linux-kernel - Re: [PATCH 3/4] x86/asm/entry/64: use smaller insns

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Message-ID: <20150326093732.GC14706@gmail.com>
Date:	Thu, 26 Mar 2015 10:37:32 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] x86/asm/entry/64: use smaller insns


* Borislav Petkov <bp@...en8.de> wrote:

> On Wed, Mar 25, 2015 at 05:05:50PM -0700, Linus Torvalds wrote:
> > so 'as' is clearly just stupid. It already takes the size of the
> > constant into account and generates different instructions. Why not
> > for the common 32-bit case too?
> 
> I think the destination register mandates which insn to use:
> 
>         mov $0x12345678, %rdi
>         mov $0x12345678, %edi
> 
> ...
> 
>   15:   48 c7 c7 78 56 34 12    mov    $0x12345678,%rdi
>   1c:   bf 78 56 34 12          mov    $0x12345678,%edi
> 
> and 'as' is perhaps not insolent enough to go and change it when 
> seeing the 32-bit constant.

Well, GAS generally has the freedom to use more optimal opcodes, as 
long as behavior on the CPU matches what the specified opcode does.

Saying that 'movq $0x1, %rdi' should map to the longer variant is 
defensible: there should probably always be a way to specify a very 
specific opcode even if it's not optimal - say I want to fill in an 
alignment space and don't want to use an extra NOP.

But here GAS generates the 10-byte opcode even if 'mov $0x12345678, 
%rdi' is used, which is an unforced error.

Thanks,

	Ingo
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