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Message-ID: <0d630dec-41fe-4968-8d6a-7faae8a554a1@antgroup.com>
Date: Thu, 26 Jun 2025 10:44:39 +0800
From: "Tiwei Bie" <tiwei.btw@...group.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
Johannes Berg <johannes@...solutions.net>
Cc: linux-kernel@...r.kernel.org, linux-um@...ts.infradead.org,
Richard Weinberger <richard@....at>,
Anton Ivanov <anton.ivanov@...bridgegreys.com>
Subject: Re: [GIT PULL] uml-for-6.16-rc4
On 2025/6/26 02:17, Linus Torvalds wrote:
> On Wed, 25 Jun 2025 at 05:15, Johannes Berg <johannes@...solutions.net> wrote:
>>
>> - reduce stack use with clang 19
>
> Interesting. The patch looks fine, I'm wondering if people made a
> clang bug report about this behavior with structure assignments?
>
> Even if most other projects likely don't have issues with stack size,
> it looks very non-optimal from a performance standpoint too to create
> a pointless temporary copy on the stack.
>
> I assume - but didn't check - that gcc didn't do the same stupid thing
> for that code?
The behavior of gcc and clang differs. Clang's behavior appears to be
related to the volatile qualifier in arch_spinlock_t:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/spinlock_types_up.h?id=b555cb66583e99158cfef8e91c025252cefae55b#n18
It can be reproduced with this code snippet:
```c
struct foo {
char a;
volatile char b;
char c[512];
};
char bar(void);
void baz(struct foo *p);
void baz(struct foo *p)
{
*p = (struct foo) { .a = bar() };
}
```
$ clang-19 --version
Ubuntu clang version 19.1.7 (++20250114103320+cd708029e0b2-1~exp1~20250114103432.75)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-19/bin
$ clang-19 -O2 -c test.c
$ objdump -dr ./test.o
./test.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <baz>:
0: 41 56 push %r14
2: 53 push %rbx
3: 48 81 ec 18 02 00 00 sub $0x218,%rsp
a: 48 89 fb mov %rdi,%rbx
d: 4c 8d 74 24 10 lea 0x10(%rsp),%r14
12: ba 01 02 00 00 mov $0x201,%edx
17: 4c 89 f7 mov %r14,%rdi
1a: 31 f6 xor %esi,%esi
1c: e8 00 00 00 00 call 21 <baz+0x21>
1d: R_X86_64_PLT32 memset-0x4
21: e8 00 00 00 00 call 26 <baz+0x26>
22: R_X86_64_PLT32 bar-0x4
26: 88 44 24 0f mov %al,0xf(%rsp)
2a: 0f b6 44 24 0f movzbl 0xf(%rsp),%eax
2f: 88 03 mov %al,(%rbx)
31: 48 ff c3 inc %rbx
34: ba 01 02 00 00 mov $0x201,%edx
39: 48 89 df mov %rbx,%rdi
3c: 4c 89 f6 mov %r14,%rsi
3f: e8 00 00 00 00 call 44 <baz+0x44>
40: R_X86_64_PLT32 memcpy-0x4
44: 48 81 c4 18 02 00 00 add $0x218,%rsp
4b: 5b pop %rbx
4c: 41 5e pop %r14
4e: c3 ret
$ clang --version
Ubuntu clang version 14.0.0-1ubuntu1.1
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
$ clang -O2 -c test.c
$ objdump -dr ./test.o
./test.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <baz>:
0: 41 56 push %r14
2: 53 push %rbx
3: 48 81 ec 18 02 00 00 sub $0x218,%rsp
a: 48 89 fb mov %rdi,%rbx
d: 4c 8d 74 24 10 lea 0x10(%rsp),%r14
12: ba 01 02 00 00 mov $0x201,%edx
17: 4c 89 f7 mov %r14,%rdi
1a: 31 f6 xor %esi,%esi
1c: e8 00 00 00 00 call 21 <baz+0x21>
1d: R_X86_64_PLT32 memset-0x4
21: e8 00 00 00 00 call 26 <baz+0x26>
22: R_X86_64_PLT32 bar-0x4
26: 88 44 24 0f mov %al,0xf(%rsp)
2a: 8a 44 24 0f mov 0xf(%rsp),%al
2e: 88 03 mov %al,(%rbx)
30: 48 83 c3 01 add $0x1,%rbx
34: ba 01 02 00 00 mov $0x201,%edx
39: 48 89 df mov %rbx,%rdi
3c: 4c 89 f6 mov %r14,%rsi
3f: e8 00 00 00 00 call 44 <baz+0x44>
40: R_X86_64_PLT32 memcpy-0x4
44: 48 81 c4 18 02 00 00 add $0x218,%rsp
4b: 5b pop %rbx
4c: 41 5e pop %r14
4e: c3 ret
$ gcc --version
gcc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ gcc -O2 -c test.c
$ objdump -dr ./test.o
./test.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <baz>:
0: f3 0f 1e fa endbr64
4: 53 push %rbx
5: 48 89 fb mov %rdi,%rbx
8: e8 00 00 00 00 call d <baz+0xd>
9: R_X86_64_PLT32 bar-0x4
d: 48 8d 7b 08 lea 0x8(%rbx),%rdi
11: 48 89 d9 mov %rbx,%rcx
14: 48 c7 03 00 00 00 00 movq $0x0,(%rbx)
1b: 48 83 e7 f8 and $0xfffffffffffffff8,%rdi
1f: 41 89 c0 mov %eax,%r8d
22: 31 c0 xor %eax,%eax
24: 48 c7 83 fa 01 00 00 movq $0x0,0x1fa(%rbx)
2b: 00 00 00 00
2f: 48 29 f9 sub %rdi,%rcx
32: 81 c1 02 02 00 00 add $0x202,%ecx
38: c1 e9 03 shr $0x3,%ecx
3b: f3 48 ab rep stos %rax,%es:(%rdi)
3e: 44 88 03 mov %r8b,(%rbx)
41: 5b pop %rbx
42: c3 ret
After 's/volatile char b;/char b;/', clang-19 produces:
./test.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <baz>:
0: 53 push %rbx
1: 48 89 fb mov %rdi,%rbx
4: e8 00 00 00 00 call 9 <baz+0x9>
5: R_X86_64_PLT32 bar-0x4
9: 88 03 mov %al,(%rbx)
b: 48 ff c3 inc %rbx
e: ba 01 02 00 00 mov $0x201,%edx
13: 48 89 df mov %rbx,%rdi
16: 31 f6 xor %esi,%esi
18: 5b pop %rbx
19: e9 00 00 00 00 jmp 1e <baz+0x1e>
1a: R_X86_64_PLT32 memset-0x4
Regards,
Tiwei
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