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From: "jsm28 at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/59666] New: IBM long double arithmetic results invalid in non-default rounding modes Date: Fri, 03 Jan 2014 16:19:00 -0000 [thread overview] Message-ID: <bug-59666-4@http.gcc.gnu.org/bugzilla/> (raw) http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59666 Bug ID: 59666 Summary: IBM long double arithmetic results invalid in non-default rounding modes Product: gcc Version: 4.9.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target Assignee: unassigned at gcc dot gnu.org Reporter: jsm28 at gcc dot gnu.org CC: dje at gcc dot gnu.org Target: powerpc*-*-linux* The IBM long double functions in ibm-ldouble.c, when called in rounding modes other than round-to-nearest, can produce results that are invalid (do not satisfy the requirements in ibm-ldouble-format on what pairs of doubles make a valid long double value, in particular as regards the high part being equal to the sum of the two parts rounded to nearest). For example: #include <fenv.h> #include <float.h> #include <stdio.h> volatile long double a = LDBL_MAX; int main (void) { fesetround (FE_TOWARDZERO); union u { long double ld; double d[2]; } x; volatile long double r = a * a; x.ld = a; printf ("LDBL_MAX: %a %a\n", x.d[0], x.d[1]); x.ld = r; printf ("LDBL_MAX * LDBL_MAX: %a %a\n", x.d[0], x.d[1]); return 0; } prints LDBL_MAX: 0x1.fffffffffffffp+1023 0x1.ffffffffffffep+969 LDBL_MAX * LDBL_MAX: 0x1.fffffffffffffp+1023 0x1.fffffffffffffp+1023 where the value for LDBL_MAX * LDBL_MAX is not a valid long double at all. (This isn't limited to overflow cases, although they may produce the greatest errors; e.g. 0x1.fffffffffffffp-1L * 0x1.fffffffffffffp-1L in FE_UPWARD mode produces (0x1.fffffffffffffp-1, -0x1.fffffffffffffp-54), where the high part is not the sum of the two parts rounded to nearest.) ISO C does not allow for arithmetic operations simply not working - producing invalid results - for some types and rounding modes, although for non-IEEE types they need not be correctly rounding. I think the right approach for a fix will probably involve setting round-to-nearest temporarily within the functions, then adjusting overflowing and underflowing results based on the original rounding mode. I don't know what performance impact that might have, and whether it might be best to avoid that performance impact in the common case by having separate __gcc_*_round functions that deal with saving / restoring the rounding mode and are only used if -frounding-math, with the existing functions (not handling rounding modes) being used otherwise.
next reply other threads:[~2014-01-03 16:19 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2014-01-03 16:19 jsm28 at gcc dot gnu.org [this message] 2023-05-01 18:06 ` [Bug target/59666] " vital.had at gmail dot com 2023-05-31 6:05 ` egallager at gcc dot gnu.org
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