From: LIU Hao <lh_mouse@126.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>,
Andrew Cooper via Gcc <gcc@gcc.gnu.org>
Subject: Re: Builtin for consulting value analysis (better ffs() code gen)
Date: Thu, 21 Mar 2024 16:15:04 +0800 [thread overview]
Message-ID: <7bcb6398-cc7a-41ab-b8e2-ce4f0e9e5b7c@126.com> (raw)
In-Reply-To: <9704774f-00f9-48d4-ad27-6cd07a816359@citrix.com>
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在 2024-03-14 23:33, Andrew Cooper via Gcc 写道:
> And for x86's arch_ffs(),
>
> unsigned int arch_ffs(unsigned int x)
> {
> unsigned int res;
>
> if ( __builtin_constant_p(x > 0) && x > 0 )
> {
> // Well defined when x is known non-zero
> asm("bsf %1, %0" : "=r"(res) : "rm"(x));
Even if you may assume that the destination operand is always destroyed, the hardware has no
knowledge about that, so this statement has a dependency on `res`, and shouldn't be declared `=r`.
I think it's better to remove this `if`. The other branch below clearly eliminates the dependency.
> }
> else
> {
> // The architects say this is safe even for 0.
> res = -1;
> asm("bsf %1, %0" : "+r"(res) : "rm"(x));
> }
>
> return res + 1;
> }
--
Best regards,
LIU Hao
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next prev parent reply other threads:[~2024-03-21 8:15 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-14 0:03 Andrew Cooper
2024-03-14 6:04 ` Alexander Monakov
2024-03-14 11:30 ` Andrew Cooper
2024-03-14 12:03 ` Andreas Schwab
2024-03-14 15:33 ` Andrew Cooper
2024-03-21 8:15 ` LIU Hao [this message]
2024-03-14 11:02 ` Florian Weimer
2024-03-14 11:52 ` Andrew Cooper
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