optimization/6065: gcc seems to reorder IEEE fpsetsticky and sqrt functions

[Daniel Villeneuve <Daniel.Villeneuve@gerad.ca>]

>Number:         6065
>Category:       optimization
>Synopsis:       gcc seems to reorder IEEE fpsetsticky and sqrt functions
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Tue Mar 26 09:46:01 PST 2002
>Closed-Date:
>Last-Modified:
>Originator:     Daniel Villeneuve
>Release:        3.0.3
>Organization:
>Environment:
System: SunOS mckinley 5.7 Generic_106541-05 sun4u sparc SUNW,Ultra-5_10
Architecture: sun4

	
host: sparc-sun-solaris2.7
build: sparc-sun-solaris2.7
target: sparc-sun-solaris2.7
configured with: ./configure --disable-shared --prefix=/home/sparc-sun-solaris2/gcc-3.0.3
>Description:

I try to access the ``inexact'' status flag after performing a call to
sqrt(), and the result is wrong for sqrt(2.0) (i.e., the inexact flags is
not set) when compiling with gcc -O3.

The program needs two arguments: a double whose square root is to be
computed, and an initial value for the inexact flag (which can be seen
to complete the floating-point value).

Test that fails:

t_sqrt 2 0

which outputs

res = 1.41421, inexact flag = 0

instead of

res = 1.41421, inexact flag = 1



>How-To-Repeat:

======================================== <Makefile>

t_sqrt: t_sqrt.o
	gcc -v -save-temps -ansi -Wall -pedantic -Wshadow -Wbad-function-cast -Wsign-compare -Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations -Wnested-externs -Winline -Wpointer-arith -W -Wwrite-strings -O3 -finline-functions -funroll-loops -Wl,-s -Wl,-R/usr/openwin/lib:/usr/dt/lib:/home/ultra-sun-solaris2/gcc-3.0.3/lib -o t_sqrt t_sqrt.o -lm

t_sqrt.o: t_sqrt.c
	gcc -v -save-temps -ansi -Wall -pedantic -Wshadow -Wbad-function-cast -Wsign-compare -Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations -Wnested-externs -Winline -Wpointer-arith -W -Wwrite-strings -O3 -finline-functions -funroll-loops -o t_sqrt.o -c t_sqrt.c

======================================== </Makefile>

======================================== <CODE>

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

#undef NDEBUG
#include <assert.h>

typedef struct S
{
  double d;
  int i;
} S;

static S *psSqrt(S *ps)
{
#ifdef VOLATILE
  double        volatile d;
#else
  double        d;
#endif
  int           i;

  d = ps->d;
  if( !d ) return ps;
  assert( d > 0 );
  i = ps->i;
  fpsetsticky(0);
  d = sqrt(d);
  if( fpgetsticky() & FP_X_IMP ) i = 1;
  ps = malloc(sizeof(S));
  ps->d = d;
  ps->i = i;
  return ps;
}

int main(int argc, char **argv)
{
  S             psArg[1], *psRes;

  assert(argc == 3);
  psArg->d = atof(argv[1]);
  psArg->i = atoi(argv[2]);

  psRes = psSqrt(psArg);

