Re: Re: [PATCH] Little memset_explicit optimisation

[Joey Pabalinas <joeypabalinas@gmail.com>]

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On Mon, Nov 26, 2018 at 07:36:19PM +0000, David CARLIER wrote:
> Sorry I m not used yet at all to LKML rules.
> 
> So here a slight difference in assembly generated between the two
> versions (amd64) :
> `
>      .loc 1 7 7
>      leaq    -12(%rbp), %rax
>      movq    %rax, -8(%rbp)
> -    .loc 1 11 2
> +    .loc 1 9 6
>      movq    -8(%rbp), %rax
>      movl    $4, %edx
>      movl    $0, %esi
>      movq    %rax, %rdi
>      call    memset@PLT
> +    movq    %rax, -8(%rbp)
>      .loc 1 13 23
>      movq    -8(%rbp), %rax
>      movl    (%rax), %eax

What is the advantage of having the added `movq %rax, -8(%rbp)` here?

The next instruction is `movq -8(%rbp), %rax` and nothing afterwords
uses the value stored in `-8(%rbp)`.

Also, is this compiled without optimization? Take a looks at the
assembly in a small test case with -O1 (making sure to use the target
variable so it isn't optimized out) and compare the assembly generated
with and without that assignment.

-- 
Cheers,
Joey Pabalinas

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