[Bug 204671] clang floating point wrong around Inf (i386)

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Thu Nov 19 06:41:51 UTC 2015


https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=204671

            Bug ID: 204671
           Summary: clang floating point wrong around Inf (i386)
           Product: Base System
           Version: 10.2-RELEASE
          Hardware: i386
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: netch at segfault.kiev.ua

Created attachment 163320
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=163320&action=edit
source files

The test program, being called as "./t <a> <operator> <b> <rounding>",
performs a single arithmetic operator with the specified rounding and
prints its results. In some cases, output is wrong.

Conditions to reproduce:
1. Clang of any available version (confirmed on 3.4 from base,
clang36-3.6.2, clang37-3.7.2 from ports). I can't get this issue with
gcc-4.8.5, gcc-5.2.0_1 from ports.
2. i386 (amd64 isn't affected, I guess, because the issue is bound to FPU
variant).
3. no high -march= ("native" causes issues to disappear, I guess, for
the same connection to FPU; clang starts emitting SSE for this CPU).
4. -O or higher optimization level (-O0 isn't affected).

The OS is: FreeBSD 10.2-RELEASE-p7 i386.
The CPU on the test machine is: AMD Athlon(tm) 64 Processor 3500+
(Origin="AuthenticAMD"  Id=0x50ff2  Family=0xf  Model=0x5f  Stepping=2).

The proper results are (as I see from available IEEE754 documents):

$ ./t1 1e308 + 1e308 0
r=inf                       ( 7F F0 00 00 00 00 00 00)
$ ./t1 1e308 + 1e308 1
r=1.797693134862316e+308    ( 7F EF FF FF FF FF FF FF)
$ ./t1 1e308 + 1e308 2
r=inf                       ( 7F F0 00 00 00 00 00 00)
$ ./t1 1e308 + 1e308 3
r=1.797693134862316e+308    ( 7F EF FF FF FF FF FF FF)

This satisties the standard requirement that, e.g., "roundTowardZero,
the result shall be the format's floating-point number closest to and no
greater in magnitude than the infinitely precise result."

The variant with t1.c from attachment when the issue is exposed
(compiled as "cc -o t1 t1.c -g -Wall -W -lm -O"):

$ ./t1 1e308 + 1e308 0
r=inf                       ( 7F F0 00 00 00 00 00 00)
$ ./t1 1e308 + 1e308 1
r=inf                       ( 7F EF FF FF FF FF FF FF)
$ ./t1 1e308 + 1e308 2
r=inf                       ( 7F F0 00 00 00 00 00 00)
$ ./t1 1e308 + 1e308 3
r=inf                       ( 7F EF FF FF FF FF FF FF)

So, the binary representation of result is correct, but the printf
output is not.

The same compilation with -DNO_HEX always prints "inf" (so, it rejects a
guess of an aliasing issue):

$ ./t1 1e308 + 1e308 0
r=inf                       ()
$ ./t1 1e308 + 1e308 1
r=inf                       ()
$ ./t1 1e308 + 1e308 2
r=inf                       ()
$ ./t1 1e308 + 1e308 3
r=inf                       ()

The variant in t2.c uses global union instead of local on-stack
one for binary printing. The behavior differs so binary representation
always shows "inf":

$ ./t2 1e308 + 1e308 0
r=inf                       ( 7F F0 00 00 00 00 00 00)
$ ./t2 1e308 + 1e308 1
r=inf                       ( 7F F0 00 00 00 00 00 00)
$ ./t2 1e308 + 1e308 2
r=inf                       ( 7F F0 00 00 00 00 00 00)
$ ./t2 1e308 + 1e308 3
r=inf                       ( 7F F0 00 00 00 00 00 00)

Again, adding -DNO_HEX causes "inf" still printed in all cases.

But: a variant with "r" declared as global variable instead of local one
(-DR_GLOBAL for both source versions) stops the issue.

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