lkml.org 
[lkml]   [2022]   [Feb]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH v2] x86: use builtins to read eflags
Date
On Tue, Dec 28 2021 at 18:12, Bill Wendling wrote:
> GCC and Clang both have builtins to read and write the EFLAGS register.
> This allows the compiler to determine the best way to generate this
> code, which can improve code generation.
>
> This issue arose due to Clang's issue with the "=rm" constraint. Clang
> chooses to be conservative in these situations, and so uses memory
> instead of registers. This is a known issue, which is currently being
> addressed.
>
> However, using builtins is benefiical in general, because it removes the
> burden of determining what's the way to read the flags register from the
> programmer and places it on to the compiler, which has the information
> needed to make that decision. Indeed, this piece of code has had several
> changes over the years, some of which were pinging back and forth to
> determine the correct constraints to use.
>
> With this change, Clang generates better code:
>
> Original code:
> movq $0, -48(%rbp)
> #APP
> # __raw_save_flags
> pushfq
> popq -48(%rbp)
> #NO_APP
> movq -48(%rbp), %rbx
>
> New code:
> pushfq
> popq %rbx
> #APP
>
> Note that the stack slot in the original code is no longer needed in the
> new code, saving a small amount of stack space.

This still lacks any information about the effect on GCC. There is a
world outside clang. It's not my job to validate that.

Thanks,

tglx

\
 
 \ /
  Last update: 2022-02-04 01:17    [W:0.271 / U:0.280 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site