RE: gcc question

["HON LUU" <hon@dreambigsemi.com>]

Hi Segher,

Following is more information that I can collect. Please let me know if you need anything else.
Thank you very much for your help.

Regards,
Hon
=====================================================================

+ this is part of the content of cdev.c, from which the error happened
...
...
struct static_key_false test;

int qdma_cdev_init(void) {
   
    if (static_branch_unlikely(&test)) {
        printk("test...\n");
    }
	return 0;
}

void qdma_cdev_cleanup(void) {
}
...

+ static_branch_unlikely is defined as followed from /lib/modules/`uname -r`/build/include/linux/jump_label.h :

#define static_branch_unlikely(x)						\
({										\
	bool branch;								\
	if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))	\
		branch = arch_static_branch_jump(&(x)->key, false);		\
	else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
		branch = arch_static_branch(&(x)->key, false);			\
	else									\
		branch = ____wrong_branch_error();				\
	unlikely(branch);							\
})

+ the error is related to arch_static_branch_jump() which is defined from /lib/modules/`uname -r`/build/arch/x86/include/asm/jump_label.h as followed:

static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
{
	asm_volatile_goto("1:"
		".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
		".pushsection __jump_table,  \"aw\" \n\t"
		_ASM_ALIGN "\n\t"
		_ASM_PTR "1b, %l[l_yes], %c0 + %c1 \n\t"
		".popsection \n\t"
		: :  "i" (key),  "i" (branch) : : l_yes);

	return false;
l_yes:
	return true;
}

+ The code compiled successfully in kernel mode with followed command:
    + make V=1 clean
    + make V=1
+ The code failed to compile when I try to compile into user space mode with the followed command:
    + make -f Makefile.user clean
    + make -f Makefile.user
+ Note that the difference between gcc command line between the 2 modes are:
    + kernel:      .... -fno-PIE ... -mcmodel=kernel ...
    + user space:  .... -fPIC    ... <  removed    > ...
+ Another interesting note is:
    + if I remove -fPIC from user space gcc command, then the code compile successfully. However -fPIC is the must for my project.


-----Original Message-----
From: Segher Boessenkool <segher@kernel.crashing.org> 
Sent: Friday, September 10, 2021 11:36 AM
To: HON LUU <hon@dreambigsemi.com>
Cc: gcc-help@gcc.gnu.org
Subject: Re: gcc question

On Fri, Sep 10, 2021 at 05:58:07PM +0000, HON  LUU wrote:
> Hi Segher,
> 
> I wonder if you have any issues to reproduce the problem.

> Attached is the tar ball of stand alone module, which duplicate the issue.
> There is a README file, should be straightforward to duplicate the issue.

> You need to show the full code of the asm_volatile_goto, and the declarations of everything used in its operands, for us to get a handle on what is going on here.  Bonus points if you manage to make a stand- alone compilable testcase that shows the problem.

If you write up a good email with all the necessary info, you will get an answer (and a good answer!) much quicker.  I do not particularly feel like doing all the work here.  Sorry.


Segher