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Q68559: Bad Code Generated for "a+b == a" FP Comparison Expressions

Article: Q68559
Product(s): See article
Version(s): 6.00 6.00a | 6.00 6.00a
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | buglist6.00 buglist6.00a conditional equals codegen | mspl13_c
Last Modified: 1-FEB-1991

The Microsoft C Compiler versions 6.00 and 6.00a will perform an
incorrect optimization on expressions of the form "a+b == a" if a and
b are floating-point variables (double or float type).

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)

; if ((a+b) == a) ;
    *** 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)

Note that the first expression gets optimized to compare variable b to
zero, rather than comparing a+b to a. In the second expression, a+b is
correctly compared to a.

The optimization performed in the above code is valid for integers but
not for floating-point numbers, and will produce incorrect results in
certain cases. One reason the expression (a+b == a) might be used is
to detect when b is negligibly small in relation to a.
The following are valid workarounds:

1. Enclose the expression "a+b" in parenthesis.

2. Make the variables a and b integers, if applicable.

3. Use the quick compile (/qc) option on the compiler command line.

Microsoft has confirmed this to be a problem with C versions 6.00 and
6.00a. We are researching this problem and will post new information
here as it becomes available.

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