Use more optimal Ord implementation for integers

[tesuji]

Closes #63758
r? @nagisa

Compare results

(godbolt link)

Old assembly:

example::cmp1:
  mov eax, dword ptr [rdi]
  mov ecx, dword ptr [rsi]
  cmp eax, ecx
  setae dl
  add dl, dl
  add dl, -1
  xor esi, esi
  cmp eax, ecx
  movzx eax, dl
  cmove eax, esi
  ret

New assembly:

example::cmp2:
  mov eax, dword ptr [rdi]
  xor ecx, ecx
  cmp eax, dword ptr [rsi]
  seta cl
  mov eax, 255
  cmovae eax, ecx
  ret

Old llvm-mca statistics:

Iterations:        100
Instructions:      1100
Total Cycles:      243
Total uOps:        1300

Dispatch Width:    6
uOps Per Cycle:    5.35
IPC:               4.53
Block RThroughput: 2.2

New llvm-mca statistics:

Iterations:        100
Instructions:      700
Total Cycles:      217
Total uOps:        1100

Dispatch Width:    6
uOps Per Cycle:    5.07
IPC:               3.23
Block RThroughput: 1.8

[nagisa]

@bors r+

[bors]

📌 Commit 0337cc1 has been approved by nagisa

[matthiaskrgr]

Could you add a comment explaining that the ordering is performance critical here? (perhaps with a link to the original ticket)
This should make sure that it es not changed back to something slower by accident.

[hellow554]

What about adding a assembly testcase as well to prevent regressions (e.g. due to other optimizations?)

[tesuji]

What about adding a assembly testcase as well to prevent regressions (e.g. due to other optimizations?)

I don't know how. Could you give a mentor?

[hellow554]

@lzutao You can take a look at https://rust-lang.github.io/rustc-guide/tests/intro.html and the codegen test cases in https://github.com/rust-lang/rust/tree/master/src/test/codegen especially at

rust/src/test/codegen/float_math.rs

Lines 1 to 50 in bea0372

	// compile-flags: -C no-prepopulate-passes
	#![crate_type = "lib"]
	#![feature(core_intrinsics)]
	use std::intrinsics::{fadd_fast, fsub_fast, fmul_fast, fdiv_fast, frem_fast};
	// CHECK-LABEL: @add
	#[no_mangle]
	pub fn add(x: f32, y: f32) -> f32 {
	// CHECK: fadd float
	// CHECK-NOT: fast
	x + y
	}
	// CHECK-LABEL: @addition
	#[no_mangle]
	pub fn addition(x: f32, y: f32) -> f32 {
	// CHECK: fadd fast float
	unsafe {
	fadd_fast(x, y)
	}
	}
	// CHECK-LABEL: @subtraction
	#[no_mangle]
	pub fn subtraction(x: f32, y: f32) -> f32 {
	// CHECK: fsub fast float
	unsafe {
	fsub_fast(x, y)
	}
	}
	// CHECK-LABEL: @multiplication
	#[no_mangle]
	pub fn multiplication(x: f32, y: f32) -> f32 {
	// CHECK: fmul fast float
	unsafe {
	fmul_fast(x, y)
	}
	}
	// CHECK-LABEL: @division
	#[no_mangle]
	pub fn division(x: f32, y: f32) -> f32 {
	// CHECK: fdiv fast float
	unsafe {
	fdiv_fast(x, y)
	}
	}

I guess

[tesuji]

The CI is green.

[nagisa]

@bors r+

[bors]

📌 Commit f5b16f6 has been approved by nagisa