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* [Bug c/52451] New: gcc w/i387 float generates fucom rather than fcom for floating point comparsons
@ 2012-03-01 15:48 bugdal at aerifal dot cx
2012-03-01 15:56 ` [Bug c/52451] " rguenth at gcc dot gnu.org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: bugdal at aerifal dot cx @ 2012-03-01 15:48 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52451
Bug #: 52451
Summary: gcc w/i387 float generates fucom rather than fcom for
floating point comparsons
Classification: Unclassified
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: bugdal@aerifal.cx
The C comparison operators (as opposed to the isgreater, etc. macros) are
specified to correspond to the IEEE "unordered-signaling predicates" which
raise the INVALID exception on comparisons involving a NaN (quiet or
signaling). However, gcc on x86 is generating the FUCOM opcode rather than
FCOM, which does not raise any exception unless it encounters a signaling NaN.
Apparently this is a regression since ancient versions of gcc, which correctly
generated FCOM.
This bug report is i387-specific; I have no idea if the same issue applies with
SSE math or non-x86-family cpu archs.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/52451] gcc w/i387 float generates fucom rather than fcom for floating point comparsons
2012-03-01 15:48 [Bug c/52451] New: gcc w/i387 float generates fucom rather than fcom for floating point comparsons bugdal at aerifal dot cx
@ 2012-03-01 15:56 ` rguenth at gcc dot gnu.org
2012-03-15 16:24 ` [Bug target/52451] " bugdal at aerifal dot cx
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: rguenth at gcc dot gnu.org @ 2012-03-01 15:56 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52451
Richard Guenther <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |WAITING
Last reconfirmed| |2012-03-01
Ever Confirmed|0 |1
--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-03-01 15:55:37 UTC ---
Please provide a testcase and specify the GCC version you are using.
Works for me with
int foo (double x, double y)
{
return x < y;
}
and GCC 4.6.2.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug target/52451] gcc w/i387 float generates fucom rather than fcom for floating point comparsons
2012-03-01 15:48 [Bug c/52451] New: gcc w/i387 float generates fucom rather than fcom for floating point comparsons bugdal at aerifal dot cx
2012-03-01 15:56 ` [Bug c/52451] " rguenth at gcc dot gnu.org
@ 2012-03-15 16:24 ` bugdal at aerifal dot cx
2012-04-23 9:38 ` bugdal at aerifal dot cx
2014-09-11 16:11 ` nszabolcs at gmail dot com
3 siblings, 0 replies; 5+ messages in thread
From: bugdal at aerifal dot cx @ 2012-03-15 16:24 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52451
--- Comment #2 from Rich Felker <bugdal at aerifal dot cx> 2012-03-15 16:23:11 UTC ---
Try foo(NAN,NAN) and then check for the INVALID exception. It's not set.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug target/52451] gcc w/i387 float generates fucom rather than fcom for floating point comparsons
2012-03-01 15:48 [Bug c/52451] New: gcc w/i387 float generates fucom rather than fcom for floating point comparsons bugdal at aerifal dot cx
2012-03-01 15:56 ` [Bug c/52451] " rguenth at gcc dot gnu.org
2012-03-15 16:24 ` [Bug target/52451] " bugdal at aerifal dot cx
@ 2012-04-23 9:38 ` bugdal at aerifal dot cx
2014-09-11 16:11 ` nszabolcs at gmail dot com
3 siblings, 0 replies; 5+ messages in thread
From: bugdal at aerifal dot cx @ 2012-04-23 9:38 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52451
--- Comment #3 from Rich Felker <bugdal at aerifal dot cx> 2012-04-23 09:37:45 UTC ---
Compiling with the -mno-ieee-fp option fixes this bug. It seems like the
behavior of this option is reversed from the documentation; -mno-ieee-fp gives
IEEE conformant comparisons (raising exception on unordered) and -mieee-fp
gives non-conformant comparisons (silent on unordered)...
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug target/52451] gcc w/i387 float generates fucom rather than fcom for floating point comparsons
2012-03-01 15:48 [Bug c/52451] New: gcc w/i387 float generates fucom rather than fcom for floating point comparsons bugdal at aerifal dot cx
` (2 preceding siblings ...)
2012-04-23 9:38 ` bugdal at aerifal dot cx
@ 2014-09-11 16:11 ` nszabolcs at gmail dot com
3 siblings, 0 replies; 5+ messages in thread
From: nszabolcs at gmail dot com @ 2014-09-11 16:11 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52451
Szabolcs Nagy <nszabolcs at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |nszabolcs at gmail dot com
--- Comment #4 from Szabolcs Nagy <nszabolcs at gmail dot com> ---
this bug is still present in gcc-4.9 on i386 and x86_64 targets
and possibly others (clang is broken too but that's another story)
the ieee-754 and thus iso c99/c11 + annex f, requires that
==, != are quiet (never raise any exception)
<, >, <=, >= are signaling (raise invalid if an operand is nan)
eg. with
int f(float x, float y)
{
return x < y;
}
int g(float x, float y)
{
return x == y;
}
on x87 (i386):
fcom* should be used for f (signaling)
fucom* should be used for g (quiet)
with sse2 (x86_64):
comis* should be used for f (signaling)
ucomis* should be used for g (quiet)
(on arm it is vcmpe.f32 vs vcmp.f32)
it is easy to check that gcc always emits quiet comparisions
and with -mno-ieee-fp it always emits signaling ones
(both of them are wrong)
-O2 -std=c99 on x86_64:
http://goo.gl/awmRGP
-O2 -std=c99 -mno-ieee-fp on x86_64:
http://goo.gl/9CHNjK
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2014-09-11 16:11 UTC | newest]
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2012-03-01 15:48 [Bug c/52451] New: gcc w/i387 float generates fucom rather than fcom for floating point comparsons bugdal at aerifal dot cx
2012-03-01 15:56 ` [Bug c/52451] " rguenth at gcc dot gnu.org
2012-03-15 16:24 ` [Bug target/52451] " bugdal at aerifal dot cx
2012-04-23 9:38 ` bugdal at aerifal dot cx
2014-09-11 16:11 ` nszabolcs at gmail dot com
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