Q44723: Side Effects in ISRs and Signal Handlers Confuse Optimizer

Article: Q44723
Product(s): See article
Version(s): 5.00 5.10 | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | S_QuicKc | mspl13_c
Last Modified: 1-JUN-1989

It can be dangerous to allow the Microsoft C or QuickC compiler to
perform loop optimization on code that contains global variables that
may be modified in interrupt service routines or signal handlers.
For example, consider the following code

    int TimerCount = 1000;

    while( TimerCount );
    printf( "Timer expired." );

where an ISR is concurrently executing the following:

    void interrupt far ISR()
    {
        TimerCount--;
    }

This is acceptable code and performs a simple task -- it waits for a
global timer variable to reach 0, indicating that the ISR has been
executed a specified number of times. However, the loop optimization
optionally performed by the compiler results in the while() loop being
completely removed from the executable code. Because the compiler does
not know that the ISR is simultaneously accessing TimerCount, it sees
no reason to keep an expression that obviously evaluates to true and
does nothing but waste time. This is NOT a problem with the compiler.
Section 2.1.2.3 of the 7 December 1988 ANSI C draft standard states
the following:

   An actual implementation need not evaluate part of an expression
   if it can deduce that its value is not used and that no needed
   side effects are produced (including any caused by calling a
   function or accessing a volatile object).

Because C 5.10 does not semantically implement the "volatile" keyword,
there is no way to indicate that TimerCount is volatile and that the
expression involving TimerCount should not be removed from the program
by the optimizer. Thus, it is reasonable for the compiler to assume
that this expression is not necessary and may be safely removed.

When writing code for Interrupt Service Routines or signal handlers
that expect conditions that the compiler cannot predict, care should
be used in the choice of optimizations; and, if needed, an assembly
code listing should be generated with the /Fc or /Fs /Fa switches and
examined for the optimization's effect. Disabling optimization with
/Od may result in slower executable programs but will prevent
optimizer side effects.

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