[llvm-bugs] [Bug 50272] New: False positive leading to "The result of the '/' expression is undefined"

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50272

            Bug ID: 50272
           Summary: False positive leading to "The result of the '/'
                    expression is undefined"
           Product: clang
           Version: 11.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Static Analyzer
          Assignee: dcoughlin at apple.com
          Reporter: daniel at haxx.se
                CC: dcoughlin at apple.com, llvm-bugs at lists.llvm.org

Using scan-build-11 in Debian unstable (based on clang 11.0.1-2) on current
curl code (in curl's git master as of May 7th).

scan-build complains on a code path that protects against integer overflows and
division by zero but still somehow scan-build is not content. No earlier
versions of scan-build (and no other code analyzer we use) warn on this code.

Here's the saved scan-build output:

 https://daniel.haxx.se/scan-build-2021-05-08-115719/report-0d7f93.html#EndPath

I'm working around this false positive by putting (the same) logic into a
function, which if it was a genuine problem also would be weird...

The code that causes two warnings look like this:

/* returns TRUE if it's time to show the progress meter */
static bool progress_calc(struct Curl_easy *data, struct curltime now)
{
  curl_off_t timespent;
  curl_off_t timespent_ms; /* milliseconds */
  curl_off_t dl = data->progress.downloaded;
  curl_off_t ul = data->progress.uploaded;
  bool timetoshow = FALSE;

  /* The time spent so far (from the start) */
  data->progress.timespent = Curl_timediff_us(now, data->progress.start);
  timespent = (curl_off_t)data->progress.timespent/1000000; /* seconds */
  timespent_ms = (curl_off_t)data->progress.timespent/1000; /* ms */

  /* The average download speed this far */
  if(dl < CURL_OFF_T_MAX/1000000)
    data->progress.dlspeed =
      (dl * 1000000 / (data->progress.timespent?data->progress.timespent:1));
  else if(dl < CURL_OFF_T_MAX/1000)
    data->progress.dlspeed = (dl * 1000 / (timespent_ms>0?timespent_ms:1));
  else
    data->progress.dlspeed = (dl / (timespent>0?timespent:1));

  /* The average upload speed this far */
  if(ul < CURL_OFF_T_MAX/1000000)
    data->progress.ulspeed =
      (ul * 1000000 / (data->progress.timespent?data->progress.timespent:1));
  else if(ul < CURL_OFF_T_MAX/1000)
    data->progress.ulspeed = (ul * 1000 / (timespent_ms>0?timespent_ms:1));
  else
    data->progress.ulspeed = (ul / (timespent>0?timespent:1));

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