PRB: Bad Code Generated for "a+b == a" FP Comparison Expression

MS-DOS                 | OS/2       | WINDOWS

The information in this article applies to:

• The Microsoft C/C++ Compiler (CL.EXE) included with:

      - Microsoft C for MS-DOS, versions 6.0, 6.0a, and 6.0ax
      - Microsoft C for OS/2, versions 6.0 and 6.0a
      - Microsoft C/C++ for MS-DOS, version 7.0
      - Microsoft Visual C++ for Windows, versions 1.0 and 1.5
  

SYMPTOMS

The compiler optimizes expressions of the form "a+b == a" if a and b are floating-point variables (double or float type).

RESOLUTION

This is expected behavior for the compiler. The proper code is generated by compiling with the /Op option, which directs the compiler to use consistent floating-point calculations. If this is not a viable option, one of the following will work around the situation:

• Enclose the expression "a+b" in parentheses.
• Make the variables a and b integers, if applicable.
• Compile with the quick compile (/qc) or fast compile (/f) options.

MORE INFORMATION

Sample code with floating-point expressions of the form "a+b == a" was compiled with optimization disabled (/Od) and the first few lines of the resulting assembly listing follows:

; double a,b; ; if (a+b == a) ;

    *** 00000b  9b d9 ee                fldz
    *** 00000e  9b dc 16 00 00          fcom    QWORD PTR _b
    *** 000013  9b dd d8                fstp    ST(0)
    *** (lines deleted)

    *** 000025  9b dd 06 00 00          fld     QWORD PTR _b
    *** 00002a  9b dc 06 00 00          fadd    QWORD PTR _a
    *** 00002f  9b dc 16 00 00          fcom    QWORD PTR _a
    *** 000034  9b dd d8                fstp    ST(0)
    *** (lines deleted)

Compiling with the /Op option generates the correct code in both cases. The expression (a+b == a) might be used with floating point numbers to detect when b is negligibly small in relation to a.