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  1. Embedded Software & Tools
  2. EXT_EP-9681

Compiler incorrectly optimizes unsigned loop counter which wraps around

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    • Icon: Bug Bug
    • Resolution: Fixed
    • Icon: Medium Medium
    • Code Generation Tools
    • CODEGEN-7227
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      C6000_8.2.0
      C2000_18.1.0.LTS
      C6000_8.3.0
      MSP430_20.2.0.LTS
      C7000_1.4.0.LTS*
      ARM_18.12.0.LTS
      PRU_2.3.0
      ARP32_1.1.0*
      ARM_20.2.0.LTS
      C2000_20.2.0.LTS
      ARM_18.1.0.LTS
      MSP430_18.1.0.LTS
      PRU_2.4.0*
      MSP430_18.12.0.LTS
      C2000_18.12.0.LTS
      Show
      C6000_8.2.0 C2000_18.1.0.LTS C6000_8.3.0 MSP430_20.2.0.LTS C7000_1.4.0.LTS* ARM_18.12.0.LTS PRU_2.3.0 ARP32_1.1.0* ARM_20.2.0.LTS C2000_20.2.0.LTS ARM_18.1.0.LTS MSP430_18.1.0.LTS PRU_2.4.0* MSP430_18.12.0.LTS C2000_18.12.0.LTS
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      ARM_18.12.6.LTS*
      C7000_1.4.0.LTS*
      MSP430_18.1.8.LTS
      C6000_8.3.7*
      ARP32_1.1.0*
      MSP430_20.2.1.LTS
      ARM_20.2.1.LTS
      C2000_18.12.6.LTS*
      PRU_2.4.0*
      ARM_18.1.8.LTS
      MSP430_18.12.6.LTS*
      C2000_18.1.8.LTS
      PRU_2.3.4*
      C6000_8.2.9
      C2000_20.2.1.LTS
      Show
      ARM_18.12.6.LTS* C7000_1.4.0.LTS* MSP430_18.1.8.LTS C6000_8.3.7* ARP32_1.1.0* MSP430_20.2.1.LTS ARM_20.2.1.LTS C2000_18.12.6.LTS* PRU_2.4.0* ARM_18.1.8.LTS MSP430_18.12.6.LTS* C2000_18.1.8.LTS PRU_2.3.4* C6000_8.2.9 C2000_20.2.1.LTS
    • default
    • Avoid writing a do-while loop in which the loop variable wraps around on the first iteration. For instance, use a signed loop variable or a while-loop.
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      A do-while loop which uses an unsigned loop variable that wraps around on the first iteration (for example, initialised to (unsigned)-1 and incremented before the first end-test) may confuse the compiler into removing the entire loop. It may interpret the -1 as a large constant that is already past the loop's end value, and fold the loop away.
      Show
      A do-while loop which uses an unsigned loop variable that wraps around on the first iteration (for example, initialised to (unsigned)-1 and incremented before the first end-test) may confuse the compiler into removing the entire loop. It may interpret the -1 as a large constant that is already past the loop's end value, and fold the loop away.

      The attached file contains an implementation of strnlen.

      size_t strnlen_wtf(const char *s, size_t maxlen)
{
    size_t n = (size_t)-1;

    do n++; while ( (*s++)  &&  (n < maxlen) );
    return n;
}

      Build it ...

      % cl2000 -o2 -s try1.c

      Inspect the resulting assembly to see that it is optimized into a function that always returns the value 0.

      Please see the associated forum thread for more context.

            syncuser TI User
            syncuser TI User
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              Created:
              Updated:
              Resolved: