digitalmars.D.bugs - [Issue 3918] New: Parameter use before its use in an AndAnd expression with reals treats NaN as false
- d-bugmail puremagic.com (65/65) Mar 09 2010 http://d.puremagic.com/issues/show_bug.cgi?id=3918
- d-bugmail puremagic.com (14/15) Mar 15 2010 http://d.puremagic.com/issues/show_bug.cgi?id=3918
- d-bugmail puremagic.com (7/7) Mar 15 2010 http://d.puremagic.com/issues/show_bug.cgi?id=3918
- d-bugmail puremagic.com (38/38) Aug 02 2010 http://d.puremagic.com/issues/show_bug.cgi?id=3918
- d-bugmail puremagic.com (14/20) Oct 28 2012 http://d.puremagic.com/issues/show_bug.cgi?id=3918
http://d.puremagic.com/issues/show_bug.cgi?id=3918 Summary: Parameter use before its use in an AndAnd expression with reals treats NaN as false Product: D Version: 2.041 Platform: x86 OS/Version: Windows Status: NEW Severity: normal Priority: P2 Component: DMD AssignedTo: nobody puremagic.com ReportedBy: crimson.magus gmail.com PST --- NaN is supposed to be treated as true when implicitly converted to bool, because it's not the same as zero. But, in the following case, the NaN is treated as false. - optimized build - a variable of type real with value NaN - the variable is used in an AndAnd (&&) or OrOr (||) expression - the variable is used before the AndAnd (&&) or OrOr (||) expression I assume this happens anywhere a real is turned into a bool, but I haven't checked. Here's a sample program: import std.stdio; void Do( float t, real u ) { writeln( u ); writeln( t && u ); } void Do2( real t, float u ) { writeln( t ); writeln( t && u ); } void main( string[] args ) { Do( float.nan, real.nan ); Do2( real.nan, float.nan ); } In the function calls to both Do and Do2, the result is "false". It should be "true". Commenting out "writeln( u )" and "writeln( t )" also make the results "true". Looking at the assembly code, I noticed that the cause seems to be an instruction that's left out when the program is optimized. Unoptimized (right): 00403C0D fld tbyte ptr [esp+8] 00403C11 fldz 00403C13 fucompp 00403C15 fnstsw ax 00403C17 sahf 00403C18 jne 00403C20 00403C1A jp 00403C20 Optimized (wrong): 00403C1E fld tbyte ptr [esp+8] 00403C22 fldz 00403C24 fucompp 00403C26 fnstsw ax 00403C28 sahf 00403C29 jne 00403C2F The JP instruction is missing. As a result, we'll fall thru to where EAX is cleared, and the result turns into false. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Mar 09 2010
http://d.puremagic.com/issues/show_bug.cgi?id=3918 Don <clugdbug yahoo.com.au> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |clugdbug yahoo.com.auNaN is supposed to be treated as true when implicitly converted to bool,because it's not the same as zero. That's surprising behaviour, I'd have expected that it needs to a non-zero value. I don't think implicit conversion float->bool should be legal at all, for exactly this reason. But obviously its behaviour should be unaffected by -O. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Mar 15 2010
http://d.puremagic.com/issues/show_bug.cgi?id=3918 --- Actually, I'm fine with NaN going either way, as long as it's consistent... and documented. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Mar 15 2010
http://d.puremagic.com/issues/show_bug.cgi?id=3918 I investigated this bug because I thought it might be related to the more difficult bug 4443. Unfortunately, they are unrelated. This one happens because the variable 'u' is recognised as a common sub expression (CSE). Then in cod3.c, jmpopcode(), around line 805, we see this code. The two lines marked with * mean that it just does a JNE, instead of a JNE/JP combination. When there's no CSE, the next return is used, "return XP|JNE", which adds the JP in. op = e->Eoper; * if (e->Ecount != e->Ecomsub) // comsubs just get Z bit set * return JNE; if (!OTrel(op)) // not relational operator { tym_t tymx = tybasic(e->Ety); if (tyfloating(tymx) && config.inline8087 && (tymx == TYldouble || tymx == TYildouble || tymx == TYcldouble || tymx == TYcdouble || tymx == TYcfloat || op == OPind)) { return XP|JNE; } return (op >= OPbt && op <= OPbts) ? JC : JNE; } ------------ How to fix this? (1) Duplicate the if(tyfloating) code in the first return, so that floating point CSEs still include a JP. But that penalizes the general case. (2) Don't detect if(x) as a CSE, when x is floating point. One way of doing this would be to change it into x!=0. (3) Entirely disallow if (x) for floating point, generating an error in the front end. Personally I think (3) is the best. I think making 'if (nan)' true leads to subtle bugs, and making it false also leads to subtle bugs. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Aug 02 2010
http://d.puremagic.com/issues/show_bug.cgi?id=3918 yebblies <yebblies gmail.com> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |yebblies gmail.comThen in cod3.c, jmpopcode(), around line 805, we see this code. The two lines marked with * mean that it just does a JNE, instead of a JNE/JP combination. When there's no CSE, the next return is used, "return XP|JNE", which adds the JP in.I hit this same bit of code when investigating the xmm codegen bugs earlier this year and tried to fix it by moving the CSE if to under the fp block, and Walter told me that "floating point expressions are not common subexpressioned." Nasty. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Oct 28 2012