linux-kernel - Re: [PATCH 2/3] time: allow gcc to fold constants when using msecs_to

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Date:	Sun, 12 Apr 2015 10:36:34 +0200
From:	Nicholas Mc Guire <der.herr@...r.at>
To:	Joe Perches <joe@...ches.com>
Cc:	Nicholas Mc Guire <hofrat@...dl.org>,
	Michal Marek <mmarek@...e.cz>,
	Masahiro Yamada <yamada.m@...panasonic.com>,
	Sam Ravnborg <sam@...nborg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Alvin" <hpa@...or.com>,
	John Stultz <john.stultz@...aro.org>,
	Andrew Hunter <ahh@...gle.com>, Paul Turner <pjt@...gle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] time: allow gcc to fold constants when using
	msecs_to_jiffies

> Your inline version has not worked with any of
> x86-64 gcc 4.4, 4.6, 4.7, or 4.9
> 
> I suggest you add some lines to
> lib/test_module.c/test_module_init like:
> 
> 	unsigned int m;
> 
> 	for (m = 10; m < 200; m += 10)
> 		pr_info("msecs_to_jiffies(%u) is %lu\n",
> 			m, msecs_to_jiffies(m));
> 
> 	pr_info("msecs_to_jiffies(%u) is %lu\n",
> 		10, msecs_to_jiffies(10));
> 	pr_info("msecs_to_jiffies(%u) is %lu\n",
> 		100, msecs_to_jiffies(100));
> 	pr_info("msecs_to_jiffies(%u) is %lu\n",
> 		1000, msecs_to_jiffies(1000));
> 
> Then it's pretty easy to look at the assembly/.lst file
> 
> Your inline function doesn't allow gcc to precompute
> the msecs_to_jiffies value.  The macro one does for all
> those gcc versions.
> 
> Try it and look at the generated .lst files with and
> without the patch I sent.
> 
I have checked it with the testcase you proposed  - and I quite sure
it is working, find the .s file snippets from the test_module.c modified 
as you suggested below. HZ is set to 300 so that it is easy to differenciate
the parameter to msecs_to_jiffies and the printed const.

there is no call and no calculation for the constant cases - just load as 
it should be.

with the patch applied:
test_module_init:
        pushq   %rbp    #
        movq    %rsp, %rbp      #,
        pushq   %rbx    #
        movl    $10, %ebx       #, m
        pushq   %rcx    #
.L4:
        movl    %ebx, %edi      # m,
        call    __msecs_to_jiffies      #
        movl    %ebx, %esi      # m,
        movq    %rax, %rdx      # D.14515,
        movq    $.LC1, %rdi     #,
        xorl    %eax, %eax      #
        addl    $10, %ebx       #, m
        call    printk  #
        cmpl    $200, %ebx      #, m
        jne     .L4     #,                <---end of for loop
        movl    $3, %edx        #,        <---msecs_to_jiffies(10)
        movl    $10, %esi       #,
        movq    $.LC1, %rdi     #,
        xorl    %eax, %eax      #
        call    printk  #
        movl    $30, %edx       #,        <---msecs_to_jiffies(100)
        movl    $100, %esi      #,
        movq    $.LC1, %rdi     #,
        xorl    %eax, %eax      #
        call    printk  #
        movl    $300, %edx      #,        <---msecs_to_jiffies(1000)
        movl    $1000, %esi     #,
        movq    $.LC1, %rdi     #,
        xorl    %eax, %eax      #
        call    printk  #

without the patch applied:

        pushq   %rbp    #
        movq    %rsp, %rbp      #,
        pushq   %rbx    #
        movl    $10, %ebx       #, m
        pushq   %rcx    #
.L2:
        movl    %ebx, %edi      # m,
        call    msecs_to_jiffies        #
        movl    %ebx, %esi      # m,
        movq    %rax, %rdx      # D.14503,
        movq    $.LC0, %rdi     #,
        xorl    %eax, %eax      #
        addl    $10, %ebx       #, m
        call    printk  #
        cmpl    $200, %ebx      #, m
        jne     .L2     #,
        movl    $10, %edi       #,
        call    msecs_to_jiffies        #
        movl    $10, %esi       #,
        movq    %rax, %rdx      # D.14504,
        movq    $.LC0, %rdi     #,
        xorl    %eax, %eax      #
        call    printk  #
        movl    $100, %edi      #,
        call    msecs_to_jiffies        #
        movl    $100, %esi      #,
        movq    %rax, %rdx      # D.14505,
        movq    $.LC0, %rdi     #,
        xorl    %eax, %eax      #
        call    printk  #
        movl    $1000, %edi     #,

could you check the .s file for you test module ?
I'm a bit lost on why you are not seeing this - also
checked with cross-build (multi_v7_defconfig) and it
looks like thats working as well.

thx!
hofrat
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