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Date: Sat, 20 Jan 2024 00:22:59 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
To: Thorsten Glaser <tg@debian.org>, Peter Zijlstra <peterz@infradead.org>
CC: x86@kernel.org, rostedt@goodmis.org, torvalds@linuxfoundation.org,
        linux-kernel@vger.kernel.org, linux-toolchains@vger.kernel.org,
        jpoimboe@redhat.com, alexei.starovoitov@gmail.com, mhiramat@kernel.org
Subject: Re: [PATCH 1/2] x86: Remove dynamic NOP selection
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On January 19, 2024 10:58:56 PM PST, Thorsten Glaser <tg@debian=2Eorg> wrot=
e:
>Peter Zijlstra dixit:
>
>>-/* generic versions from gas
>[=E2=80=A6]
>>-   3: leal 0x00(%esi),%esi
>>-   4: leal 0x00(,%esi,1),%esi
>>-   6: leal 0x00000000(%esi),%esi
>>-   7: leal 0x00000000(,%esi,1),%esi
>
>vs=2E
>
>>+ * Generic 32bit nops from GAS:
>[=E2=80=A6]
>>+ * 3: leal 0x0(%esi),%esi
>>+ * 4: leal 0x0(%esi,%eiz,1),%esi
>>+ * 5: leal %ds:0x0(%esi,%eiz,1),%esi
>>+ * 6: leal 0x0(%esi),%esi
>>+ * 7: leal 0x0(%esi,%eiz,1),%esi
>>+ * 8: leal %ds:0x0(%esi,%eiz,1),%esi
>
>I think there=E2=80=99s some mistake introduced=2E The BYTES_* are
>identical for e=2Eg=2E #7, but %eiz must be wrong, it=E2=80=99s not
>a register=2E Indeed, gas (on Debian bullseye) does not
>assemble that either=2E
>
>(Awful AT&T syntax aside=E2=80=A6)
>
>bye,
>//mirabilos

%eiz was something that binutils used to put in when disassembling certain=
 redundant encodings with SIB at some point=2E