Re: [PATCH] configure: Improve alias attribute check

[Gavin Shan <gshan@redhat.com>]

Hi Richard and Paolo,

On 3/22/21 5:23 AM, Richard Henderson wrote:
> On 3/21/21 11:46 AM, Paolo Bonzini wrote:
>> HRM, what about biting the bullet and making exec-vary.c a C++ source?... Then instead of making it conditional an attribute((alias)), we make it conditional on having a C++ compiler.
> 
> Doesn't help.  The gcc bug I filed talks about c++, because that's the closest analogy.
> 
> But set_preferred_target_page_bits is called *much* later than a constructor. Though still before any use of the variable in question, for which we have an --enable-debug-tcg assertion.
> 

It looks this issue can be avoided after "volatile" is applied to
@target_page. However, I'm not sure if it's the correct fix to have.
If it is, I can post a formal patch so that it can be included.

--- a/exec-vary.c
+++ b/exec-vary.c
-extern const TargetPageBits target_page
+extern const TargetPageBits volatile target_page
      __attribute__((alias("init_target_page")));
--- a/include/exec/cpu-all.h
+++ b/include/exec/cpu-all.h
-extern const TargetPageBits target_page;
+extern const TargetPageBits volatile target_page;


According to the experiments I did, it would be function call
to set_preferred_target_page_bits() is dropped when the machine
is created. The following c files are used in the experiment:

--- a.c ---

static int x;
const extern int VOLATILE y __attribute__((alias("x")));
extern int read_y(void);

void write_x(int val) { x = 1; }
int main(void) { return read_y(); }

--- b.c---

extern const int VOLATILE y;
extern void write_x(int val);

int read_y(void) { write_x(1); return y; }

# gcc a.c b.c -O2 -flto=auto -DVOLATILE= -o a; ./a; echo $?
0
# gdb -nw ./a
(gdb) disassem main
Dump of assembler code for function main:
    0x0000000000400480 <+0>:	adrp	x1, 0x420000 <__libc_start_main@got.plt>
    0x0000000000400484 <+4>:	mov	w2, #0x1                   	// #1
    0x0000000000400488 <+8>:	mov	w0, #0x0                   	// #0
    0x000000000040048c <+12>:	str	w2, [x1, #32]
    0x0000000000400490 <+16>:	ret
End of assembler dump.

# gcc a.c b.c -O2 -flto=auto -DVOLATILE=volatile -o a; ./a; echo $?
1
# gdb -nw ./a
(gdb) disassem main
Dump of assembler code for function main:
    0x0000000000400480 <+0>:	adrp	x1, 0x420000 <__libc_start_main@got.plt>
    0x0000000000400484 <+4>:	mov	w2, #0x1                   	// #1
    0x0000000000400488 <+8>:	adrp	x0, 0x420000 <__libc_start_main@got.plt>
    0x000000000040048c <+12>:	str	w2, [x1, #32]
    0x0000000000400490 <+16>:	ldr	w0, [x0, #32]
    0x0000000000400494 <+20>:	ret


Thanks,
Gavin