Override try_[r]fold for RangeInclusive

[scottmcm]

Because the last item needs special handling, it seems that LLVM has trouble canonicalizing the loops in external iteration. With the override, it becomes obvious that the start==end case exits the loop (as opposed to the one after that exiting the loop in external iteration).

Demo adapted from #45222

#[no_mangle]
pub fn foo3r(n: u64) -> u64 {
    let mut count = 0;
    (0..n).for_each(|_| {
        (0 ..= n).rev().for_each(|j| {
            count += j;
        })
    });
    count
}

Current nightly ASM, 100 lines (https://play.rust-lang.org/?gist=f5674c702c6e2045c3aab5d03763e5f6&version=nightly&mode=release)

foo3r:
	pushq	%rbx
.Lcfi0:
.Lcfi1:
	testq	%rdi, %rdi
	je	.LBB0_1
	testb	$1, %dil
	jne	.LBB0_4
	xorl	%eax, %eax
	xorl	%r8d, %r8d
	cmpq	$1, %rdi
	jne	.LBB0_11
	jmp	.LBB0_23
.LBB0_1:
	xorl	%eax, %eax
	popq	%rbx
	retq
.LBB0_4:
	xorl	%r8d, %r8d
	movq	$-1, %r9
	xorl	%eax, %eax
	movq	%rdi, %r11
	xorl	%r10d, %r10d
	jmp	.LBB0_5
.LBB0_8:
	addq	%r11, %rax
	movq	%rsi, %r11
	movq	%rdx, %r10
.LBB0_5:
	cmpq	%r11, %r10
	movl	$1, %ecx
	cmovbq	%r9, %rcx
	cmoveq	%r8, %rcx
	testq	%rcx, %rcx
	movl	$0, %esi
	movl	$1, %edx
	je	.LBB0_8
	cmpq	$-1, %rcx
	jne	.LBB0_9
	leaq	-1(%r11), %rsi
	movq	%r10, %rdx
	jmp	.LBB0_8
.LBB0_9:
	movl	$1, %r8d
	cmpq	$1, %rdi
	je	.LBB0_23
.LBB0_11:
	xorl	%r9d, %r9d
	movq	$-1, %r10
.LBB0_12:
	movq	%rdi, %rsi
	xorl	%r11d, %r11d
	jmp	.LBB0_13
.LBB0_16:
	addq	%rsi, %rax
	movq	%rcx, %rsi
	movq	%rbx, %r11
.LBB0_13:
	cmpq	%rsi, %r11
	movl	$1, %edx
	cmovbq	%r10, %rdx
	cmoveq	%r9, %rdx
	testq	%rdx, %rdx
	movl	$0, %ecx
	movl	$1, %ebx
	je	.LBB0_16
	cmpq	$-1, %rdx
	jne	.LBB0_17
	leaq	-1(%rsi), %rcx
	movq	%r11, %rbx
	jmp	.LBB0_16
.LBB0_17:
	movq	%rdi, %rcx
	xorl	%r11d, %r11d
	jmp	.LBB0_18
.LBB0_21:
	addq	%rcx, %rax
	movq	%rsi, %rcx
	movq	%rbx, %r11
.LBB0_18:
	cmpq	%rcx, %r11
	movl	$1, %edx
	cmovbq	%r10, %rdx
	cmoveq	%r9, %rdx
	testq	%rdx, %rdx
	movl	$0, %esi
	movl	$1, %ebx
	je	.LBB0_21
	cmpq	$-1, %rdx
	jne	.LBB0_22
	leaq	-1(%rcx), %rsi
	movq	%r11, %rbx
	jmp	.LBB0_21
.LBB0_22:
	addq	$2, %r8
	cmpq	%rdi, %r8
	jne	.LBB0_12
.LBB0_23:
	popq	%rbx
	retq
.Lfunc_end0:

With this PR:

foo3r:
	test	rcx, rcx
	je	.LBB3_1
	lea	r8, [rcx - 1]
	lea	rdx, [rcx - 2]
	mov	rax, r8
	mul	rdx
	shld	rdx, rax, 63
	imul	r8, r8
	add	r8, rcx
	sub	r8, rdx
	imul	r8, rcx
	mov	rax, r8
	ret
.LBB3_1:
	xor	r8d, r8d
	mov	rax, r8
	ret

[rust-highfive]

r? @alexcrichton

(rust_highfive has picked a reviewer for you, use r? to override)

[alexcrichton]

@bors: r+

Thanks!

[bors]

📌 Commit 1b1e887 has been approved by alexcrichton