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Github Optimize DST field access by mahkoh · Pull Request #80200 · rust-lang/rus...

 3 years ago
source link: https://github.com/rust-lang/rust/pull/80200
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Contributor

mahkoh commented 27 days ago 

struct X<T: ?Sized>(T)
struct Y<T: ?Sized>(u8, T)

the offset of the unsized field is

0
mem::align_of_val(&self.1)

respectively. This patch changes the expression used to compute these
offsets so that the optimizer can perform this optimization.

Consider

fn f(x: &X<dyn Any>) -> &dyn Any {
    &x.0
}

Before:

test:
	movq	%rsi, %rdx
	movq	16(%rsi), %rax
	leaq	-1(%rax), %rcx
	negq	%rax
	andq	%rcx, %rax
	addq	%rdi, %rax
	retq

After:

test:
	movq	%rsi, %rdx
	movq	%rdi, %rax
	retq

report

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