Auto merge of #95695 - the8472:vec-codegen-tests, r=Mark-Simulacrum

Add codegen tests for additional cases where noop iterators get optimized away

Optimizations have improved over time and now LLVM manages to optimize more in-place-collect noop-iterators to O(1) functions. This updates the codegen test to match.

Many but not all cases reported in #79308 work now.

src/test/codegen/vec-in-place.rs

@@ -1,13 +1,72 @@
+// min-llvm-version: 14.0
 // ignore-debug: the debug assertions get in the way
-// compile-flags: -O
+// compile-flags: -O -Z merge-functions=disabled
 #![crate_type = "lib"]
 
 // Ensure that trivial casts of vec elements are O(1)
 
-// CHECK-LABEL: @vec_iterator_cast
+pub struct Wrapper<T>(T);
+
+#[repr(C)]
+pub struct Foo {
+    a: u64,
+    b: u64,
+    c: u64,
+    d: u64,
+}
+
+// Going from an aggregate struct to another type currently requires Copy to
+// enable the TrustedRandomAccess specialization. Without it optimizations do not yet
+// reliably recognize the loops as noop for for repr(C) or non-Copy structs.
+#[derive(Copy, Clone)]
+pub struct Bar {
+    a: u64,
+    b: u64,
+    c: u64,
+    d: u64,
+}
+
+// CHECK-LABEL: @vec_iterator_cast_primitive
+#[no_mangle]
+pub fn vec_iterator_cast_primitive(vec: Vec<i8>) -> Vec<u8> {
+    // CHECK-NOT: loop
+    // CHECK-NOT: call
+    vec.into_iter().map(|e| e as u8).collect()
+}
+
+// CHECK-LABEL: @vec_iterator_cast_wrapper
+#[no_mangle]
+pub fn vec_iterator_cast_wrapper(vec: Vec<u8>) -> Vec<Wrapper<u8>> {
+    // CHECK-NOT: loop
+    // CHECK-NOT: call
+    vec.into_iter().map(|e| Wrapper(e)).collect()
+}
+
+// CHECK-LABEL: @vec_iterator_cast_unwrap
+#[no_mangle]
+pub fn vec_iterator_cast_unwrap(vec: Vec<Wrapper<u8>>) -> Vec<u8> {
+    // CHECK-NOT: loop
+    // CHECK-NOT: call
+    vec.into_iter().map(|e| e.0).collect()
+}
+
+// CHECK-LABEL: @vec_iterator_cast_aggregate
 #[no_mangle]
-pub fn vec_iterator_cast(vec: Vec<isize>) -> Vec<usize> {
+pub fn vec_iterator_cast_aggregate(vec: Vec<[u64; 4]>) -> Vec<Foo> {
     // CHECK-NOT: loop
     // CHECK-NOT: call
-    vec.into_iter().map(|e| e as usize).collect()
+    vec.into_iter().map(|e| unsafe { std::mem::transmute(e) }).collect()
+}
+
+// CHECK-LABEL: @vec_iterator_cast_deaggregate
+#[no_mangle]
+pub fn vec_iterator_cast_deaggregate(vec: Vec<Bar>) -> Vec<[u64; 4]> {
+    // CHECK-NOT: loop
+    // CHECK-NOT: call
+
+    // Safety: For the purpose of this test we assume that Bar layout matches [u64; 4].
+    // This currently is not guaranteed for repr(Rust) types, but it happens to work here and
+    // the UCG may add additional guarantees for homogenous types in the future that would make this
+    // correct.
+    vec.into_iter().map(|e| unsafe { std::mem::transmute(e) }).collect()
 }