linux-kernel - RE: [PATCH v4] x86: use builtins to read eflags

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Date:   Fri, 11 Feb 2022 16:40:01 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Bill Wendling' <morbo@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <nathan@...nel.org>,
        "Nick Desaulniers" <ndesaulniers@...gle.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v4] x86: use builtins to read eflags

From: Bill Wendling
> Sent: 10 February 2022 22:32
> 
> 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 beneficial 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.

Except that neither gcc nor clang attempt to make that decision.
They always do pushf; pop ax;

...
> v4: - Clang now no longer generates stack frames when using these builtins.
>     - Corrected misspellings.

While clang 'head' has been fixed, it seems a bit premature to say
it is 'fixed' enough for all clang builds to use the builtin.

Seems better to change it (back) to "=r" and comment that this
is currently as good as __builtin_ia32_readeflags_u64() and that
clang makes a 'pigs breakfast' of "=rm" - which has only marginal
benefit.

Changing to __builtin_ia32_readeflags_u64() may be worth while
if/when the compilers will generate pushf; pop mem; for it.

	David

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