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Fix #15184 FN zerodiv with compound assignment #9006

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Fix #15184 FN zerodiv with compound assignment #9006

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fd40e83
Update vf_settokenvalue.cpp
chrchr-github Nov 9, 2026
e7154cb
Update calculate.h
chrchr-github Nov 9, 2026
98b5640
Update testvalueflow.cpp
chrchr-github Nov 9, 2026
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10 changes: 10 additions & 0 deletions lib/calculate.h
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Original file line number Diff line number Diff line change
Expand Up @@ -60,43 +60,53 @@ R calculate(const std::string& s, const T& x, const T& y, bool* error = nullptr)
constexpr MathLib::bigint maxBitsSignedShift = maxBitsShift - 1;
switch (MathLib::encodeMultiChar(s)) {
case '+':
case '+=':
return wrap(x + y);
case '-':
case '-=':
return wrap(x - y);
case '*':
case '*=':
return wrap(x * y);
case '/':
case '/=':
if (isZero(y) || (std::is_signed<T>{} && y < 0)) {
if (error)
*error = true;
return R{};
}
return wrap(x / y);
case '%':
case '%=':
if (isZero(MathLib::bigint(y)) || (std::is_signed<T>{} && MathLib::bigint(y) < 0)) {
if (error)
*error = true;
return R{};
}
return wrap(MathLib::bigint(x) % MathLib::bigint(y));
case '&':
case '&=':
return wrap(MathLib::bigint(x) & MathLib::bigint(y));
case '|':
case '|=':
return wrap(MathLib::bigint(x) | MathLib::bigint(y));
case '^':
case '^=':
return wrap(MathLib::bigint(x) ^ MathLib::bigint(y));
case '>':
return wrap(x > y);
case '<':
return wrap(x < y);
case '<<':
case '<<=':
if (y >= maxBitsSignedShift || y < 0 || x < 0) {
if (error)
*error = true;
return R{};
}
return wrap(MathLib::bigint(x) << MathLib::bigint(y));
case '>>':
case '>>=':
if (y >= maxBitsSignedShift || y < 0 || x < 0) {
if (error)
*error = true;
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2 changes: 1 addition & 1 deletion lib/vf_settokenvalue.cpp
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Expand Up @@ -486,7 +486,7 @@ namespace ValueFlow
}

// Calculations..
else if ((parent->isArithmeticalOp() || parent->isComparisonOp() || (parent->tokType() == Token::eBitOp) ||
else if ((parent->isArithmeticalOp() || parent->isComparisonOp() || (parent->tokType() == Token::eBitOp) || (Token::Match(parent, "%assign%") && parent->str().size() > 1) ||

@dmcppcheck dmcppcheck Nov 9, 2026

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AI review

For compound assignments, the computed result has to be converted to the type of the left operand. Today the value of c += 1 is computed like the value of c + 1, which is promoted to int, so it is never truncated.

int f(void) {
    unsigned char c = 255;
    int x = (c += 1);   // PR: `+=` always 256 (should be 0)
    if (x == 0)         // PR: new FP knownConditionTrueFalse "always false"
        return 1;
    return 0;
}
void g(int a[10]) { unsigned char c = 255; a[c += 1] = 0; } // PR: new FP arrayIndexOutOfBounds index 256

unsigned int u = 0; (u -= 1) likewise gets the value -1. Casting the result with castValue or setTokenValueCast, using the valueType() of parent->astOperand1(), would fix this. A test with an unsigned char or unsigned left operand would cover it.

(parent->tokType() == Token::eLogicalOp)) &&
parent->astOperand1() && parent->astOperand2()) {

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6 changes: 6 additions & 0 deletions test/testvalueflow.cpp
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Expand Up @@ -1403,6 +1403,12 @@ class TestValueFlow : public TestFixture {
"}\n";
ASSERT_NO_THROW(tokenValues(code, "<=>"));

code = "int f() {\n" // #15184
" int i = -1;\n"
" return 1 / (i += 1);\n"
"}\n";
ASSERT_EQUALS(true, testKnownValueOfTok(code, "+=", 0));

// Comparison of string
values = removeImpossible(tokenValues("f(\"xyz\" == \"xyz\");\n", "==")); // implementation defined
ASSERT_EQUALS(0U, values.size()); // <- no value
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