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SubjectRe: [PATCH] WARN_ONCE(): use bool for boolean flag
Cesar Eduardo Barros escreveu:
> Daniel Walker escreveu:
>> On Sun, 2009-09-27 at 14:24 -0300, Cesar Eduardo Barros wrote:
>>
>>> In fact, I was expecting no change at all, since gcc should be able
>>> to see it is being treated as a boolean (perhaps I am trusting gcc
>>> too much). And to make matters even more confusing, my own test
>>> changing all __ret_warn_once to bool and dropping the !! caused an
>>> _increase_ of 598 bytes (x86-64 defconfig).
>>>
>>> text data bss dec hex filename
>>> 8100553 1207148 991988 10299689 9d2929 vmlinux.warnret.before
>>> 8101119 1207180 991988 10300287 9d2b7f vmlinux.warnret.after
>>>
>>> (And yes, data increased again.)
>>
>> If this was just your regular base line config , then that is odd .. I
>> also would think worse case would be no size reduction .. I did my
>> compile test on x86-32 btw..
>
> I will try looking at the first function which shows a difference in
> size (which appears to be handle_irq) and see what I can find.

I just took a quick look, and it does seem to be bad code generation
(the gcc on this machine is a bit old). The question is, is the gain in
less buggy gcc versions enough to offset the loss in older and buggier
gcc versions?

The function in question (stack_overflow_check() in
arch/x86/kernel/irq_64.c) has a somewhat complex expression in the call
to WARN_ON, which gcc seems to be pessimizing in this case (it is
storing the boolean in a register just to test it again).

I will send the patch I am using in the next email.

gcc (Ubuntu 4.3.2-1ubuntu12) 4.3.2

--- /dev/fd/63 2009-09-27 14:59:26.124947107 -0300
+++ /dev/fd/62 2009-09-27 14:59:26.144947152 -0300
@@ -246,14 +246,14 @@
pushq %rbp
#APP
# 14
"/scratch/build/cesarb/linux/linux-2.6/arch/x86/include/asm/current.h" 1
- movq %gs:per_cpu__current_task,%rcx
+ movq %gs:per_cpu__current_task,%rax
# 0 "" 2
#NO_APP
movq %rsp, %rbp
pushq %rbx
movl %edi, %ebx
subq $8, %rsp
- movq 8(%rcx), %r8
+ movq 8(%rax), %r8
movq 152(%rsi), %rdx
cmpq %r8, %rdx
jb .L24
@@ -262,28 +262,40 @@
ja .L24
leaq 400(%r8), %rax
cmpq %rax, %rdx
- jae .L24
+ setb %al
+ movzbl %al, %eax
+ jmp .L25
+.L24:
+ xorl %eax, %eax
+.L25:
+ testl %eax, %eax
+ je .L26
cmpb $0, __warned.21424(%rip)
- jne .L24
+ jne .L26
movq %rdx, %r9
- addq $1112, %rcx
- movq $.LC3, %rdx
movl $47, %esi
+ movq $.LC3, %rdx
+#APP
+# 14
"/scratch/build/cesarb/linux/linux-2.6/arch/x86/include/asm/current.h" 1
+ movq %gs:per_cpu__current_task,%rcx
+# 0 "" 2
+#NO_APP
movq $.LC0, %rdi
+ addq $1112, %rcx
xorl %eax, %eax
call warn_slowpath_fmt
movb $1, __warned.21424(%rip)
-.L24:
+.L26:
movl %ebx, %edi
call irq_to_desc
xorl %edx, %edx
testq %rax, %rax
- je .L26
+ je .L28
movq %rax, %rsi
movl %ebx, %edi
call *24(%rax)
movb $1, %dl
-.L26:
+.L28:
movb %dl, %al
popq %rdx
popq %rbx

--
Cesar Eduardo Barros
cesarb@cesarb.net
cesar.barros@gmail.com


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