IV36069: Exception bit folded incorrectly with -qfloat=fenv -O2

Fixes are available

APAR status

Closed as program error.

Error description

With options -qfloat=fenv -O2, the exception bit is folded
incorrectly sometimes, which may cause libraries or programs to
mal-behave.
The following test case outputs bits 32:55 of the FPSCR in
binary before and after the test expression is executed.
It tests 4 idioms:  1/0, 0/0, inf-inf, (inf+x)-inf.  The
exception bit is bit 32 (the left most bit).  It should be 0
before and 1 after the exception call.
For C, the FPSCR is being set correctly for all but the inf-inf
case.  This is the expected result.
$ cat testmain.c
#include <math.h>
void printb (int val) {
  int j;
  printf ("fpscr=");
  for (int j = 31; j >= 0; j --) {
    if (val & 0x80000000) printf("1");
    else printf("0");
    if (j%4==0) printf(" ");
    val<<=1;
  }
  printf ("\n");
}
void printd (long double val) {
  printf ("retval=%e\n", val);
}
main(){
  int i;
  double x, y;
  union {double d; int i[2];} u;
  extern double test_inf(double), test_nan1(double),
test_nan2(double),
    test_nan3(double);
  x = 1.0;
  for (i=0; i<4; i++) {
    u.d = __readflm();
    u.i[1] &= ~0xFFFFFF00;  // clear exception bits
    u.d = __setflm (u.d);

    u.d = __readflm();
    printb (u.i[1]);

    switch (i) {
      case 0:
        y = test_inf(x);
        break;
      case 1:
        y = test_nan1(x);
        break;
      case 3:
        y = test_nan2(x);
        break;
      case 4:
        y = test_nan3(x);
        break;
    }

    u.d = __readflm();
    printb (u.i[1]);

    printd (y);
  }
}
$ cat testfuncs.c
double test_inf(double x) {
  return 1.0/0.0;
}
double test_nan1(double x) {
  return 0.0/0.0;
}
double test_nan2(double x) {
  double inf = 1.0/0.0;
  return inf - inf;
}
double test_nan3(double x) {
  double inf = 1.0/0.0;
  return (inf+x) - inf;
}
Actual Output:
+ xlc -O2 -qfloat=fenv -c testmain.c -qfloat=fenv
+ xlc -O2 -qfloat=fenv testfuncs.c testmain.o
"testfuncs.c", line 2.14: 1506-232 (I) Divisor for modulus or
division operator cannot be zero.
"testfuncs.c", line 6.14: 1506-232 (I) Divisor for modulus or
division operator cannot be zero.
"testfuncs.c", line 10.20: 1506-232 (I) Divisor for modulus or
division operator cannot be zero.
"testfuncs.c", line 15.20: 1506-232 (I) Divisor for modulus or
division operator cannot be zero.
+ ./a.out
fpscr=0000 0000 0000 0000 0000 0000 0000 0000
fpscr=0000 0000 0000 0000 0000 0000 0000 0000
retval=INF
fpscr=0000 0000 0000 0000 0000 0000 0000 0000
fpscr=0000 0000 0000 0000 0000 0000 0000 0000
retval=NaNQ
fpscr=0000 0000 0000 0000 0000 0000 0000 0000
fpscr=0000 0000 0000 0000 0000 0000 0000 0000
retval=NaNQ
fpscr=0000 0000 0000 0000 0000 0000 0000 0000
fpscr=0000 0000 0000 0000 0000 0000 0000 0000
retval=NaNQ
Expected Output:
+ xlc -O2 -qfloat=fenv -c testmain.c -qfloat=fenv
+ xlc -O2 -qfloat=fenv testfuncs.c testmain.o
"testfuncs.c", line 2.14: 1506-232 (I) Divisor for modulus or
division operator cannot be zero.
"testfuncs.c", line 6.14: 1506-232 (I) Divisor for modulus or
division operator cannot be zero.
"testfuncs.c", line 10.20: 1506-232 (I) Divisor for modulus or
division operator cannot be zero.
"testfuncs.c", line 15.20: 1506-232 (I) Divisor for modulus or
division operator cannot be zero.
+ ./a.out
fpscr=0000 0000 0000 0000 0000 0000 0000 0000
fpscr=1000 0100 0000 0000 0101 0000 0000 0000
retval=INF
fpscr=0000 0000 0000 0000 0000 0000 0000 0000
fpscr=1010 0000 0010 0001 0001 0000 0000 0000
retval=NaNQ
fpscr=0000 0000 0000 0000 0000 0000 0000 0000
fpscr=0000 0000 0000 0000 0000 0000 0000 0000
retval=NaNQ
fpscr=0000 0000 0000 0000 0000 0000 0000 0000
fpscr=1010 0100 1000 0001 0001 0000 0000 0000
retval=NaNQ

Local fix

```
Use -qfloat=nofold
```

Problem summary

PROBLEM DESCRIPTION: Expressions that may set FPSCR bits may be
folded away under -qfloat=fenv

USERS AFFECTED: Those who require the value of the FPSCR to be
faithfully updated upon evaluation of floating-point
expressions at -O2 and up.

Problem conclusion

Fixed compile-time floating point folding to respect
-qfloat=fenv at -O2 and up, and preserve expressions that would
modify the FPSCR. Apply provided service.

Temporary fix

Comments

APAR Information

APAR number
IV36069
Reported component name
XL C/C++ AIX
Reported component ID
5724X1300
Reported release
B10
Status
CLOSED PER
PE
NoPE
HIPER
NoHIPER
Special Attention
NoSpecatt
Submitted date
2013-01-29
Closed date
2013-08-13
Last modified date
2013-08-13

APAR is sysrouted FROM one or more of the following:
APAR is sysrouted TO one or more of the following:

IV49627 LI77652

Fix information

Fixed component name
XL C FOR AIX
Fixed component ID
5724X1200

Applicable component levels

[{"Business Unit":{"code":"BU058","label":"IBM Infrastructure w\/TPS"},"Product":{"code":"SSGH2K","label":"XL C for AIX"},"Component":"","ARM Category":[],"Platform":[{"code":"PF025","label":"Platform Independent"}],"Version":"11.1","Edition":"","Line of Business":{"code":"LOB57","label":"Power"}}]

Document Information

Modified date:
13 August 2013

Tips

IV36069: Exception bit folded incorrectly with -qfloat=fenv -O2

Fixes are available

Subscribe

APAR status

Closed as program error.

Error description

Local fix

Problem summary

Problem conclusion

Temporary fix

Comments

APAR Information

APAR number

Reported component name

Reported component ID

Reported release

Status

PE

HIPER

Special Attention

Submitted date

Closed date

Last modified date

APAR is sysrouted FROM one or more of the following:

APAR is sysrouted TO one or more of the following:

Fix information

Fixed component name

Fixed component ID

Applicable component levels

Document Information

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