Is this use of the Effective Type rule strictly conforming?

问题

The Effective Type rule in C99 and C11 provides that storage with no declared type may be written with any type and, that storing a value of a non-character type will set the Effective Type of the storage accordingly.

Setting aside the fact that INT_MAX might be less than 123456789, would the following code's use of the Effective Type rule be strictly conforming?

#include <stdlib.h>
#include <stdio.h>

/* Performs some calculations using using int, then float,
  then int.

    If both results are desired, do_test(intbuff, floatbuff, 1);
    For int only, do_test(intbuff, intbuff, 1);
    For float only, do_test(floatbuff, float_buff, 0);

  The latter two usages require storage with no declared type.    
*/

void do_test(void *p1, void *p2, int leave_as_int)
{
  *(int*)p1 = 1234000000;

  float f = *(int*)p1;
  *(float*)p2 = f*2-1234000000.0f;

  if (leave_as_int)
  {
    int i = *(float*)p2;
    *(int*)p1 = i+567890;
  }
}

void (*volatile test)(void *p1, void *p2, int leave_as_int) = do_test;

int main(void)
{
  int iresult;
  float fresult;
  void *p = malloc(sizeof(int) + sizeof(float));
  if (p)
  {
    test(p,p,1);
    iresult = *(int*)p;
    test(p,p,0);
    fresult = *(float*)p;
    free(p);
    printf("%10d %15.2f\n", iresult,fresult);
  }
  return 0;
}

From my reading of the Standard, all three usages of the function described in the comment should be strictly conforming (except for the integer-range issue). The code should thus output 1234567890 1234000000.00. GCC 7.2, however, outputs 1234056789 1157904.00. I think that when leave_as_int is 0, it's storing 123400000 to *p1 after it stores 123400000.0f to *p2, but I see nothing in the Standard that would authorize such behavior. Am I missing anything, or is gcc non-conforming?

回答1:

Yes, this is a gcc bug. I've filed it (with a simplified testcase) as https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82697 .

回答2:

The generated machine code unconditionally writes to both pointers:

do_test:
        cmpl    $1, %edx
        movl    $0x4e931ab1, (%rsi)
        sbbl    %eax, %eax
        andl    $-567890, %eax
        addl    $1234567890, %eax
        movl    %eax, (%rdi)
        ret

This is a GCC bug because all stores are expected to change the dynamic type of the memory accessed. I do not think this behavior is mandated by the standard; it is a GCC extension.

Can you file a GCC bug?

来源：https://stackoverflow.com/questions/46592132/is-this-use-of-the-effective-type-rule-strictly-conforming