[Bug middle-end/102930] New: equal values appear to be different due to missing correct rounding in libc

["vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102930

            Bug ID: 102930
           Summary: equal values appear to be different due to missing
                    correct rounding in libc
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

Due to missing correct rounding in libc, expressions like sin(x) may give
different values at compile time and at run time. With GCC optimization, this
makes equal values appear to be different (e.g. an integer variable having two
different values at the same time).

Here's a testcase, inspired by PR85957 Comment 7 (which was about the x87
extended precision).

#include <stdio.h>
#include <math.h>

#define D1 0x1.005023d32fee5p+1
#define D2 0x0.2ef652eba3771p-1

__attribute__((noinline,noclone))
static double opaque(void)
{
  return D1;
}

__attribute__((noinline,noclone))
static void t1(void)
{
  double a = D1, b = opaque();

  if (a != b) printf("uneq");

  double sa = sin(a), sb = sin(b);

  printf("t1: %a %s %a\n", sa, sa == sb ? "==" : "!=", sb);
}

__attribute__((noinline,noclone))
static void t2(void)
{
  double a = D1, b = opaque();

  if (a != b) printf("uneq");

  int ia = sin(a) + D2, ib = sin(b) + D2;

  printf("t2: %d %s %d\n", ia, ia == ib ? "==" : "!=", ib);
}

__attribute__((noinline,noclone))
static void t3(void)
{
  double a = D1, b = opaque();

  if (a != b) printf("uneq");

  int ia = sin(a) + D2, ib = sin(b) + D2;

  printf("t3: ib = %d\n", ib);
  if (ia == ib)
    printf("t3: ib = %d\n", ib);
}

int main(void)
{
  t1();
  t2();
  t3();
  return 0;
}

Compile with the following options: -O2 -lm -fdisable-tree-dom3

This gives:

t1: 0x1.d109ad145c88fp-1 == 0x1.d109ad145c88ep-1
t2: 1 == 0
t3: ib = 0
t3: ib = 1

Tested GCC versions under Debian/unstable:
* gcc-8 (Debian 8.4.0-7) 8.4.0
* gcc-9 (Debian 9.4.0-3) 9.4.0
* gcc-10 (Debian 10.3.0-11) 10.3.0
* gcc-11 (Debian 11.2.0-10) 11.2.0
* gcc (Debian 20210918-1) 12.0.0 20210918 (experimental) [master
r12-3644-g7afcb534239]

If -fdisable-tree-dom3 is not provided, t1 still fails with GCC 8 to 11.