  printf("res = %g, inexact flag = %d\n", psRes->d, psRes->i);
  return 0;
}

======================================== </CODE>

======================================== <Makefile OUTPUT>

gcc -v -save-temps -ansi -Wall -pedantic -Wshadow -Wbad-function-cast -Wsign-compare -Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations -Wnested-externs -Winline -Wpointer-arith -W -Wwrite-strings -O3 -finline-functions -funroll-loops -o t_sqrt.o -c t_sqrt.c
Reading specs from /home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/specs
Configured with: ./configure --disable-shared --prefix=/home/sparc-sun-solaris2/gcc-3.0.3
Thread model: posix
gcc version 3.0.3
 /home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/cpp0 -lang-c -std=c89 -v -iprefix /home/gencol/ref/gcm-3.0/bin/ultra-sun-solaris2/../lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/ -D__GNUC__=3 -D__GNUC_MINOR__=0 -D__GNUC_PATCHLEVEL__=3 -D__sparc__ -D__sun__ -D__unix__ -D__svr4__ -D__SVR4 -D__sparc -D__sun -D__unix -Asystem=unix -Asystem=svr4 -D__OPTIMIZE__ -D__STDC_HOSTED__=1 -Wall -Wshadow -Wbad-function-cast -Wsign-compare -Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations -Wnested-externs -Winline -Wpointer-arith -W -Wwrite-strings -pedantic -D__GCC_NEW_VARARGS__ -Acpu=sparc -Amachine=sparc t_sqrt.c t_sqrt.i
GNU CPP version 3.0.3 (cpplib) (sparc)
ignoring nonexistent directory "/home/gencol/ref/gcm-3.0/bin/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include"
ignoring nonexistent directory "/home/gencol/ref/gcm-3.0/bin/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/../../../../sparc-sun-solaris2.7/include"
ignoring nonexistent directory "/usr/local/include"
ignoring nonexistent directory "/home/sparc-sun-solaris2/gcc-3.0.3/sparc-sun-solaris2.7/include"
#include "..." search starts here:
#include <...> search starts here:
 /home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include
 /usr/include
End of search list.
 /home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/cc1 -fpreprocessed t_sqrt.i -quiet -dumpbase t_sqrt.c -ansi -O3 -Wall -Wshadow -Wbad-function-cast -Wsign-compare -Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations -Wnested-externs -Winline -Wpointer-arith -W -Wwrite-strings -pedantic -ansi -version -finline-functions -funroll-loops -o t_sqrt.s
GNU CPP version 3.0.3 (cpplib) (sparc)
GNU C version 3.0.3 (sparc-sun-solaris2.7)
	compiled by GNU C version 3.0.3.
 /usr/ccs/bin/as -V -Qy -s -o t_sqrt.o t_sqrt.s
/usr/ccs/bin/as: WorkShop Compilers 5.0 98/12/21
gcc -v -save-temps -ansi -Wall -pedantic -Wshadow -Wbad-function-cast -Wsign-compare -Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations -Wnested-externs -Winline -Wpointer-arith -W -Wwrite-strings -O3 -finline-functions -funroll-loops -Wl,-s -Wl,-R/usr/openwin/lib:/usr/dt/lib:/home/ultra-sun-solaris2/gcc-3.0.3/lib -o t_sqrt t_sqrt.o -lm
Reading specs from /home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/specs
Configured with: ./configure --disable-shared --prefix=/home/sparc-sun-solaris2/gcc-3.0.3
Thread model: posix
gcc version 3.0.3
 /home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/collect2 -V -Y P,/usr/ccs/lib:/usr/lib -Qy -o t_sqrt /home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/crt1.o /home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/crti.o /usr/ccs/lib/values-Xc.o /home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/crtbegin.o -L/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3 -L/usr/ccs/bin -L/usr/ccs/lib -L/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/../../.. -s -R/usr/openwin/lib:/usr/dt/lib:/home/ultra-sun-solaris2/gcc-3.0.3/lib t_sqrt.o -lm -lgcc -lc -lgcc /home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/crtend.o /home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/crtn.o
ld: Software Generation Utilities - Solaris/ELF (3.0)

======================================== </Makefile OUTPUT>

======================================== <CPP>

# 1 "t_sqrt.c"
# 1 "/usr/include/ieeefp.h" 1 3
# 19 "/usr/include/ieeefp.h" 3
#pragma ident "@(#)ieeefp.h	1.15	97/11/22 SMI"
# 64 "/usr/include/ieeefp.h" 3
typedef enum fpclass_t {
        FP_SNAN = 0,
        FP_QNAN = 1,
        FP_NINF = 2,
        FP_PINF = 3,
        FP_NDENORM = 4,
        FP_PDENORM = 5,
        FP_NZERO = 6,
        FP_PZERO = 7,
        FP_NNORM = 8,
        FP_PNORM = 9
} fpclass_t;


extern fpclass_t fpclass(double);
extern int finite(double);
extern int unordered(double, double);
# 113 "/usr/include/ieeefp.h" 3
typedef enum fp_rnd {
        FP_RN = 0,
        FP_RZ = 1,
        FP_RP = 2,
        FP_RM = 3
} fp_rnd;




extern fp_rnd fpsetround(fp_rnd);
extern fp_rnd fpgetround(void);
# 191 "/usr/include/ieeefp.h" 3
extern int fpgetmask(void);
extern int fpsetmask(int);
extern int fpgetsticky(void);
extern int fpsetsticky(int);
# 209 "/usr/include/ieeefp.h" 3
extern int isnanf(float);
extern int isnand(double);
# 2 "t_sqrt.c" 2
# 1 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/math.h" 1 3
# 27 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/math.h" 3
#pragma ident "@(#)math.h	2.7	98/01/27"
# 46 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/math.h" 3
typedef union _h_val {
        unsigned long _i[sizeof(double) / sizeof(unsigned long)];
        double _d;
} _h_val;


