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Signed integer optimisations; Also warn about 'x+y<x'

This commit is contained in:
Daniel Marjamäki 2021-01-03 22:12:19 +01:00
parent 6375fb4f08
commit 52491f1c53
2 changed files with 61 additions and 12 deletions
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45
lib/checktype.cpp
View File

@ -228,15 +228,18 @@ void CheckType::integerOverflowError(const Token *tok, const ValueFlow::Value &v
void CheckType::checkIntegerOverflowOptimisations()
{
// Various patterns are defined here:
// Interesting blogs:
// https://kristerw.blogspot.com/2016/02/how-undefined-signed-overflow-enables.html
// https://research.checkpoint.com/2020/optout-compiler-undefined-behavior-optimizations/
// These optimisations seem to generate "unwanted" output
// x + c < x -> false
// x + c <= x -> false
// x + c > x -> true
// x + c >= x -> true
// x + y < x -> y < 0
if (!mSettings->isEnabled(Settings::PORTABILITY))
return;
@ -244,21 +247,39 @@ void CheckType::checkIntegerOverflowOptimisations()
if (!Token::Match(tok, "<|<=|>=|>") || !tok->isBinaryOp())
continue;
if (Token::Match(tok->astOperand1(), "[+-]") &&
tok->astOperand1()->valueType() &&
tok->astOperand1()->valueType()->isIntegral() &&
tok->astOperand1()->valueType()->sign == ValueType::Sign::SIGNED &&
tok->astOperand1()->isBinaryOp() &&
tok->astOperand1()->astOperand2()->isNumber() &&
Token::Match(tok->astOperand1()->astOperand1(), "%var%") &&
tok->astOperand1()->astOperand1()->str() == tok->astOperand2()->str()) {
const std::string cmp = tok->str();
const Token * const lhs = tok->astOperand1();
if (!Token::Match(lhs, "[+-]") || !lhs->isBinaryOp() || !lhs->valueType() || !lhs->valueType()->isIntegral() || lhs->valueType()->sign != ValueType::Sign::SIGNED)
continue;
const Token *expr = lhs->astOperand1();
const Token *other = lhs->astOperand2();
if (expr->varId() != lhs->astSibling()->varId())
continue;
// x [+-] c cmp x
if (other->isNumber() && other->getKnownIntValue() > 0) {
bool result;
if (tok->astOperand1()->str() == "+")
result = Token::Match(tok, ">|>=");
result = (cmp == ">" || cmp == ">=");
else
result = Token::Match(tok, "<|<=");
result = (cmp == "<" || cmp == "<=");
integerOverflowOptimisationError(tok, result ? "true" : "false");
}
// x + y cmp x
if (lhs->str() == "+" && other->varId() > 0) {
const std::string result = other->str() + cmp + "0";
integerOverflowOptimisationError(tok, result);
}
// x - y cmp x
if (lhs->str() == "-" && other->varId() > 0) {
std::string cmp2 = cmp;
cmp2[0] = (cmp[0] == '<') ? '>' : '<';
const std::string result = other->str() + cmp2 + "0";
integerOverflowOptimisationError(tok, result);
}
}
}

28
test/testtype.cpp
View File

@ -253,6 +253,8 @@ private:
Settings settings;
settings.addEnabled("warning");
// x + c < x
check("int f(int x) { return x + 10 > x; }", &settings);
ASSERT_EQUALS("[test.cpp:1]: (portability) There is a danger that 'x+10>x' will be optimised into 'true'. Signed integer overflow is undefined behavior.\n", errout.str());
@ -276,6 +278,32 @@ private:
check("int f(int x) { return x - 10 <= x; }", &settings);
ASSERT_EQUALS("[test.cpp:1]: (portability) There is a danger that 'x-10<=x' will be optimised into 'true'. Signed integer overflow is undefined behavior.\n", errout.str());
// x + y < x
check("int f(int x, int y) { return x + y < x; }", &settings);
ASSERT_EQUALS("[test.cpp:1]: (portability) There is a danger that 'x+y<x' will be optimised into 'y<0'. Signed integer overflow is undefined behavior.\n", errout.str());
check("int f(int x, int y) { return x + y <= x; }", &settings);
ASSERT_EQUALS("[test.cpp:1]: (portability) There is a danger that 'x+y<=x' will be optimised into 'y<=0'. Signed integer overflow is undefined behavior.\n", errout.str());
check("int f(int x, int y) { return x + y > x; }", &settings);
ASSERT_EQUALS("[test.cpp:1]: (portability) There is a danger that 'x+y>x' will be optimised into 'y>0'. Signed integer overflow is undefined behavior.\n", errout.str());
check("int f(int x, int y) { return x + y >= x; }", &settings);
ASSERT_EQUALS("[test.cpp:1]: (portability) There is a danger that 'x+y>=x' will be optimised into 'y>=0'. Signed integer overflow is undefined behavior.\n", errout.str());
// x - y < x
check("int f(int x, int y) { return x - y < x; }", &settings);
ASSERT_EQUALS("[test.cpp:1]: (portability) There is a danger that 'x-y<x' will be optimised into 'y>0'. Signed integer overflow is undefined behavior.\n", errout.str());
check("int f(int x, int y) { return x - y <= x; }", &settings);
ASSERT_EQUALS("[test.cpp:1]: (portability) There is a danger that 'x-y<=x' will be optimised into 'y>=0'. Signed integer overflow is undefined behavior.\n", errout.str());
check("int f(int x, int y) { return x - y > x; }", &settings);
ASSERT_EQUALS("[test.cpp:1]: (portability) There is a danger that 'x-y>x' will be optimised into 'y<0'. Signed integer overflow is undefined behavior.\n", errout.str());
check("int f(int x, int y) { return x - y >= x; }", &settings);
ASSERT_EQUALS("[test.cpp:1]: (portability) There is a danger that 'x-y>=x' will be optimised into 'y<=0'. Signed integer overflow is undefined behavior.\n", errout.str());
}
void signConversion() {