Auto merge of #111113 - scottmcm:assume-align-offset, r=thomcc

`assume` the runtime range of `align_offset`

Found when I saw code with `align_to` having extraneous checks.

Demo that LLVM can't do this today: <https://rust.godbolt.org/z/6dnG749bq>

(It's filed as https://github.com/llvm/llvm-project/issues/62502.)

1 files changed, 78 insertions, 0 deletions

diff --git a/tests/codegen/align-offset.rs b/tests/codegen/align-offset.rs
new file mode 100644
index 00000000000..7c7660c5a55
--- /dev/null
+++ b/tests/codegen/align-offset.rs
@@ -0,0 +1,78 @@
+// compile-flags: -O
+// min-llvm-version: 15.0 (because we're using opaque pointers)
+// ignore-debug (debug assertions in `slice::from_raw_parts` block optimizations)
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @align8
+#[no_mangle]
+pub fn align8(p: *const u8) -> bool {
+    // CHECK: ret i1 true
+    p.align_offset(8) < 8
+}
+
+#[repr(align(4))]
+pub struct Align4([u8; 4]);
+
+// CHECK-LABEL: @align_to4
+#[no_mangle]
+pub fn align_to4(x: &[u8]) -> bool {
+    // CHECK: ret i1 true
+    let (prefix, _middle, suffix) = unsafe { x.align_to::<Align4>() };
+    prefix.len() < 4 && suffix.len() < 4
+}
+
+// CHECK-LABEL: @align_offset_byte_ptr(ptr{{.+}}%ptr)
+#[no_mangle]
+pub fn align_offset_byte_ptr(ptr: *const u8) -> usize {
+    // CHECK: %[[ADDR:.+]] = ptrtoint ptr %ptr to [[USIZE:i[0-9]+]]
+    // CHECK: %[[UP:.+]] = add [[USIZE]] %[[ADDR]], 31
+    // CHECK: %[[ALIGNED:.+]] = and [[USIZE]] %[[UP]], -32
+    // CHECK: %[[OFFSET:.+]] = sub [[USIZE]] %[[ALIGNED]], %[[ADDR]]
+
+    // Since we're offsetting a byte pointer, there's no further fixups
+    // CHECK-NOT: shr
+    // CHECK-NOT: div
+    // CHECK-NOT: select
+
+    // CHECK: ret [[USIZE]] %[[OFFSET]]
+    ptr.align_offset(32)
+}
+
+// CHECK-LABEL: @align_offset_word_slice(ptr{{.+}}align 4{{.+}}%slice.0
+#[no_mangle]
+pub fn align_offset_word_slice(slice: &[Align4]) -> usize {
+    // CHECK: %[[ADDR:.+]] = ptrtoint ptr %slice.0 to [[USIZE]]
+    // CHECK: %[[UP:.+]] = add [[USIZE]] %[[ADDR]], 31
+    // CHECK: %[[ALIGNED:.+]] = and [[USIZE]] %[[UP]], -32
+    // CHECK: %[[BOFFSET:.+]] = sub [[USIZE]] %[[ALIGNED]], %[[ADDR]]
+    // CHECK: %[[OFFSET:.+]] = lshr exact [[USIZE]] %[[BOFFSET]], 2
+
+    // Slices are known to be aligned, so we don't need the "maybe -1" path
+    // CHECK-NOT: select
+
+    // CHECK: ret [[USIZE]] %[[OFFSET]]
+    slice.as_ptr().align_offset(32)
+}
+
+
+// CHECK-LABEL: @align_offset_word_ptr(ptr{{.+}}%ptr
+#[no_mangle]
+pub fn align_offset_word_ptr(ptr: *const Align4) -> usize {
+    // CHECK: %[[ADDR:.+]] = ptrtoint ptr %ptr to [[USIZE]]
+    // CHECK: %[[UP:.+]] = add [[USIZE]] %[[ADDR]], 31
+    // CHECK: %[[ALIGNED:.+]] = and [[USIZE]] %[[UP]], -32
+    // CHECK: %[[BOFFSET:.+]] = sub [[USIZE]] %[[ALIGNED]], %[[ADDR]]
+
+    // While we can always get a *byte* offset that will work, if the original
+    // pointer is unaligned it might be impossible to return an *element* offset
+    // that will make it aligned. We want it to be a `select`, not a `br`, so
+    // that the assembly will be branchless.
+    // CHECK: %[[LOW:.+]] = and [[USIZE]] %[[ADDR]], 3
+    // CHECK: %[[ORIGINAL_ALIGNED:.+]] = icmp eq [[USIZE]] %[[LOW]], 0
+    // CHECK: %[[OFFSET:.+]] = lshr exact [[USIZE]] %[[BOFFSET]], 2
+    // CHECK: %[[R:.+]] = select i1 %[[ORIGINAL_ALIGNED]], [[USIZE]] %[[OFFSET]], [[USIZE]] -1
+
+    // CHECK: ret [[USIZE]] %[[R]]
+    ptr.align_offset(32)
+}