Hi Dror,
The hardware instruction does not know anything about errno, which is set to EDOM when the input argument is less than 0. Thus, in order to get the correct behavior, the compiler needs to check the result (the __save_sqrt_temp_1 != __save_sqrt_temp_1 check), and if the result does not match, it calls the library implementation instead. (IIRC, floating point compare is different than integer compare, because NaN != NaN, and the compiler can't fold this if statement.) Note that you can turn off this behavior (if your code does not care errno) with the -fno-math-errno flag. I don't have open64 installed on my current machine, but it seems to work with gcc. If you want to dig deeper, you can add the statement "dz[29] = -1;" above the for loop, run the program under a debugger, and set a breakpoint at sqrtf. You will see that at iteration 29, the program hits the breakpoint. If you re-compile the code with the -fno-math-errno flag, then the sqrtf call is not even generated. Rayson ================================= Grid Engine / Open Grid Scheduler http://gridscheduler.sourceforge.net/ Scalable Grid Engine Support Program http://www.scalablelogic.com/ ----- Original Message ----- From: Dror Maydan <may...@tensilica.com> To: open64-devel@lists.sourceforge.net Cc: Sent: Monday, November 14, 2011 8:49 PM Subject: [Open64-devel] sqrtf Given code such as #include <math.h> float dz[100]; int foo() { int i; for (i=0; i<100; i++) { dz[i] = sqrtf(dz[i]); } } Open64 generates (on X86) a sqrt instruction followed by a conditional call to the sqrtf library function. __sqrt_arg_temp_0 = dz[i]; __save_sqrt_temp_1 = _F4SQRT(__sqrt_arg_temp_0); if(__save_sqrt_temp_1 != __save_sqrt_temp_1) { __save_sqrt_temp_1 = sqrtf(__sqrt_arg_temp_0); } Does anyone know the motivation for the library call? Is something missing in the X86 hardware instruction, some error value in the library that needs to be set, something else? Seems like a pretty high performance penalty for this behavior. Dror PS gcc seems to do the same thing ------------------------------------------------------------------------------ RSA(R) Conference 2012 Save $700 by Nov 18 Register now http://p.sf.net/sfu/rsa-sfdev2dev1 _______________________________________________ Open64-devel mailing list Open64-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/open64-devel ------------------------------------------------------------------------------ RSA(R) Conference 2012 Save $700 by Nov 18 Register now http://p.sf.net/sfu/rsa-sfdev2dev1 _______________________________________________ Open64-devel mailing list Open64-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/open64-devel
