clang is generating very inefficient code for native_save_fl() which is used for local_irq_save() in 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 nothing for gcc, but for clang saves ~20000 bytes of text even though more functions are inlined. $ size vmlinux.gcc.before vmlinux.gcc.after vmlinux.clang.before vmlinux.clang.after text data bss dec hex filename 45565821 25005462 4704800 75276083 47c9f33 vmlinux.gcc.before 45565821 25005462 4704800 75276083 47c9f33 vmlinux.gcc.after 45121072 24638617 5533040 75292729 47ce039 vmlinux.clang.before 45093887 24638633 5536808 75269328 47c84d0 vmlinux.clang.after $ scripts/bloat-o-meter -t vmlinux.clang.before vmlinux.clang.after add/remove: 1/2 grow/shrink: 21/533 up/down: 2250/-22112 (-19862) Signed-off-by: Eric Dumazet <edumazet@google.com> Cc: Uros Bizjak <ubizjak@gmail.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>