From 02603334ae22971409c11e547a3568d70b9c60ac Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Steinbrink?= <bsteinbr@gmail.com>
Date: Fri, 15 May 2015 03:14:16 +0200
Subject: [PATCH] Fix major compile time regression

The assume intrinsic has a strong, negative impact on compile times, so
we're currently only using it in places where LLVM can simplify it to
nonnull metadata on a load intruction. Unfortunately a recent change
that fixed invalid assume calls introduce new assume calls for which
this simplification can not happen, leading to a massive regression in
compile times in certain cases.

Moving the assumptions from the middle of the function to the beginning
allows the simplification to happen again, bringing compile times back
to their old levels.

Fixes #25393
---
 src/libcore/slice.rs | 32 ++++++++++++++++----------------
 1 file changed, 16 insertions(+), 16 deletions(-)

diff --git a/src/libcore/slice.rs b/src/libcore/slice.rs
index 9db1ceddf0d75..546e0bc806e12 100644
--- a/src/libcore/slice.rs
+++ b/src/libcore/slice.rs
@@ -665,14 +665,14 @@ macro_rules! iterator {
             #[inline]
             fn next(&mut self) -> Option<$elem> {
                 // could be implemented with slices, but this avoids bounds checks
-                if self.ptr == self.end {
-                    None
-                } else {
-                    unsafe {
-                        if mem::size_of::<T>() != 0 {
-                            ::intrinsics::assume(!self.ptr.is_null());
-                            ::intrinsics::assume(!self.end.is_null());
-                        }
+                unsafe {
+                    if mem::size_of::<T>() != 0 {
+                        assume(!self.ptr.is_null());
+                        assume(!self.end.is_null());
+                    }
+                    if self.ptr == self.end {
+                        None
+                    } else {
                         let old = self.ptr;
                         self.ptr = slice_offset!(self.ptr, 1);
                         Some(slice_ref!(old))
@@ -710,15 +710,15 @@ macro_rules! iterator {
             #[inline]
             fn next_back(&mut self) -> Option<$elem> {
                 // could be implemented with slices, but this avoids bounds checks
-                if self.end == self.ptr {
-                    None
-                } else {
-                    unsafe {
+                unsafe {
+                    if mem::size_of::<T>() != 0 {
+                        assume(!self.ptr.is_null());
+                        assume(!self.end.is_null());
+                    }
+                    if self.end == self.ptr {
+                        None
+                    } else {
                         self.end = slice_offset!(self.end, -1);
-                        if mem::size_of::<T>() != 0 {
-                            ::intrinsics::assume(!self.ptr.is_null());
-                            ::intrinsics::assume(!self.end.is_null());
-                        }
                         Some(slice_ref!(self.end))
                     }
                 }