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* [Bug analyzer/107948] New: GCC Static Analyzer doesn't realize `0 - width <= 0` is always true when `width > 0` and `width is int` type，
@ 2022-12-01 14:07 geoffreydgr at icloud dot com
  2022-12-02  2:31 ` [Bug analyzer/107948] " cvs-commit at gcc dot gnu.org
                   ` (3 more replies)
  0 siblings, 4 replies; 5+ messages in thread
From: geoffreydgr at icloud dot com @ 2022-12-01 14:07 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107948

            Bug ID: 107948
           Summary: GCC Static Analyzer doesn't realize `0 - width <= 0`
                    is always true when `width > 0` and `width is int`
                    type，
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: geoffreydgr at icloud dot com
  Target Milestone: ---

I got a false negative error when compiling the following program with
gcc(trunk) -fanalyzer -O0. https://godbolt.org/z/vneenabc5

```
extern void __analyzer_eval (int);

void foo(int width) {
    int i = 0;
    int base;
    if (width > 0){
        __analyzer_eval(i == 0);
        __analyzer_eval(width > 0);
        __analyzer_eval(width - i > 0);
        __analyzer_eval(i - width <= 0);
        if (i - width <= 0) {
        base = 512;
        }
    }
    base+=1;
}
```

Output:
```
<source>: In function 'foo':
<source>:7:9: warning: TRUE
    7 |         __analyzer_eval(i == 0);
      |         ^~~~~~~~~~~~~~~~~~~~~~~
<source>:8:9: warning: TRUE
    8 |         __analyzer_eval(width > 0);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:9:9: warning: TRUE
    9 |         __analyzer_eval(width - i > 0);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:10:9: warning: UNKNOWN
   10 |         __analyzer_eval(i - width <= 0);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:15:9: warning: use of uninitialized value 'base' [CWE-457]
[-Wanalyzer-use-of-uninitialized-value]
   15 |     base+=1;
      |     ~~~~^~~
  'foo': events 1-3
    |
    |    5 |     int base;
    |      |         ^~~~
    |      |         |
    |      |         (1) region created on stack here
    |      |         (2) capacity: 4 bytes
    |......
    |   15 |     base+=1;
    |      |     ~~~~~~~
    |      |         |
    |      |         (3) use of uninitialized value 'base' here

```

GCC Static Analyzer doesn't realize `0 - width <= 0` is always true when `width
> 0` and `width is int type`，hence it reports a wrong
use-of-uninitialized-value warning.

The analysis result shows that analyzer knows `width - i > 0` is true but does
not know the equivalence formula ` i - width <= 0` is also true.

^ permalink raw reply	[flat|nested] 5+ messages in thread
end of thread, other threads:[~2023-03-29 18:18 UTC | newest]

Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-12-01 14:07 [Bug analyzer/107948] New: GCC Static Analyzer doesn't realize `0 - width <= 0` is always true when `width > 0` and `width is int` type， geoffreydgr at icloud dot com
2022-12-02  2:31 ` [Bug analyzer/107948] " cvs-commit at gcc dot gnu.org
2022-12-02  2:39 ` dmalcolm at gcc dot gnu.org
2022-12-03  6:57 ` geoffreydgr at icloud dot com
2023-03-29 18:18 ` cvs-commit at gcc dot gnu.org

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