the same layout,
+    // and the validity invariant of pointers is the same as that of integers
+    unsafe { mem::transmute::<_, *const T>(0usize).wrapping_byte_add(addr) }
 }
 
 /// Creates a new pointer that is dangling, but well-aligned.
@@ -632,11 +635,14 @@ pub const fn dangling<T>() -> *const T {
 #[unstable(feature = "strict_provenance", issue = "95228")]
 pub const fn without_provenance_mut<T>(addr: usize) -> *mut T {
     // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
-    // We use transmute rather than a cast so tools like Miri can tell that this
-    // is *not* the same as from_exposed_addr.
-    // SAFETY: every valid integer is also a valid pointer (as long as you don't dereference that
-    // pointer).
-    unsafe { mem::transmute(addr) }
+    // FIXME(maybe): simply transmute the address once codegen lowers that correctly.
+    // We offset the null pointer instead of using a cast so that this lowers to
+    // getelementptr instead of inttoptr.
+    // We use transmute instead of casting so that in miri the pointer truly
+    // has no provenance.
+    // SAFETY: on all current platforms, usize and pointers have the same layout,
+    // and the validity invariant of pointers is the same as that of integers
+    unsafe { mem::transmute::<_, *mut T>(0usize).wrapping_byte_add(addr) }
 }
 
 /// Creates a new pointer that is dangling, but well-aligned.

tests/codegen/strict-provenance.rs

@@ -0,0 +1,16 @@
+//@ compile-flags: -O
+
+#![crate_type = "lib"]
+#![feature(strict_provenance)]
+
+use std::ptr;
+
+// CHECK-LABEL: ptr @without_provenance(
+// CHECK-SAME: [[USIZE:i[0-9]+]] noundef %addr)
+#[no_mangle]
+fn without_provenance(addr: usize) -> *const () {
+    // CHECK: start
+    // CHECK-NEXT: %0 = getelementptr i8, ptr null, [[USIZE]] %addr
+    // CHECK-NEXT: ret ptr %0
+    ptr::without_provenance(addr)
+}