[Bug c/38354] New: Spurious error: initializer element is not computable at load time

[Bug c/38354] New: Spurious error: initializer element is not computable at load time

[gnu at behdad dot org]

The following code is from gcc docs and works as expected:

#include <stdio.h>

int
main (int argc, char **argv)
{
  static const int array[] = { &&foo-&&foo, &&bar-&&foo, &&hack-&&foo };
  int i = argc - 1;

  goto *(array[i]+&&foo);

foo:
  printf ("foo\n");
  return 0;

bar:
  printf ("bar\n");
  return 0;

hack:
  printf ("hack\n");
  return 0;
}


However, if I change the array type from:

  static const int array[] = { &&foo-&&foo, &&bar-&&foo, &&hack-&&foo };

to:

  static const void *array[] = { &&foo-&&foo, &&bar-&&foo, &&hack-&&foo };

I get the error:

label-value.c:6: error: initializer element is not computable at load time

This doesn't make any sense to me.


-- 
           Summary: Spurious error: initializer element is not computable at
                    load time
           Product: gcc
           Version: 4.3.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: gnu at behdad dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38354

[Bug c/38354] Spurious error: initializer element is not computable at load time

[jakub at gcc dot gnu dot org]

------- Comment #1 from jakub at gcc dot gnu dot org  2008-12-01 23:33 -------
What's the rationale for using a pointer type for something that is integral
(difference of 2 pointers)?
I don't see why we should support that as a valid constant initializer.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38354

[Bug c/38354] Spurious error: initializer element is not computable at load time

[gnu at behdad dot org]

------- Comment #2 from gnu at behdad dot org  2008-12-01 23:38 -------
It's not a useful use case, agreed, but I don't see how that affects the
computability of a value at "load time", whatever that means.  It did trick me:
I was converting a vtable to use label values, and before converting the
function pointer type of the array to int, I tried a compile and got the error.
 I didn't expect the error to go away if I replace the function pointer type
with an int.  To my surprise, it did.

What I'm saying is that the statement in the error message is false.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38354

[Bug c/38354] Spurious error: initializer element is not computable at load time

[adam at consulting dot net dot nz]

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------- Comment #3 from adam at consulting dot net dot nz  2010-08-17 06:28 -------
AMD64 non-large code model. Functions occupy lower 2GB of the address space.
Cannot compile an array of 32-bit function addresses:

#include <stddef.h>
#include <stdint.h>
#include <stdio.h>

void fn() {
  printf("Hello, World\n");
}

uint32_t computable_at_load_time_a[]={fn};
uint32_t computable_at_load_time_b[]={(uint32_t) (ptrdiff_t) fn};

int main() {
  return 0;
}

$ gcc -m32 computable_at_load_time.c
computable_at_load_time.c:9: warning: initialization makes integer from pointer
without a cast

$ gcc computable_at_load_time.c
computable_at_load_time.c:9: warning: initialization makes integer from pointer
without a cast
computable_at_load_time.c:9: error: initializer element is not computable at
load time
computable_at_load_time.c:9: error: (near initialization for
‘computable_at_load_time_a[0]’)
computable_at_load_time.c:10: error: initializer element is not constant
computable_at_load_time.c:10: error: (near initialization for
‘computable_at_load_time_b[0]’)

The very good reason for wanting to compile a table of 32-bit function pointers
is to halve the data cache footprint of threaded code.


-- 

adam at consulting dot net dot nz changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |adam at consulting dot net
                   |                            |dot nz


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38354