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Fix #15032 FP knownConditionTrueFalse for bitwise negation of char #8941

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Fix #15032 FP knownConditionTrueFalse for bitwise negation of char #8941

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cf062a9
Update vf_common.h
chrchr-github Oct 14, 2026
5c0277e
Update vf_analyzers.cpp
chrchr-github Oct 14, 2026
29745e9
Update vf_common.cpp
chrchr-github Oct 14, 2026
1d21203
Update vf_settokenvalue.cpp
chrchr-github Oct 14, 2026
a74ef74
Update valueflow.cpp
chrchr-github Oct 14, 2026
d19541a
Update testvalueflow.cpp
chrchr-github Oct 14, 2026
9221dc5
Update testvalueflow.cpp
chrchr-github Oct 14, 2026
22d1c8c
Update testvalueflow.cpp
chrchr-github Oct 14, 2026
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2 changes: 1 addition & 1 deletion lib/valueflow.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -4131,7 +4131,7 @@ static std::list<ValueFlow::Value> truncateValues(std::list<ValueFlow::Value> va
}

if (value.isIntValue() && sz > 0 && sz < sizeof(MathLib::biguint))
value.intvalue = ValueFlow::truncateIntValue(value.intvalue, sz, dst->sign);
value.intvalue = ValueFlow::truncateIntValue(value.intvalue, sz, dst, settings.platform);
}
return values;
}
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2 changes: 1 addition & 1 deletion lib/vf_analyzers.cpp
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Expand Up @@ -357,7 +357,7 @@ struct ValueFlowAnalyzer : Analyzer {
if (dst) {
const size_t sz = dst->getSizeOf(settings, ValueType::Accuracy::ExactOrZero, ValueType::SizeOf::Pointer);
if (sz > 0 && sz < sizeof(MathLib::biguint)) {
MathLib::bigint newvalue = ValueFlow::truncateIntValue(value->intvalue, sz, dst->sign);
MathLib::bigint newvalue = ValueFlow::truncateIntValue(value->intvalue, sz, dst, settings.platform);

/* Handle overflow/underflow for value bounds */
if (value->bound != ValueFlow::Value::Bound::Point) {
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4 changes: 2 additions & 2 deletions lib/vf_common.cpp
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Expand Up @@ -96,15 +96,15 @@ namespace ValueFlow
return true;
}

MathLib::bigint truncateIntValue(MathLib::bigint value, size_t value_size, const ValueType::Sign dst_sign)
MathLib::bigint truncateIntValue(MathLib::bigint value, size_t value_size, const ValueType* dst, const Platform& platform)
{
if (value_size == 0)
return value;

const MathLib::biguint unsignedMaxValue = std::numeric_limits<MathLib::biguint>::max() >> ((sizeof(unsignedMaxValue) - value_size) * 8);
const MathLib::biguint signBit = 1ULL << ((value_size * 8) - 1);
value &= unsignedMaxValue;
if (dst_sign == ValueType::Sign::SIGNED && (value & signBit))
if ((dst->sign == ValueType::Sign::SIGNED || (dst->sign == ValueType::Sign::UNKNOWN_SIGN && dst->type == ValueType::Type::CHAR && platform.defaultSign == 's')) && (value & signBit))

@dmcppcheck dmcppcheck Oct 14, 2026

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AI review

When platform.defaultSign == '\0' (--platform=unspecified), plain char still goes through unsigned truncation. So char c = -1; if (~c == -256) {} still reports Condition '~c==-256' is always true, and char c = 0; c--; if (c == -1) {} reports "always false". When signedness is unknown, a value that sets the sign bit has no single known result.

Also, the cause seems to be that parsedecl (symboldatabase.cpp:7751) leaves char variables with UNKNOWN_SIGN, while symboldatabase.cpp:6929/8337 already apply mDefaultSignedness. Applying it in parsedecl would fix this for every reader of sign, not only these truncation call sites. It would also change the UNKNOWN_SIGN checks at symboldatabase.cpp:1934 and :6020.

value |= ~unsignedMaxValue;

return value;
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2 changes: 1 addition & 1 deletion lib/vf_common.h
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Expand Up @@ -41,7 +41,7 @@ namespace ValueFlow
{
bool getMinMaxValues(const ValueType* vt, const Platform& platform, MathLib::bigint& minValue, MathLib::bigint& maxValue);

MathLib::bigint truncateIntValue(MathLib::bigint value, size_t value_size, ValueType::Sign dst_sign);
MathLib::bigint truncateIntValue(MathLib::bigint value, size_t value_size, const ValueType* dst, const Platform& platform);

Token * valueFlowSetConstantValue(Token *tok, const Settings &settings);

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4 changes: 2 additions & 2 deletions lib/vf_settokenvalue.cpp
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Expand Up @@ -665,7 +665,7 @@ namespace ValueFlow
const ValueType *dst = tok->valueType();
if (dst) {
const size_t sz = dst->getSizeOf(settings, ValueType::Accuracy::ExactOrZero, ValueType::SizeOf::Pointer);
MathLib::bigint newvalue = ValueFlow::truncateIntValue(v.intvalue + 1, sz, dst->sign);
MathLib::bigint newvalue = ValueFlow::truncateIntValue(v.intvalue + 1, sz, dst, settings.platform);
if (v.bound != ValueFlow::Value::Bound::Point) {
if (newvalue < v.intvalue) {
v.invertBound();
Expand Down Expand Up @@ -695,7 +695,7 @@ namespace ValueFlow
const ValueType *dst = tok->valueType();
if (dst) {
const size_t sz = dst->getSizeOf(settings, ValueType::Accuracy::ExactOrZero, ValueType::SizeOf::Pointer);
MathLib::bigint newvalue = ValueFlow::truncateIntValue(v.intvalue - 1, sz, dst->sign);
MathLib::bigint newvalue = ValueFlow::truncateIntValue(v.intvalue - 1, sz, dst, settings.platform);
if (v.bound != ValueFlow::Value::Bound::Point) {
if (newvalue > v.intvalue) {
v.invertBound();
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7 changes: 7 additions & 0 deletions test/testvalueflow.cpp
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Expand Up @@ -1389,6 +1389,13 @@ class TestValueFlow : public TestFixture {
ASSERT_EQUALS(22, values.back().intvalue);
}

code = "void f() {\n" // #15032
" char c = -1;\n"
" if (~c == -256) {}\n"
"}\n";
const int expected = std::numeric_limits<char>::is_signed ? 0 : -256;
ASSERT_EQUALS(true, testKnownValueOfTok(code, "~", expected));

// #10251 starship operator
code = "struct X {};\n"
"auto operator<=>(const X & a, const X & b) -> decltype(1 <=> 2) {\n"
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