* feraiseexcept does not raise traps in Cygwin @ 2023-10-25 9:21 Bruno Haible 2023-10-25 22:39 ` feraiseexcept does not raise traps in newlib on x86/_64 [was Cygwin] Brian Inglis 2023-10-31 12:33 ` feraiseexcept does not raise traps in Cygwin Corinna Vinschen 0 siblings, 2 replies; 3+ messages in thread From: Bruno Haible @ 2023-10-25 9:21 UTC (permalink / raw) To: cygwin Hi, Seen on - Cygwin 3.4.6 or 2.9.0 on x86_64. - Cygwin 2.9.0 on i386. According to ISO C 23 § 7.6.4.3 "The feraiseexcept function attempts to raise the supported floating-point exceptions represented by its argument. 266) Footnote 266) The effect is intended to be similar to that of floating-point exceptions raised by arithmetic operations. Hence, implementation extensions associated with raising a floating-point exception (for example, enabled traps or IEC 60559 alternate exception handling) should be honored." This does not work. How to reproduce: =================================== foo.c =================================== #define _GNU_SOURCE 1 #include <fenv.h> #include <assert.h> int main () { /* Clear FE_INVALID exceptions from past operations. */ if (feclearexcept (FE_INVALID)) return 1; /* An FE_INVALID exception shall trigger a SIGFPE signal, which by default terminates the program. */ if (feenableexcept (FE_INVALID) == -1) return 2; if (feraiseexcept (FE_INVALID)) return 3; return 0; } ============================================================================= For x86_64: $ x86_64-pc-cygwin-gcc -Wall foo.c $ ./a.exe; echo $? For i386: $ i686-pc-cygwin-gcc -Wall foo.c $ ./a.exe; echo $? Expected result (like seen e.g. on Linux/glibc): Floating-point exception (core dumped) 136 Actual result: 0 The workaround for x86_64 is to redefine feeraiseexcept in the same way as glibc does. This modified test program includes the workaround: =================================== foo.c =================================== #define _GNU_SOURCE 1 #include <fenv.h> #include <assert.h> /* The floating-point environment of the 387 unit. */ typedef struct { /* 7 32-bit words: */ unsigned short __control_word; /* fctrl register */ unsigned short __reserved1; unsigned short __status_word; /* fstat register */ unsigned short __reserved2; unsigned int more[5]; } x86_387_fenv_t; int feraiseexcept (int exceptions) { exceptions &= FE_ALL_EXCEPT; if ((exceptions & ~(FE_INVALID | FE_DIVBYZERO)) == 0 && 0) { /* First: invalid exception. */ if (exceptions & FE_INVALID) { double volatile a; double volatile b; a = 0; b = a / a; (void) b; } /* Next: division by zero. */ if (exceptions & FE_DIVBYZERO) { double volatile a, b; double volatile c; a = 1; b = 0; c = a / b; (void) c; } } else { /* The general case. */ /* Set the bits in the 387 unit. */ x86_387_fenv_t env; unsigned short orig_status_word; __asm__ __volatile__ ("fnstenv %0" : "=m" (*&env)); orig_status_word = env.__status_word; env.__status_word |= exceptions; if (env.__status_word != orig_status_word) { __asm__ __volatile__ ("fldenv %0" : : "m" (*&env)); /* A trap (if enabled) is triggered only at the next floating-point instruction. Force it to occur here. */ __asm__ __volatile__ ("fwait"); } } return 0; } int main () { /* Clear FE_INVALID exceptions from past operations. */ if (feclearexcept (FE_INVALID)) return 1; /* An FE_INVALID exception shall trigger a SIGFPE signal, which by default terminates the program. */ if (feenableexcept (FE_INVALID) == -1) return 2; if (feraiseexcept (FE_INVALID)) return 3; return 0; } ============================================================================= (This workaround *should* also work on i386, but it doesn't. I don't know why.) Bruno ^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: feraiseexcept does not raise traps in newlib on x86/_64 [was Cygwin] 2023-10-25 9:21 feraiseexcept does not raise traps in Cygwin Bruno Haible @ 2023-10-25 22:39 ` Brian Inglis 2023-10-31 12:33 ` feraiseexcept does not raise traps in Cygwin Corinna Vinschen 1 sibling, 0 replies; 3+ messages in thread From: Brian Inglis @ 2023-10-25 22:39 UTC (permalink / raw) To: cygwin, newlib; +Cc: Bruno Haible On 2023-10-25 03:21, Bruno Haible via Cygwin wrote: > Hi, > > Seen on > - Cygwin 3.4.6 or 2.9.0 on x86_64. > - Cygwin 2.9.0 on i386. > > According to ISO C 23 § 7.6.4.3 > "The feraiseexcept function attempts to raise the supported floating-point > exceptions represented by its argument. 266) > Footnote 266) The effect is intended to be similar to that of floating-point > exceptions raised by arithmetic operations. Hence, implementation extensions > associated with raising a floating-point exception (for example, enabled > traps or IEC 60559 alternate exception handling) should be honored." > > This does not work. How to reproduce: > =================================== foo.c =================================== > #define _GNU_SOURCE 1 > #include <fenv.h> > #include <assert.h> > > int > main () > { > /* Clear FE_INVALID exceptions from past operations. */ > if (feclearexcept (FE_INVALID)) > return 1; > > /* An FE_INVALID exception shall trigger a SIGFPE signal, which by default > terminates the program. */ > if (feenableexcept (FE_INVALID) == -1) > return 2; > > if (feraiseexcept (FE_INVALID)) > return 3; > > return 0; > } > ============================================================================= > > For x86_64: > > $ x86_64-pc-cygwin-gcc -Wall foo.c > $ ./a.exe; echo $? > > For i386: > > $ i686-pc-cygwin-gcc -Wall foo.c > $ ./a.exe; echo $? > > Expected result (like seen e.g. on Linux/glibc): > Floating-point exception (core dumped) > 136 > > Actual result: > 0 > > The workaround for x86_64 is to redefine feeraiseexcept in the same way > as glibc does. This modified test program includes the workaround: > > =================================== foo.c =================================== > #define _GNU_SOURCE 1 > #include <fenv.h> > #include <assert.h> #ifdef REDEFFE > /* The floating-point environment of the 387 unit. */ > typedef struct > { > /* 7 32-bit words: */ > unsigned short __control_word; /* fctrl register */ > unsigned short __reserved1; > unsigned short __status_word; /* fstat register */ > unsigned short __reserved2; > unsigned int more[5]; > } > x86_387_fenv_t; > > int > feraiseexcept (int exceptions) > { > exceptions &= FE_ALL_EXCEPT; > if ((exceptions & ~(FE_INVALID | FE_DIVBYZERO)) == 0 && 0) > { > /* First: invalid exception. */ > if (exceptions & FE_INVALID) > { > double volatile a; > double volatile b; > a = 0; b = a / a; > (void) b; > } > /* Next: division by zero. */ > if (exceptions & FE_DIVBYZERO) > { > double volatile a, b; > double volatile c; > a = 1; b = 0; c = a / b; > (void) c; > } > } > else > { > /* The general case. */ > > /* Set the bits in the 387 unit. */ > x86_387_fenv_t env; > unsigned short orig_status_word; > __asm__ __volatile__ ("fnstenv %0" : "=m" (*&env)); > orig_status_word = env.__status_word; > env.__status_word |= exceptions; > if (env.