[Bug 1831808] [NEW] nexttowardl for ppc64el generates wrong results

[Hubert Tong]

Public bug reported:

On PPC, the non-IEEE 128-bit long double is represented as a pair of
doubles. This means that for 1.L, the next representable value in the
direction of 2.L should be 1.L + DBL_EPSILON. The nexttowardl function
does not seem to produce such a result.

To reproduce, compile and link the following C program and run the
resulting executable. The actual output and the expected output is
provided below.

### SOURCE (<stdin>):
long double nexttowardl(long double, long double);
int printf(const char *, ...);

int main(void) {
  long double eps = nexttowardl(0.L, 1.L);
  long double ans = nexttowardl(1.L, 2.L);
  long double ref = 1.L + eps;
  typedef unsigned char *ucp;
  for (ucp p = (ucp)&ref; p < (ucp)&1[&ref]; ++p) {
    printf("%02hhx", *p);
  }
  printf("\n");
  for (ucp p = (ucp)&ans; p < (ucp)&1[&ans]; ++p) {
    printf("%02hhx", *p);
  }
  printf("\n");
}


### COMPILER INVOCATION:
gcc -xc - -lm


### RUN OUTPUT:
000000000000f03f0100000000000000
000000000000f03f0000000000006039


### EXPECTED OUTPUT:
000000000000f03f0100000000000000
000000000000f03f0100000000000000


### VERSION INFO:
> lsb_release -rd
Description:    Ubuntu 16.04.2 LTS
Release:        16.04
Return:  0x00:0

> apt-cache policy libc6
libc6:
  Installed: 2.23-0ubuntu7
  Candidate: 2.23-0ubuntu11
  Version table:
     2.23-0ubuntu11 500
        500 http://ports.ubuntu.com/ubuntu-ports xenial-updates/main ppc64el Packages
     2.23-0ubuntu10 500
        500 http://ports.ubuntu.com/ubuntu-ports xenial-security/main ppc64el Packages
 *** 2.23-0ubuntu7 100
        100 /var/lib/dpkg/status
     2.23-0ubuntu3 500
        500 http://ports.ubuntu.com/ubuntu-ports xenial/main ppc64el Packages
Return:  0x00:0

** Affects: glibc (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1831808

Title:
  nexttowardl for ppc64el generates wrong results

Status in glibc package in Ubuntu:
  New

Bug description:
  On PPC, the non-IEEE 128-bit long double is represented as a pair of
  doubles. This means that for 1.L, the next representable value in the
  direction of 2.L should be 1.L + DBL_EPSILON. The nexttowardl function
  does not seem to produce such a result.

  To reproduce, compile and link the following C program and run the
  resulting executable. The actual output and the expected output is
  provided below.

  ### SOURCE (<stdin>):
  long double nexttowardl(long double, long double);
  int printf(const char *, ...);

  int main(void) {
    long double eps = nexttowardl(0.L, 1.L);
    long double ans = nexttowardl(1.L, 2.L);
    long double ref = 1.L + eps;
    typedef unsigned char *ucp;
    for (ucp p = (ucp)&ref; p < (ucp)&1[&ref]; ++p) {
      printf("%02hhx", *p);
    }
    printf("\n");
    for (ucp p = (ucp)&ans; p < (ucp)&1[&ans]; ++p) {
      printf("%02hhx", *p);
    }
    printf("\n");
  }

  
  ### COMPILER INVOCATION:
  gcc -xc - -lm

  
  ### RUN OUTPUT:
  000000000000f03f0100000000000000
  000000000000f03f0000000000006039

  
  ### EXPECTED OUTPUT:
  000000000000f03f0100000000000000
  000000000000f03f0100000000000000

  
  ### VERSION INFO:
  > lsb_release -rd
  Description:    Ubuntu 16.04.2 LTS
  Release:        16.04
  Return:  0x00:0

  > apt-cache policy libc6
  libc6:
    Installed: 2.23-0ubuntu7
    Candidate: 2.23-0ubuntu11
    Version table:
       2.23-0ubuntu11 500
          500 http://ports.ubuntu.com/ubuntu-ports xenial-updates/main ppc64el Packages
       2.23-0ubuntu10 500
          500 http://ports.ubuntu.com/ubuntu-ports xenial-security/main ppc64el Packages
   *** 2.23-0ubuntu7 100
          100 /var/lib/dpkg/status
       2.23-0ubuntu3 500
          500 http://ports.ubuntu.com/ubuntu-ports xenial/main ppc64el Packages
  Return:  0x00:0

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