user0/rust
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

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

Browse source 
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.
This commit is contained in:
bors 2017-08-06 08:09:59 +00:00
commit a9c24fd579
4 changed files with 108 additions and 4 deletions
Download patch file Download diff file Expand all files Collapse all files

74
src/test/codegen/slice-init.rs Normal file
View file

@ -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);
}