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Message-ID: <0214B313-79AF-411F-AE04-8FF2705C5A92@zytor.com>
Date: Thu, 18 Dec 2025 02:12:59 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Eric Dumazet <edumazet@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org
CC: linux-kernel <linux-kernel@...r.kernel.org>,
Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH] x86/irqflags: Force a register output in native_save_fl()
On December 18, 2025 1:54:33 AM PST, Eric Dumazet <edumazet@...gle.com> wrote:
>clang is generating very inefficient code for native_save_fl() which
>is used for local_irq_save() in few critical spots.
>
>Allowing the "pop %0" to use memory :
>
>1) forces the compiler to add annoying stack canaries when
> CONFIG_STACKPROTECTOR_STRONG=y in many places.
>
>2) Almost always is followed by an immediate "move memory,register"
>
>One good example is _raw_spin_lock_irqsave, with 8 extra instructions
>
>ffffffff82067a30 <_raw_spin_lock_irqsave>:
>ffffffff82067a30: ...
>ffffffff82067a39: 53 push %rbx
>
>// Three instructions to ajust the stack, read the per-cpu canary
>// and copy it to 8(%rsp)
>ffffffff82067a3a: 48 83 ec 10 sub $0x10,%rsp
>ffffffff82067a3e: 65 48 8b 05 da 15 45 02 mov %gs:0x24515da(%rip),%rax # <__stack_chk_guard>
>ffffffff82067a46: 48 89 44 24 08 mov %rax,0x8(%rsp)
>
>ffffffff82067a4b: 9c pushf
>
>// instead of pop %rbx, compiler uses 2 instructions.
>ffffffff82067a4c: 8f 04 24 pop (%rsp)
>ffffffff82067a4f: 48 8b 1c 24 mov (%rsp),%rbx
>
>ffffffff82067a53: fa cli
>ffffffff82067a54: b9 01 00 00 00 mov $0x1,%ecx
>ffffffff82067a59: 31 c0 xor %eax,%eax
>ffffffff82067a5b: f0 0f b1 0f lock cmpxchg %ecx,(%rdi)
>ffffffff82067a5f: 75 1d jne ffffffff82067a7e <_raw_spin_lock_irqsave+0x4e>
>
>// three instructions to check the stack canary
>ffffffff82067a61: 65 48 8b 05 b7 15 45 02 mov %gs:0x24515b7(%rip),%rax # <__stack_chk_guard>
>ffffffff82067a69: 48 3b 44 24 08 cmp 0x8(%rsp),%rax
>ffffffff82067a6e: 75 17 jne ffffffff82067a87
>
>...
>
>// One extra instruction to adjust the stack.
>ffffffff82067a73: 48 83 c4 10 add $0x10,%rsp
>...
>
>// One more instruction in case the stack was mangled.
>ffffffff82067a87: e8 a4 35 ff ff call ffffffff8205b030 <__stack_chk_fail>
>
>This patch changes almost nothing for gcc, but saves 23153 bytes
>of text with clang, while allowing more functions to be inlined.
>
>$ size vmlinux.gcc.before vmlinux.gcc.after vmlinux.clang.before vmlinux.clang.after
> text data bss dec hex filename
>45564911 25005415 4708896 75279221 47cab75 vmlinux.gcc.before
>45564901 25005414 4708896 75279211 47cab6b vmlinux.gcc.after
>45115647 24638569 5537072 75291288 47cda98 vmlinux.clang.before
>45092494 24638585 5545000 75276079 47c9f2f vmlinux.clang.after
>
>$ scripts/bloat-o-meter -t vmlinux.clang.before vmlinux.clang.after
>add/remove: 0/3 grow/shrink: 22/533 up/down: 5162/-25088 (-19926)
>Function old new delta
>__noinstr_text_start 640 3584 +2944
>wakeup_cpu_via_vmgexit 1002 1447 +445
>rcu_tasks_trace_pregp_step 1052 1454 +402
>snp_kexec_finish 1290 1527 +237
>check_all_holdout_tasks_trace 909 1106 +197
>x2apic_send_IPI_mask_allbutself 38 198 +160
>hpet_set_rtc_irq_bit 118 265 +147
>x2apic_send_IPI_mask 38 184 +146
>ring_buffer_poll_wait 261 405 +144
>rb_watermark_hit 253 386 +133
>...
>tcp_wfree 402 332 -70
>stacktrace_trigger 133 62 -71
>w1_touch_bit 418 343 -75
>w1_triplet 446 370 -76
>link_create 980 902 -78
>drain_dead_softirq_workfn 425 347 -78
>kcryptd_queue_crypt 253 174 -79
>perf_event_aux_pause 448 368 -80
>idle_worker_timeout 320 240 -80
>srcu_funnel_exp_start 418 333 -85
>call_rcu 751 666 -85
>enable_IR_x2apic 279 191 -88
>bpf_link_free 432 342 -90
>synchronize_rcu 497 403 -94
>identify_cpu 2665 2569 -96
>ftrace_modify_all_code 355 258 -97
>load_gs_index 212 104 -108
>verity_end_io 369 257 -112
>bpf_prog_detach 672 555 -117
>__x2apic_send_IPI_mask 552 275 -277
>snp_cleanup_vmsa 284 - -284
>__end_rodata 606208 602112 -4096
>Total: Before=28577927, After=28558001, chg -0.07%
>
>Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>---
> arch/x86/include/asm/irqflags.h | 7 +------
> 1 file changed, 1 insertion(+), 6 deletions(-)
>
>diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
>index b30e5474c18e1be63b7c69354c26ae6a6cb02731..a06bdc51b6e3d04c06352c28389b10ec66551a0e 100644
>--- a/arch/x86/include/asm/irqflags.h
>+++ b/arch/x86/include/asm/irqflags.h
>@@ -18,14 +18,9 @@ extern __always_inline unsigned long native_save_fl(void)
> {
> unsigned long flags;
>
>- /*
>- * "=rm" is safe here, because "pop" adjusts the stack before
>- * it evaluates its effective address -- this is part of the
>- * documented behavior of the "pop" instruction.
>- */
> asm volatile("# __raw_save_flags\n\t"
> "pushf ; pop %0"
>- : "=rm" (flags)
>+ : "=r" (flags)
> : /* no input */
> : "memory");
>
Maybe report a bug to the clang team?
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