extern const _h_val __huge_val;
# 134 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/math.h" 3
extern double acos (double);
extern double asin (double);
extern double atan (double);
extern double atan2 (double, double);
extern double cos (double);
extern double sin (double);
extern double tan (double);

extern double cosh (double);
extern double sinh (double);
extern double tanh (double);

extern double exp (double);
extern double frexp (double, int *);
extern double ldexp (double, int);
extern double log (double);
extern double log10 (double);
extern double modf (double, double *);

extern double pow (double, double);
extern double sqrt (double);

extern double ceil (double);
extern double fabs (double);
extern double floor (double);
extern double fmod (double, double);
# 3 "t_sqrt.c" 2
# 1 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdio.h" 1 3
# 14 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdio.h" 3
# 1 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdarg.h" 1 3
# 43 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdarg.h" 3
typedef __builtin_va_list __gnuc_va_list;
# 15 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdio.h" 2 3
# 34 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdio.h" 3
#pragma ident "@(#)stdio.h	1.69	98/07/13 SMI"

# 1 "/usr/include/sys/feature_tests.h" 1 3
# 13 "/usr/include/sys/feature_tests.h" 3
#pragma ident "@(#)feature_tests.h	1.17	97/12/04 SMI"

# 1 "/usr/include/sys/isa_defs.h" 1 3
# 9 "/usr/include/sys/isa_defs.h" 3
#pragma ident "@(#)isa_defs.h	1.15	97/11/22 SMI"
# 16 "/usr/include/sys/feature_tests.h" 2 3
# 37 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdio.h" 2 3
# 1 "/usr/include/sys/va_list.h" 1 3
# 9 "/usr/include/sys/va_list.h" 3
#pragma ident "@(#)va_list.h	1.11	97/11/22 SMI"
# 26 "/usr/include/sys/va_list.h" 3
typedef void *__va_list;
# 38 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdio.h" 2 3
# 1 "/usr/include/stdio_tag.h" 1 3
# 9 "/usr/include/stdio_tag.h" 3
#pragma ident "@(#)stdio_tag.h	1.3	98/04/20 SMI"
# 21 "/usr/include/stdio_tag.h" 3
typedef struct __FILE __FILE;
# 39 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdio.h" 2 3
# 1 "/usr/include/stdio_impl.h" 1 3
# 9 "/usr/include/stdio_impl.h" 3
#pragma ident "@(#)stdio_impl.h	1.7	98/04/17 SMI"

# 1 "/usr/include/sys/isa_defs.h" 1 3
# 12 "/usr/include/stdio_impl.h" 2 3
# 22 "/usr/include/stdio_impl.h" 3
typedef int ssize_t;
# 38 "/usr/include/stdio_impl.h" 3
struct __FILE
{




        ssize_t _cnt;
        unsigned char *_ptr;

        unsigned char *_base;
        unsigned char _flag;
        unsigned char _file;
        unsigned __orientation:2;
        unsigned __filler:6;
};
# 40 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdio.h" 2 3
# 55 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdio.h" 3
typedef __FILE FILE;







typedef unsigned int size_t;
# 103 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdio.h" 3
typedef long fpos_t;
# 203 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdio.h" 3
extern FILE __iob[20];
# 215 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdio.h" 3
extern unsigned char _sibuf[], _sobuf[];
# 244 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdio.h" 3
extern unsigned char *_bufendtab[];
extern FILE *_lastbuf;
# 275 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdio.h" 3
extern int remove(const char *);
extern int rename(const char *, const char *);
extern FILE *tmpfile(void);
extern char *tmpnam(char *);



extern int fclose(FILE *);
extern int fflush(FILE *);
extern FILE *fopen(const char *, const char *);
extern FILE *freopen(const char *, const char *, FILE *);
extern void setbuf(FILE *, char *);





extern int setvbuf(FILE *, char *, int, size_t);

extern int fprintf(FILE *, const char *, ...);

extern int fscanf(FILE *, const char *, ...);

extern int printf(const char *, ...);

extern int scanf(const char *, ...);







extern int sprintf(char *, const char *, ...);

extern int sscanf(const char *, const char *, ...);
extern int vfprintf(FILE *, const char *, __gnuc_va_list);
extern int vprintf(const char *, __gnuc_va_list);





