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: [email protected]
Reporter: [email protected]
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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