__status_word != orig_status_word) > { > __asm__ __volatile__ ("fldenv %0" : : "m" (*&env)); > /* A trap (if enabled) is triggered only at the next floating-point > instruction. Force it to occur here. */ > __asm__ __volatile__ ("fwait"); > } > } > return 0; > } #endif /* REDEFFE */ > int > main () > { > /* Clear FE_INVALID exceptions from past operations. */ > if (feclearexcept (FE_INVALID)) > return 1; > > /* An FE_INVALID exception shall trigger a SIGFPE signal, which by default > terminates the program. */ > if (feenableexcept (FE_INVALID) == -1) > return 2; > > if (feraiseexcept (FE_INVALID)) > return 3; > > return 0; > } > ============================================================================= > > (This workaround *should* also work on i386, but it doesn't. I don't know > why.) Thanks for the report Bruno, Confirmed on Cygwin current stable with #ifdef added for ease of rebuild: $ uname -srvmo CYGWIN_NT-10.0-19045 3.4.9-1.x86_64 2023-09-06 11:19 UTC x86_64 Cygwin $ gcc -UREDEFFE -o feraiseexcept{,.c} && ./feraiseexcept; echo $? 0 $ gcc -DREDEFFE -o feraiseexcept{,.c} && ./feraiseexcept; echo $? Floating point exception (core dumped) 136 Cygwin no longer supports x86/i686 32 bit. Anyone have a Cygwin 3.5.0 test release environment available, just in case it might be different in some way, and could confirm? The code is in newlib, so redirecting there: https://sourceware.org/cgit/newlib-cygwin/tree/newlib/libm/machine/shared_x86/fenv.c#n253 That only has similar common code as in glibc, not the specific exceptions. Please, could anyone test on a non-Cygwin newlib x86/_64 build? The BSDs store, add exceptions, then load first the X87 then SSE MXCSR environments: https://cgit.freebsd.org/src/tree/lib/msun/amd64/fenv.c#n32 https://github.com/NetBSD/src/blob/trunk/lib/libm/arch/x86_64/fenv.c#L195 https://github.com/openbsd/src/blob/master/lib/libm/arch/amd64/fenv.c#L116 which is handled in newlib fenv.c functions by fgetenv/fsetenv *except* in feraiseexcept! -- Take care. Thanks, Brian Inglis Calgary, Alberta, Canada La perfection est atteinte Perfection is achieved non pas lorsqu'il n'y a plus rien à ajouter not when there is no more to add mais lorsqu'il n'y a plus rien à retirer but when there is no more to cut -- Antoine de Saint-Exupéry ^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: feraiseexcept does not raise traps in Cygwin 2023-10-25 9:21 feraiseexcept does not raise traps in Cygwin Bruno Haible 2023-10-25 22:39 ` feraiseexcept does not raise traps in newlib on x86/_64 [was Cygwin] Brian Inglis @ 2023-10-31 12:33 ` Corinna Vinschen 1 sibling, 0 replies; 3+ messages in thread From: Corinna Vinschen @ 2023-10-31 12:33 UTC (permalink / raw) To: Bruno Haible; +Cc: cygwin Hi Bruno, On Oct 25 11:21, Bruno Haible via Cygwin wrote: > Hi, > > Seen on > - Cygwin 3.4.6 or 2.9.0 on x86_64. > - Cygwin 2.9.0 on i386. > > According to ISO C 23 § 7.6.4.3 > "The feraiseexcept function attempts to raise the supported floating-point > exceptions represented by its argument. 266) > Footnote 266) The effect is intended to be similar to that of floating-point > exceptions raised by arithmetic operations. Hence, implementation extensions > associated with raising a floating-point exception (for example, enabled > traps or IEC 60559 alternate exception handling) should be honored." > > This does not work. How to reproduce: > [...] Thanks for the report and the simple testcase. I checked in a patch. It will be in the test build cygwin-3.5.0-0.450.g2ef627c46eb2 in an hour or two, if you want to give it a try. Thanks again, Corinna ^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2023-10-31 12:34 UTC | newest] Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2023-10-25 9:21 feraiseexcept does not raise traps in Cygwin Bruno Haible 2023-10-25 22:39 ` feraiseexcept does not raise traps in newlib on x86/_64 [was Cygwin] Brian Inglis 2023-10-31 12:33 ` feraiseexcept does not raise traps in Cygwin Corinna Vinschen
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