storing in a variable results in needless copying

[tspiteri]

With this lib.rs

pub struct S([i64; 2]);
pub fn foo(create: fn() -> S) -> S {
    create()
}
pub fn bar(create: fn() -> S) -> S {
    let s = create();
    s
}

rustc lib.rs --crate-type lib -C opt-level=3 --emit=asm produces this lib.s (cleaned up)

foo:
	pushq	%rbx
	movq	%rdi, %rbx
	callq	*%rsi
	movq	%rbx, %rax
	popq	%rbx
	retq
bar:
	pushq	%rbx
	subq	$16, %rsp
	movq	%rdi, %rbx
	movq	%rsp, %rdi
	callq	*%rsi
	movups	(%rsp), %xmm0
	movups	%xmm0, (%rbx)
	movq	%rbx, %rax
	addq	$16, %rsp
	popq	%rbx
	retq

The assembly for bar should not be longer than the assembly for foo.

This is similar to #42870. I don't know if it's a duplicate or just similar.

[dotdash]

http://lists.llvm.org/pipermail/llvm-dev/2018-March/121577.html

[steveklabnik]

Triage: no change.

[slightlyoutofphase]

Currently those do generate identical assembly when you actually call them in a program. Playground link to demonstrate.

[tspiteri]

This is fixed in nightly, possibly due to #72205.