Auto merge of rust-lang#130998 - the8472:bail-before-memcpy, r=<try>

avoid phi node for pointers flowing into Vec appends

related discussion: https://rust-lang.zulipchat.com/#narrow/stream/187780-t-compiler.2Fwg-llvm/topic/nocapture.20and.20allocation.20elimination

r? ghost

Author: bors

library/alloc/src/vec/mod.rs

@@ -2519,6 +2519,14 @@ 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 in cases
+            // where LLVM sees all the way to the allocation site of `other`
+            // this can avoid a phi-node merging the two different pointers
+            // when zero-length allocations are special-cased.
+            // That in turn can enable more optimizations around the memcpy below.
+            return;
+        }
         self.reserve(count);
         let len = self.len();
         unsafe { ptr::copy_nonoverlapping(other as *const T, self.as_mut_ptr().add(len), count) };