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Fix #12609 FN redundantCopyLocalConst (copy from variable) #8251

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Merged
chrchr-github merged 6 commits into from
Feb 24, 2026
Merged

Fix #12609 FN redundantCopyLocalConst (copy from variable) #8251

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6295044
Fix #12609 FN redundantCopyLocalConst (copy from variable)
web-flow Feb 23, 2026
871b7e6
Add test
web-flow Feb 23, 2026
15e1882
Format
web-flow Feb 23, 2026
73f8b9c
Fix
web-flow Feb 23, 2026
392f09e
Simplify
web-flow Feb 23, 2026
3de3155
Fix
web-flow Feb 23, 2026
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83 changes: 55 additions & 28 deletions lib/checkother.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3290,10 +3290,63 @@ static bool constructorTakesReference(const Scope * const classScope)
});
}

static bool isLargeObject(const Variable* var, const Settings& settings)
{
return var && !var->isGlobal() &&
(!var->type() || !var->type()->classScope ||
(var->valueType() && var->valueType()->getSizeOf(settings, ValueType::Accuracy::LowerBound, ValueType::SizeOf::Pointer) > 2 * settings.platform.sizeof_pointer));
}

//---------------------------------------------------------------------------
// This check rule works for checking the "const A a = getA()" usage when getA() returns "const A &" or "A &".
// In most scenarios, "const A & a = getA()" will be more efficient.
//---------------------------------------------------------------------------
static bool checkFunctionReturnsRef(const Token* tok, const Settings& settings)
{
if (!Token::Match(tok->previous(), "%name% ("))
return false;
if (!Token::Match(tok->link(), ") )|}| ;")) // bailout for usage like "const A a = getA()+3"
return false;
const Token* dot = tok->astOperand1();
if (Token::simpleMatch(dot, ".")) {
const Token* varTok = dot->astOperand1();
const int indirect = varTok->valueType() ? varTok->valueType()->pointer : 0;
if (isVariableChanged(tok, tok->scope()->bodyEnd, indirect, varTok->varId(), /*globalvar*/ true, settings))
return false;
if (isTemporary(dot, &settings.library, /*unknown*/ true))
return false;
}
if (exprDependsOnThis(tok->previous()))
return false;
const Function* func = tok->previous()->function();
if (func && func->tokenDef->strAt(-1) == "&") {
const Scope* fScope = func->functionScope;
if (fScope && fScope->bodyEnd && Token::Match(fScope->bodyEnd->tokAt(-3), "return %var% ;")) {
const Token* varTok = fScope->bodyEnd->tokAt(-2);
if (isLargeObject(varTok->variable(), settings))
return true;
}
}
return false;
}

static bool checkVariableAssignment(const Token* tok, const Settings& settings)
{
if (!Token::Match(tok, "%var% ;"))
return false;
const Variable* var = tok->variable();
if (!var || !isLargeObject(var, settings))
return false;
if (findVariableChanged(tok->tokAt(2), tok->scope()->bodyEnd, /*indirect*/ 0, var->declarationId(), /*globalvar*/ false, settings))
return false;
if (var->isLocal() || (var->isArgument() && !var->isReference()))
return true;
const Scope* scope = tok->scope();
while (scope && scope->type != ScopeType::eFunction)
scope = scope->nestedIn;
return scope && scope->function && (!scope->functionOf || scope->function->isConst() || scope->function->isStatic());
}

void CheckOther::checkRedundantCopy()
{
if (!mSettings->severity.isEnabled(Severity::performance) || mTokenizer->isC() || !mSettings->certainty.isEnabled(Certainty::inconclusive))
Expand Down Expand Up @@ -3326,35 +3379,9 @@ void CheckOther::checkRedundantCopy()
const Token* tok = startTok->next()->astOperand2();
if (!tok)
continue;
if (!Token::Match(tok->previous(), "%name% ("))
continue;
if (!Token::Match(tok->link(), ") )|}| ;")) // bailout for usage like "const A a = getA()+3"
if (!checkFunctionReturnsRef(tok, *mSettings) && !checkVariableAssignment(tok, *mSettings))
continue;

