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Improvements for Clang warning -Wtautological-overlap-compare #13473

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llvmbot opened this issue Jun 13, 2012 · 4 comments
Open

Improvements for Clang warning -Wtautological-overlap-compare #13473

llvmbot opened this issue Jun 13, 2012 · 4 comments
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bugzilla Issues migrated from bugzilla clang:diagnostics New/improved warning or error message in Clang, but not in clang-tidy or static analyzer

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@llvmbot
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llvmbot commented Jun 13, 2012 •
edited by philnik777

Bugzilla Link 13101
Version unspecified
OS All
Reporter LLVM Bugzilla Contributor
CC @chandlerc,@nico,@Weverything

Extended Description

The simple test case, where

p[0] != 'h' || p[0] != 'H'
is always true, is not warned by clang++ and g++ too.

It will nice, If we have some flag like -Walways-true, which should warn on such usage.
This will churn out silly bugs.

tested with the following code.

int main()
{
        char *p ="Hello World\n";
        if(p[0] != 'h' || p[0] != 'H')
        {
                cout << "Word does not start with h/H\n";
        }
        return 0;
}
@chandlerc
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Just fixed a bug in LLVM that this warning would have caught in r285930.

It would be great to actually implement this. Especially for binary expressions with ||, and the false variant for &&.

A much simpler set of test cases:

bool f(int i) {
return i != 1 || i != 2;
}

bool g(int i) {
return i == 1 && i == 2;
}

@Weverything
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Since this original bug was filed, a new warning group, -Wtautological-overlap-compare, a sub-group of -Wtautological-compare, has been added to clang which can detect when a logical expression with multiple sub-expressions will always evaluate to true or always evaluate to false. This is in r205665.

From the test cases provided, there's at least three areas for improvement:

  1. The logical expression in return or assignment statements
  2. Handling character literals and other non-integer literal values as the constant
  3. Handling variables that are specific array indexes

@Weverything
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https://reviews.llvm.org/rL290920

  1. has been fixed in r290920

@llvmbot
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llvmbot commented Jan 9, 2017

hmm... I think the warning message "overlapping comparisons always evaluate to false" is unfortunate. Comparisons such as "x < 2" and "x > 5" are not "overlapping".

@llvmbot llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 3, 2021
@philnik777 philnik777 added clang:diagnostics New/improved warning or error message in Clang, but not in clang-tidy or static analyzer and removed clang:driver 'clang' and 'clang++' user-facing binaries. Not 'clang-cl' labels Aug 15, 2023
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bugzilla Issues migrated from bugzilla clang:diagnostics New/improved warning or error message in Clang, but not in clang-tidy or static analyzer
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@chandlerc @philnik777 @Weverything @llvmbot