Enables conformance to the ANSI C and IEEE 754 standards for floating-point arithmetic.
Windows: C/C++ > Optimization > Floating-point Precision Improvement
Linux: None
Mac OS: None
IA-32, IntelŪ EM64T, IntelŪ ItaniumŪ architecture
| Linux and Mac OS: | None |
| Windows: | /Op |
| /Op- |
None
| OFF |
This option enables conformance to the ANSI C and IEEE 754 standards for floating-point arithmetic.
It restricts some optimizations to maintain declared precision and to ensure that floating-point arithmetic conforms more closely to the ANSI and IEEE standards. Floating point intermediate results are kept in full 10-byte internal precision. All spills and reloads of the x87 floating-point registers utilize this internal format to prevent accidental loss of precision.
For most programs, specifying this option adversely affects performance. If you are not sure whether your application needs this option, try compiling and running your program both with and without it to evaluate the effects on performance versus precision. Alternatives to /Op include /QxN (for the IntelŪ Pentium® 4 processor or newer) and /Qprec.
Specifying the /Op option has the following effects on program compilation:
User variables declared as floating-point types are not assigned to registers.
Whenever an expression is spilled (moved from a register to memory), it is spilled as 80 bits (extended precision), not 64 bits (double precision).
Floating-point arithmetic comparisons conform to the IEEE 754 specification except for NaN behavior.
The exact operations specified in the code are performed. For example, division is never changed to multiplication by the reciprocal.
The compiler performs floating-point operations in the order specified without re-association.
The compiler does not perform the constant-folding optimization on floating-point values. Constant folding also eliminates any multiplication by 1, division by 1, and addition or subtraction of 0. For example, code that adds 0.0 to a number is executed exactly as written. Compile-time floating-point arithmetic is not performed to ensure that floating-point exceptions are also maintained.
Floating-point operations conform to ANSI C. When assignments to type float and double are made, the precision is rounded from 80 bits (extended) down to 32 bits (float) or 64 bits ( double ). When you do not specify /Op, the extra bits of precision are not always rounded before the variable is reused.
It sets the /Oi- option, which disables inline functions expansion.
The recommended method to control the semantics of floating-point calculations is to use option /fp.
None
fp-model, fp compiler option