const Token* dot = tok->astOperand1();
if (Token::simpleMatch(dot, ".")) {
const Token* varTok = dot->astOperand1();
const int indirect = varTok->valueType() ? varTok->valueType()->pointer : 0;
if (isVariableChanged(tok, tok->scope()->bodyEnd, indirect, varTok->varId(), /*globalvar*/ true, *mSettings))
continue;
if (isTemporary(dot, &mSettings->library, /*unknown*/ true))
continue;
}
if (exprDependsOnThis(tok->previous()))
continue;

const Function* func = tok->previous()->function();
if (func && func->tokenDef->strAt(-1) == "&") {
const Scope* fScope = func->functionScope;
if (fScope && fScope->bodyEnd && Token::Match(fScope->bodyEnd->tokAt(-3), "return %var% ;")) {
const Token* varTok = fScope->bodyEnd->tokAt(-2);
if (varTok->variable() && !varTok->variable()->isGlobal() &&
(!varTok->variable()->type() || !varTok->variable()->type()->classScope ||
(varTok->variable()->valueType() &&
varTok->variable()->valueType()->getSizeOf(*mSettings, ValueType::Accuracy::LowerBound, ValueType::SizeOf::Pointer) > 2 * mSettings->platform.sizeof_pointer)))
redundantCopyError(startTok, startTok->str());
}
}
redundantCopyError(startTok, startTok->str());
}
}

Expand Down
41 changes: 40 additions & 1 deletion test/testother.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2702,7 +2702,9 @@ class TestOther : public TestFixture {
check("void f(std::string str) {\n"
" std::string s2 = str;\n"
"}");
ASSERT_EQUALS("[test.cpp:1:20]: (performance) Function parameter 'str' should be passed by const reference. [passedByValue]\n", errout_str());
ASSERT_EQUALS("[test.cpp:2:17]: (performance, inconclusive) Use const reference for 's2' to avoid unnecessary data copying. [redundantCopyLocalConst]\n"
"[test.cpp:1:20]: (performance) Function parameter 'str' should be passed by const reference. [passedByValue]\n",
errout_str());

check("void f(std::string str) {\n"
" std::string& s2 = str;\n"
Expand Down Expand Up @@ -9884,6 +9886,43 @@ class TestOther : public TestFixture {
" if (s.empty()) {}\n"
"}\n");
ASSERT_EQUALS("[test.cpp:6:16]: (performance, inconclusive) Use const reference for 's' to avoid unnecessary data copying. [redundantCopyLocalConst]\n", errout_str());

check("void f1(const std::string& s) {\n"
" std::string s1 = s;\n"
" (void)s1;\n"
"}\n"
"void f2() {\n"
" const std::string s;\n"
" std::string s1 = s;\n"
" (void)s1;\n"
"}\n");
ASSERT_EQUALS("[test.cpp:2:17]: (performance, inconclusive) Use const reference for 's1' to avoid unnecessary data copying. [redundantCopyLocalConst]\n"
"[test.cpp:7:17]: (performance, inconclusive) Use const reference for 's1' to avoid unnecessary data copying. [redundantCopyLocalConst]\n",
errout_str());

check("struct S {\n"
" std::string m;\n"
" int f(const std::string& s);\n"
"};\n"
"int S::f(const std::string& s) {\n"
" std::string c = s;\n"
" m.clear();\n"
" return c.size();\n"
"}\n");
ASSERT_EQUALS("", errout_str());

check("struct S {\n"
" std::string m;\n"
" int f(std::string s);\n"
"};\n"
"int S::f(std::string s) {\n"
" s += m;\n"
" std::string c = s;\n"
" m.clear();\n"
" return c.size();\n"
"}\n");
ASSERT_EQUALS("[test.cpp:7:17]: (performance, inconclusive) Use const reference for 'c' to avoid unnecessary data copying. [redundantCopyLocalConst]\n",
errout_str());
}

void checkNegativeShift() {
Expand Down
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