On 02/19/2015 03:09 PM, Jakub Jelinek wrote:
If you have hw where NULL is mapped and you know your code violates the
C/C++ standards by placing objects at that address, simply do use
the option that is designed for that purpose.
As I pointed out earlier, though, that won't help you if your program
(perhaps implicitly) uses library code that's been built without the
magic option.
I thought that declaring an object explicitly placed at address 0 as
"volatile" ought to be sufficient to prevent writes to it from being
optimized away and replaced with traps, but alas, that is not the case.
Also, a structure copy to such a volatile object is implicitly
invoking the regular memcpy function without complaining about casting
away volatile-ness, as an explicit call would do. I've attached a
couple quick test cases and .s output from arm-none-eabi-gcc -O2, using
a mainline build from last night.
-Sandra
static int * const x0 = (int *) 0x00000000;
static volatile int * const xv = (int *) 0x00000000;
static int * const xn = (int *) 0x10000000;
void f0 (int *data)
{
*x0 = *data;
}
void fv (int *data)
{
*xv = *data;
}
void fn (int *data)
{
*xn = *data;
}
.cpu arm7tdmi
.fpu softvfp
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 1
.eabi_attribute 30, 2
.eabi_attribute 34, 0
.eabi_attribute 18, 4
.arm
.syntax divided
.file "test.c"
.text
.align 2
.global f0
.type f0, %function
f0:
@ Function supports interworking.
@ Volatile: function does not return.
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
mov r3, #0
str r3, [r3]
.inst 0xe7f000f0
.size f0, .-f0
.align 2
.global fv
.type fv, %function
fv:
@ Function supports interworking.
@ Volatile: function does not return.
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
mov r3, #0
str r3, [r3]
.inst 0xe7f000f0
.size fv, .-fv
.align 2
.global fn
.type fn, %function
fn:
@ Function supports interworking.
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
mov r3, #268435456
ldr r2, [r0]
str r2, [r3]
bx lr
.size fn, .-fn
.ident "GCC: (GNU) 5.0.0 20150219 (experimental)"
struct big {
int a, b;
char c [100];
};
static struct big * const x0 = (struct big *) 0x00000000;
static volatile struct big * const xv = (struct big *) 0x00000000;
static struct big * const xn = (struct big *) 0x10000000;
void f0 (struct big *data)
{
*x0 = *data;
}
void fv (struct big *data)
{
*xv = *data;
}
void fn (struct big *data)
{
*xn = *data;
}
.cpu arm7tdmi
.fpu softvfp
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 1
.eabi_attribute 30, 2
.eabi_attribute 34, 0
.eabi_attribute 18, 4
.arm
.syntax divided
.file "test2.c"
.text
.align 2
.global f0
.type f0, %function
f0:
@ Function supports interworking.
@ Volatile: function does not return.
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
mov r1, r0
stmfd sp!, {r4, lr}
mov r2, #108
mov r0, #0
bl memcpy
.inst 0xe7f000f0
.size f0, .-f0
.align 2
.global fv
.type fv, %function
fv:
@ Function supports interworking.
@ Volatile: function does not return.
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
mov r1, r0
stmfd sp!, {r4, lr}
mov r2, #108
mov r0, #0
bl memcpy
.inst 0xe7f000f0
.size fv, .-fv
.align 2
.global fn
.type fn, %function
fn:
@ Function supports interworking.
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
stmfd sp!, {r4, lr}
mov r1, r0
mov r2, #108
mov r0, #268435456
bl memcpy
ldmfd sp!, {r4, lr}
bx lr
.size fn, .-fn
.ident "GCC: (GNU) 5.0.0 20150219 (experimental)"
