[erlang-bugs] math:exp() erlang bug on SPARC

[Matt Keenan]

Mikael,

This is great, I'll attempt applying your code changes below on a local 
Solaris build of Erlang and verify if it fixes the issue for me.

thanks

Matt

On 16/04/2016 20:12, Mikael Pettersson wrote:
> Matt Keenan writes:
>   > Hi,
>   >
>   > Just wanted to report a SPARC specific issue with Erlang.
>   >
>   > See discussions:
>   >    https://github.com/rabbitmq/rabbitmq-server/issues/132
>   >    https://groups.google.com/forum/#!topic/rabbitmq-users/Gca8vW52gB8
>   >
>   > Only happens on SPARC systems does not occur on X86 architecture.
>   > e.g.:
>   >
>   > On s11u3 x86:
>   > 1> math:exp(-1162.102134881488).
>   > 0.0
>   >
>   > On s11u3 SPARC:
>   > 1> math:exp(-1162.102134881488).
>   > ** exception error: an error occurred when evaluating an arithmetic
>   > expression in function math:exp/1 called as math:exp(-1162.102134881488)
>   >
>   > Both systems are running 64-bit Erlang 17.5 in a 64-bit environment.
>   >
>   > Has an issue related to this been posted before ?
>
> After reproducing this on Solaris 10 / SPARC I recalled:
> http://erlang.org/pipermail/erlang-bugs/2014-December/004731.html
>
> First, this call to exp(-1162.1...) goes straight to libm's exp(), which
> detects an underflow and returns an error on both Solaris (SPARC) and Linux
> (x86_64, sparc64, ARMv7) [all glibc].  In this case, exp() returns 0,
> sets errno, and sets flag bits testable by fetestexcept().
>
> On Solaris, libm also calls matherr() as per ancient SVID, and ERTS treats
> that as an implicit FP exception.
>
> After exp() returns, ERTS checks (1) if the return value is !isfinite()
> [but here it's a finite 0], or (2) if the thread's FP exception flag is set.
> On Solaris it's set due to the matherr() call, so the call is treated as
> an error.  On Linux we see a finite return value, don't check the error
> indicators, and go on as if no error occurred.  (That may be correct for
> underflow errors, I don't know.)
>
> The older email thread was about a similar issue with math:pow/2.
>
> Our treatment of matherr() is completely wrong.  With FP exceptions disabled,
> we derive no information from matherr() calls, so all error checking is
> expressed in the code around those libm calls.  For libm calls, checking for
> real FP exceptions is needed to reset parts of the FPU controls, but that
> isn't needed for the fake FP exceptions derived from matherr() calls.
>
> In short, I think matherr() should be a no-op regardless of NO_FPE_SIGNALS.
>
> The path below should fix your issue on Solaris.  I'll issue a PR later.
>
> /Mikael
>
> diff --git a/erts/emulator/sys/ose/sys_float.c b/erts/emulator/sys/ose/sys_float.c
> index 5187579..764c35b 100644
> --- a/erts/emulator/sys/ose/sys_float.c
> +++ b/erts/emulator/sys/ose/sys_float.c
> @@ -836,10 +836,5 @@ sys_chars_to_double(char* buf, double* fp)
>   int
>   matherr(struct exception *exc)
>   {
> -#if !defined(NO_FPE_SIGNALS)
> -    volatile unsigned long *fpexnp = erts_get_current_fp_exception();
> -    if (fpexnp != NULL)
> -	*fpexnp = (unsigned long)__builtin_return_address(0);
> -#endif
>       return 1;
>   }
> diff --git a/erts/emulator/sys/unix/sys_float.c b/erts/emulator/sys/unix/sys_float.c
> index 8fe7e59..beb5cb5 100644
> --- a/erts/emulator/sys/unix/sys_float.c
> +++ b/erts/emulator/sys/unix/sys_float.c
> @@ -838,11 +838,6 @@ sys_chars_to_double(char* buf, double* fp)
>   int
>   matherr(struct exception *exc)
>   {
> -#if !defined(NO_FPE_SIGNALS)
> -    volatile unsigned long *fpexnp = erts_get_current_fp_exception();
> -    if (fpexnp != NULL)
> -	*fpexnp = (unsigned long)__builtin_return_address(0);
> -#endif
>       return 1;
>   }
>
> diff --git a/erts/emulator/sys/win32/sys_float.c b/erts/emulator/sys/win32/sys_float.c
> index 86e822d..14d4fa0 100644
> --- a/erts/emulator/sys/win32/sys_float.c
> +++ b/erts/emulator/sys/win32/sys_float.c
> @@ -139,8 +139,7 @@ sys_double_to_chars_ext(double fp, char *buffer, size_t buffer_size, size_t deci
>   int
>   matherr(struct _exception *exc)
>   {
> -    erl_fp_exception = 1;
> -    DEBUGF(("FP exception (matherr) (0x%x) (%d)\n", exc->type, erl_fp_exception));
> +    DEBUGF(("FP exception (matherr) (0x%x)\n", exc->type));
>       return 1;
>   }
>
>