[Bug tree-optimization/118729] New: dead store elimination with function attributes for read / write and similar functions

[uecker at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118729

            Bug ID: 118729
           Summary: dead store elimination with function attributes for
                    read / write and similar functions
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: uecker at gcc dot gnu.org
  Target Milestone: ---

In the following example, the initialization (also with
-ftrivial-auto-var-init...) is not removed although it is dead. But the
compiler can not know this. One solution would be to have some function
attributes that tell the compiler how much is read/written to the buffer. But
then this would also not sufficient as the second example shows, where
everything is known.

#define BUFSIZE 4096

void doit(int in_fd, int out_fd) 
{
    while (1) {
        char buf[BUFSIZE] = { 0 };
        ssize_t len = read(in_fd, buf, BUFSIZE);
        if (len <= 0)
            break;
        write(out_fd, buf, len);
    }
}

---------------------------------------
#define BUFSIZE 4096
int g(void);

void doit(char b2[BUFSIZE]) 
{
    while (1) {
        char buf[BUFSIZE] = { 0 };
        int len = g();
        for (int i = 0; i < len; i++)
            buf[i] = 0;
        if (len <= 0)
            break;
        for (int i = 0; i < len; i++)
            b2[i] = buf[i];
    }
}