linux-kernel - Re: [tip:x86/asm] objtool: Handle GCC stack pointer adjustment bug

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Message-ID: <e6eb6a11-ad65-dccb-2c79-485e58e316a2@zytor.com>
Date:   Wed, 30 Aug 2017 12:23:24 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     arnd@...db.de, linux-kernel@...r.kernel.org, mingo@...nel.org,
        fengguang.wu@...el.com, torvalds@...ux-foundation.org,
        jpoimboe@...hat.com, tglx@...utronix.de, peterz@...radead.org,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/asm] objtool: Handle GCC stack pointer adjustment bug

On 08/30/17 02:43, tip-bot for Josh Poimboeuf wrote:
> 
> Those warnings are caused by an unusual GCC non-optimization where it
> uses an intermediate register to adjust the stack pointer.  It does:
> 
>   lea    0x8(%rsp), %rcx
>   ...
>   mov    %rcx, %rsp
> 
> Instead of the obvious:
> 
>   add    $0x8, %rsp
> 
> It makes no sense to use an intermediate register, so I opened a GCC bug
> to track it:
> 
>   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81813
> 
> But it's not exactly a high-priority bug and it looks like we'll be
> stuck with this issue for a while.  So for now we have to track register
> values when they're loaded with stack pointer offsets.
> 

This seems like a good reason to try to extract this information from
the DWARF data *if available*?

	-hpa