linux-kernel - RE: [PATCH -next V7 0/7] riscv: Optimize function trace

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Date:   Thu, 9 Feb 2023 09:00:13 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Guo Ren' <guoren@...nel.org>
CC:     Mark Rutland <mark.rutland@....com>,
        Evgenii Shatokhin <e.shatokhin@...ro.com>,
        "suagrfillet@...il.com" <suagrfillet@...il.com>,
        "andy.chiu@...ive.com" <andy.chiu@...ive.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Guo Ren <guoren@...ux.alibaba.com>,
        "anup@...infault.org" <anup@...infault.org>,
        "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
        "palmer@...belt.com" <palmer@...belt.com>,
        "conor.dooley@...rochip.com" <conor.dooley@...rochip.com>,
        "heiko@...ech.de" <heiko@...ech.de>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "mhiramat@...nel.org" <mhiramat@...nel.org>,
        "jolsa@...hat.com" <jolsa@...hat.com>, "bp@...e.de" <bp@...e.de>,
        "jpoimboe@...nel.org" <jpoimboe@...nel.org>,
        "linux@...ro.com" <linux@...ro.com>
Subject: RE: [PATCH -next V7 0/7] riscv: Optimize function trace

From: Guo Ren
> Sent: 09 February 2023 01:51
...
> Yours cost one more instruction, right?
>          addr-12  auipc
>          addr-8    jalr
>          addr-4    // Literal (32-bits)
>          addr+0   nop or jmp addr-n // one more?
>          addr+4   function_code

Yes, it is 4 bytes larger but there is one less
instruction executed (only one nop) when ftrace is disabled.
That probably matters more than anything in the ftrace
'prologue' code.

I also suspect that you can use a 32bit integer as
a table index in 64bit mode to save a word there.

	David

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