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Re: [PATCH 3/4] tools/tests/x86_emulator: fix build on clang-21
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 23 Feb 2026 17:21:06 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Edwin Török <edwin.torok@xxxxxxxxxx>
- Delivery-date: Mon, 23 Feb 2026 16:21:17 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 23.02.2026 17:06, Andrew Cooper wrote:
> On 23/02/2026 4:02 pm, Jan Beulich wrote:
>> On 23.02.2026 11:04, Edwin Török wrote:
>>> clang-21's built-in assembler is enabled by default, but it doesn't
>>> support some mnemonics:
>>> ```
>>> test_x86_emulator.c:2636:36: error: invalid instruction mnemonic 'fsaves'
>>> 2636 | "fidivs %1\n\t"
>>> test_x86_emulator.c:2640:24: error: invalid instruction mnemonic 'frstors'
>>> 2640 | asm volatile ( "frstors %0" :: "m" (res[25]) : "memory" );
>>> | ^
>>> test_x86_emulator.c:4251:24: error: invalid instruction mnemonic
>>> 'vpcmpestriq'
>>> 4251 | put_insn(vpcmpestri,
>>> | ^
>>> ```
>> Was this reported to them as a bug (or perhaps even two)?
>
> Looking at just one of these, the mnemonic is FRSTOR without an S, and
> Clang 21 is happy with that.
>
> What is the trailing S supposed to signify to GAS?
"short", i.e. the want for a 16-bit operand size prefix. Just like in
vpcmpestriq the request is for a REX64 prefix. Suffixes are the way to
go in AT&T syntax when operands alone can't disambiguate operand size.
The less nice alternative are data16 prefixes; not sure if Clang would
support those.
Jan
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