avoid phi node for pointers flowing into Vec appends

library/alloc/src/slice.rs

@@ -160,6 +160,12 @@ pub(crate) mod hack {
     impl<T: Copy> ConvertVec for T {
         #[inline]
         fn to_vec<A: Allocator>(s: &[Self], alloc: A) -> Vec<Self, A> {
+            if s.is_empty() {
+                // The early return is not necessary for correctness, but it helps
+                // LLVM by avoiding phi nodes flowing into memcpy.
+                // See codegen/lib-optimizations/append-elements.rs
+                return Vec::new_in(alloc);
+            }
             let mut v = Vec::with_capacity_in(s.len(), alloc);
             // SAFETY:
             // allocated above with the capacity of `s`, and initialize to `s.len()` in

library/alloc/src/vec/mod.rs

@@ -2519,6 +2519,12 @@ impl<T, A: Allocator> Vec<T, A> {
     #[inline]
     unsafe fn append_elements(&mut self, other: *const [T]) {
         let count = unsafe { (*other).len() };
+        if count == 0 {
+            // The early return is not necessary for correctness, but it helps
+            // LLVM by avoiding phi nodes flowing into memcpy.
+            // See codegen/lib-optimizations/append-elements.rs
+            return;
+        }
         self.reserve(count);
         let len = self.len();
         unsafe { ptr::copy_nonoverlapping(other as *const T, self.as_mut_ptr().add(len), count) };

tests/codegen/lib-optimizations/append-elements.rs

@@ -0,0 +1,37 @@
+//@ compile-flags: -O
+#![crate_type = "lib"]
+
+//! Check that pointer flowing into vec.append comes straight from an allocation
+//! and not through a phi node that merges the allocation and zero-length cases.
+//! With this and https://github.com/llvm/llvm-project/pull/110280 the intermediate
+//! allocation should be optimized away in the future.
+
+// CHECK-LABEL: @vec_append_with_temp_alloc
+#[no_mangle]
+pub fn vec_append_with_temp_alloc(dst: &mut Vec<u8>, src: &[u8]) {
+    // CHECK: %[[TEMP:.+]] = tail call noalias noundef ptr @__rust_alloc
+
+    // First memcpy, it uses the src pointer directly
+    // CHECK: call void @llvm.memcpy.{{.*}}%src.0
+    let temp = src.to_vec();
+
+    // final memcpy to destination
+    // CHECK: call void @llvm.memcpy.{{.*}}%dst.i{{.*}}%[[TEMP]]
+    dst.extend(&temp);
+    // CHECK: ret
+}
+
+// CHECK-LABEL: @string_append_with_temp_alloc
+#[no_mangle]
+pub fn string_append_with_temp_alloc(dst: &mut String, src: &str) {
+    // CHECK: %[[TEMP:.+]] = tail call noalias noundef ptr @__rust_alloc
+
+    // First memcpy, it uses the src pointer directly
+    // CHECK: call void @llvm.memcpy.{{.*}}%src.0
+    let temp = src.to_string();
+
+    // final memcpy to destination
+    // CHECK: call void @llvm.memcpy.{{.*}}%dst.i{{.*}}%[[TEMP]]
+    dst.push_str(&temp);
+    // CHECK: ret
+}