Unconditional trap

Unconditional trap

[William Tambe]

I am currently using following for trap instruction:

(define_insn "trap"
  [(trap_if (const_int 1) (const_int 0))]
  ""
  "brk")

However it causes the compiler to generate a trap when address 0 is
used; in the example below, I have a function that prints characters
through the UART using address 0; but the resulting assembly generates
a function that traps instead.

#define UART_ADDR 0
void uart_print (char *s) {
        while (*s) {
                *(volatile char *)UART_ADDR = *s;
                ++s;
        }
}

How can one implement unconditional trap ?

Re: Unconditional trap

[Marc Glisse]

On Sat, 12 Sep 2020, William Tambe via Gcc-help wrote:

> However it causes the compiler to generate a trap when address 0 is
> used; in the example below, I have a function that prints characters
> through the UART using address 0; but the resulting assembly generates
> a function that traps instead.

Are you using -fno-delete-null-pointer-checks?

-- 
Marc Glisse

Re: Unconditional trap

[William Tambe]

On Sat, Sep 12, 2020 at 3:26 PM Marc Glisse <marc.glisse@inria.fr> wrote:
>
> On Sat, 12 Sep 2020, William Tambe via Gcc-help wrote:
>
> > However it causes the compiler to generate a trap when address 0 is
> > used; in the example below, I have a function that prints characters
> > through the UART using address 0; but the resulting assembly generates
> > a function that traps instead.
>
> Are you using -fno-delete-null-pointer-checks?

Using -fno-delete-null-pointer-checks helps.

Is there a way to make it permanent in the GCC source such that
providing that option is not needed ?

>
> --
> Marc Glisse

Re: Unconditional trap

[Marc Glisse]

On Sat, 12 Sep 2020, William Tambe via Gcc-help wrote:

> Using -fno-delete-null-pointer-checks helps.
>
> Is there a way to make it permanent in the GCC source such that
> providing that option is not needed ?

$ grep -R delete-null-pointer-checks gcc/config
gcc/config/nios2/elf.h:/* Default -fdelete-null-pointer-checks to off, to prevent the compiler
gcc/config/cr16/cr16.c:  /* Disable -fdelete-null-pointer-checks option for CR16 target.

TARGET_OPTION_OVERRIDE seems to be the way if you want to disable it for 
some specific target. If you want to change the default for all targets, 
you could edit common.opt instead.

-- 
Marc Glisse

Re: Unconditional trap

[Segher Boessenkool]

Hi!

On Sat, Sep 12, 2020 at 03:17:03PM -0500, William Tambe via Gcc-help wrote:
> I am currently using following for trap instruction:
> 
> (define_insn "trap"
>   [(trap_if (const_int 1) (const_int 0))]
>   ""
>   "brk")

This looks fine.

> However it causes the compiler to generate a trap when address 0 is
> used; in the example below, I have a function that prints characters
> through the UART using address 0; but the resulting assembly generates
> a function that traps instead.
> 
> #define UART_ADDR 0
> void uart_print (char *s) {
>         while (*s) {
>                 *(volatile char *)UART_ADDR = *s;
>                 ++s;
>         }
> }
> 
> How can one implement unconditional trap ?

You should implement TARGET_ADDR_SPACE_ZERO_ADDRESS_VALID to make 0 a
valid address.


Segher