extern int vsprintf(char *, const char *, __gnuc_va_list);
extern int fgetc(FILE *);
extern char *fgets(char *, int, FILE *);
extern int fputc(int, FILE *);
extern int fputs(const char *, FILE *);
extern int getc(FILE *);
extern int getchar(void);
extern char *gets(char *);
extern int putc(int, FILE *);
extern int putchar(int);
extern int puts(const char *);
extern int ungetc(int, FILE *);
extern size_t fread(void *, size_t, size_t, FILE *);
extern size_t fwrite(const void *, size_t, size_t, FILE *);
extern int fgetpos(FILE *, fpos_t *);
extern int fseek(FILE *, long, int);
extern int fsetpos(FILE *, const fpos_t *);
extern long ftell(FILE *);
extern void rewind(FILE *);
extern void clearerr(FILE *);
extern int feof(FILE *);
extern int ferror(FILE *);
extern void perror(const char *);


extern int __filbuf(FILE *);
extern int __flsbuf(int, FILE *);
# 4 "t_sqrt.c" 2
# 1 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdlib.h" 1 3
# 25 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdlib.h" 3
#pragma ident "@(#)stdlib.h	1.44	98/01/22 SMI"

# 1 "/usr/include/sys/feature_tests.h" 1 3
# 28 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdlib.h" 2 3
# 38 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdlib.h" 3
typedef struct {
        int quot;
        int rem;
} div_t;

typedef struct {
        long quot;
        long rem;
} ldiv_t;
# 77 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdlib.h" 3
typedef long uid_t;
# 107 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdlib.h" 3
typedef long int wchar_t;
# 148 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdlib.h" 3
extern unsigned char __ctype[];



extern double atof(const char *);
extern int atoi(const char *);
extern long int atol(const char *);
extern double strtod(const char *, char **);
extern long int strtol(const char *, char **, int);
extern unsigned long int strtoul(const char *, char **, int);

extern int rand(void);
extern void srand(unsigned int);





extern void *calloc(size_t, size_t);
extern void free(void *);
extern void *malloc(size_t);
extern void *realloc(void *, size_t);

extern void abort(void);
extern int atexit(void (*)(void));
extern void exit(int);
extern void _exithandle(void);
extern char *getenv(const char *);
extern int system(const char *);

extern void *bsearch(const void *, const void *, size_t, size_t,
        int (*)(const void *, const void *));
extern void qsort(void *, size_t, size_t,
        int (*)(const void *, const void *));

extern int abs(int);
extern div_t div(int, int);
extern long int labs(long);
extern ldiv_t ldiv(long, long);

extern int mbtowc(wchar_t *, const char *, size_t);
extern int mblen(const char *, size_t);
extern int wctomb(char *, wchar_t);

extern size_t mbstowcs(wchar_t *, const char *, size_t);
extern size_t wcstombs(char *, const wchar_t *, size_t);
# 219 "/home/sparc-sun-solaris2/gcc-3.0.3/lib/gcc-lib/sparc-sun-solaris2.7/3.0.3/include/stdlib.h" 3
extern int mkstemp(char *);
# 5 "t_sqrt.c" 2


# 1 "/usr/include/assert.h" 1 3
# 11 "/usr/include/assert.h" 3
#pragma ident "@(#)assert.h	1.9	92/07/14 SMI"






extern void __assert(const char *, const char *, int);
# 8 "t_sqrt.c" 2

typedef struct S
{
  double d;
  int i;
} S;

static S *psSqrt(S *ps)
{



  double d;

  int i;

  d = ps->d;
  if( !d ) return ps;
  (void)((d > 0) || (__assert("d > 0", "t_sqrt.c", 26), 0));
  i = ps->i;
  fpsetsticky(0);
  d = sqrt(d);
  if( fpgetsticky() & 0x01 ) i = 1;
  ps = malloc(sizeof(S));
  ps->d = d;
  ps->i = i;
  return ps;
}

int main(int argc, char **argv)
{
  S psArg[1], *psRes;

  (void)((argc == 3) || (__assert("argc == 3", "t_sqrt.c", 41), 0));
  psArg->d = atof(argv[1]);
  psArg->i = atoi(argv[2]);

  psRes = psSqrt(psArg);

  printf("res = %g, inexact flag = %d\n", psRes->d, psRes->i);
  return 0;
}

======================================== </CPP>

>Fix:

Declare as volatile all ``double'' variables used in computations that
might affect IEEE status flags (when we want to access those flags, of
course).  I am not sure this is always correct: it happened to work on my
restricted test case.

I am aware that C99 provides the FENV_ACCESS pragma and
the fe(clear|test)except functions, but they are still
not widely available.

Is there a better way (compiler options, pragmas, attributes) than using
`volatile' to tell gcc not to `over'-optimize the code in such specific
circumstances?
>Release-Note:
>Audit-Trail:
>Unformatted: