From 894236f66d08a7078fb8029e58882fd0a09c388b Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <nnethercote@mozilla.com>
Date: Mon, 3 Aug 2020 18:19:51 +1000
Subject: [PATCH] Avoid `Layout::array()` in `raw_vec.rs`.

A non-generic equivalent results in less LLVM IR and faster compilation.
---
 library/alloc/src/raw_vec.rs | 27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/library/alloc/src/raw_vec.rs b/library/alloc/src/raw_vec.rs
index ed81ce71ddfac..5bb5524ecb322 100644
--- a/library/alloc/src/raw_vec.rs
+++ b/library/alloc/src/raw_vec.rs
@@ -1,7 +1,7 @@
 #![unstable(feature = "raw_vec_internals", reason = "implementation detail", issue = "none")]
 #![doc(hidden)]
 
-use core::alloc::{LayoutErr, MemoryBlock};
+use core::alloc::MemoryBlock;
 use core::cmp;
 use core::mem::{self, ManuallyDrop, MaybeUninit};
 use core::ops::Drop;
@@ -172,7 +172,7 @@ impl<T, A: AllocRef> RawVec<T, A> {
         } else {
             // We avoid `unwrap_or_else` here because it bloats the amount of
             // LLVM IR generated.
-            let layout = match Layout::array::<T>(capacity) {
+            let layout = match array_layout(Layout::new::<T>(), capacity) {
                 Ok(layout) => layout,
                 Err(_) => capacity_overflow(),
             };
@@ -423,10 +423,10 @@ impl<T, A: AllocRef> RawVec<T, A> {
         };
         let cap = cmp::max(min_non_zero_cap, cap);
 
-        let new_layout = Layout::array::<T>(cap);
+        let elem_layout = Layout::new::<T>();
 
         // `finish_grow` is non-generic over `T`.
-        let memory = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
+        let memory = finish_grow(cap, elem_layout, self.current_memory(), &mut self.alloc)?;
         self.set_memory(memory);
         Ok(())
     }
@@ -442,10 +442,11 @@ impl<T, A: AllocRef> RawVec<T, A> {
         }
 
         let cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
-        let new_layout = Layout::array::<T>(cap);
+
+        let elem_layout = Layout::new::<T>();
 
         // `finish_grow` is non-generic over `T`.
-        let memory = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
+        let memory = finish_grow(cap, elem_layout, self.current_memory(), &mut self.alloc)?;
         self.set_memory(memory);
         Ok(())
     }
@@ -478,7 +479,8 @@ impl<T, A: AllocRef> RawVec<T, A> {
 // significant, because the number of different `A` types seen in practice is
 // much smaller than the number of `T` types.)
 fn finish_grow<A>(
-    new_layout: Result<Layout, LayoutErr>,
+    cap: usize,
+    elem_layout: Layout,
     current_memory: Option<(NonNull<u8>, Layout)>,
     alloc: &mut A,
 ) -> Result<MemoryBlock, TryReserveError>
@@ -486,7 +488,7 @@ where
     A: AllocRef,
 {
     // Check for the error here to minimize the size of `RawVec::grow_*`.
-    let new_layout = new_layout.map_err(|_| CapacityOverflow)?;
+    let new_layout = array_layout(elem_layout, cap)?;
 
     alloc_guard(new_layout.size())?;
 
@@ -501,6 +503,15 @@ where
     Ok(memory)
 }
 
+// This is equivalent to Layout::array, but is non-generic and has a different
+// error type in its result. It helps reduce the amount of LLVM IR generated.
+#[inline]
+fn array_layout(elem_layout: Layout, n: usize) -> Result<Layout, TryReserveError> {
+    let (new_layout, offset) = elem_layout.repeat(n).map_err(|_| CapacityOverflow)?;
+    debug_assert_eq!(offset, elem_layout.size());
+    Ok(new_layout.pad_to_align())
+}
+
 unsafe impl<#[may_dangle] T, A: AllocRef> Drop for RawVec<T, A> {
     /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
     fn drop(&mut self) {