Auto merge of #43488 - Florob:repeat-opt, r=arielb1

Optimize initialization of arrays using repeat expressions

This PR was inspired by [this thread](https://www.reddit.com/r/rust/comments/6o8ok9/understanding_rust_performances_a_newbie_question/) on Reddit.
It tries to bring array initialization in the same ballpark as `Vec::from_elem()` for unoptimized builds.
For optimized builds this should relieve LLVM of having to figure out the construct we generate is in fact a `memset()`.

To that end this emits `llvm.memset()` when:
* the array is of integer type and all elements are zero (`Vec::from_elem()` also explicitly optimizes for this case)
* the array elements are byte sized

If the array is zero-sized initialization is omitted entirely.

1 files changed, 74 insertions, 0 deletions

diff --git a/src/test/codegen/slice-init.rs b/src/test/codegen/slice-init.rs
new file mode 100644
index 00000000000..569d937c812
--- /dev/null
+++ b/src/test/codegen/slice-init.rs
@@ -0,0 +1,74 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -C no-prepopulate-passes
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @zero_sized_elem
+#[no_mangle]
+pub fn zero_sized_elem() {
+    // CHECK-NOT: br label %slice_loop_header{{.*}}
+    // CHECK-NOT: call void @llvm.memset.p0i8
+    let x = [(); 4];
+    drop(&x);
+}
+
+// CHECK-LABEL: @zero_len_array
+#[no_mangle]
+pub fn zero_len_array() {
+    // CHECK-NOT: br label %slice_loop_header{{.*}}
+    // CHECK-NOT: call void @llvm.memset.p0i8
+    let x = [4; 0];
+    drop(&x);
+}
+
+// CHECK-LABEL: @byte_array
+#[no_mangle]
+pub fn byte_array() {
+    // CHECK: call void @llvm.memset.p0i8.i[[WIDTH:[0-9]+]](i8* {{.*}}, i8 7, i[[WIDTH]] 4
+    // CHECK-NOT: br label %slice_loop_header{{.*}}
+    let x = [7u8; 4];
+    drop(&x);
+}
+
+#[allow(dead_code)]
+#[derive(Copy, Clone)]
+enum Init {
+    Loop,
+    Memset,
+}
+
+// CHECK-LABEL: @byte_enum_array
+#[no_mangle]
+pub fn byte_enum_array() {
+    // CHECK: call void @llvm.memset.p0i8.i[[WIDTH:[0-9]+]](i8* {{.*}}, i8 {{.*}}, i[[WIDTH]] 4
+    // CHECK-NOT: br label %slice_loop_header{{.*}}
+    let x = [Init::Memset; 4];
+    drop(&x);
+}
+
+// CHECK-LABEL: @zeroed_integer_array
+#[no_mangle]
+pub fn zeroed_integer_array() {
+    // CHECK: call void @llvm.memset.p0i8.i[[WIDTH:[0-9]+]](i8* {{.*}}, i8 0, i[[WIDTH]] 16
+    // CHECK-NOT: br label %slice_loop_header{{.*}}
+    let x = [0u32; 4];
+    drop(&x);
+}
+
+// CHECK-LABEL: @nonzero_integer_array
+#[no_mangle]
+pub fn nonzero_integer_array() {
+    // CHECK: br label %slice_loop_header{{.*}}
+    // CHECK-NOT: call void @llvm.memset.p0i8
+    let x = [0x1a_2b_3c_4d_u32; 4];
+    drop(&x);
+}