On Fri, Sep 8, 2017 at 4:48 AM, Sam McCall via cfe-commits
<cfe-commits@lists.llvm.org> wrote:
> Nice fix! It catches a lot of new cases on our codebase, all technically
> correct so far.
>
> A couple of issues though:
> A) Rollout - until we've completely cleaned up, we need to disable
> -Wtautological-compare entirely, which is a valuable check. I imagine anyone
> else using -Werror is in the same boat.
> What do you think about putting the new warnings behind a subcategory? (e.g.
> -Wtautological-compare-unsigned-zero, implied by -Wtautological-compare)
> It's an ugly artifact of the history here, but allows this fix to be rolled
> out in a controlled way.
>
> B) Enums (not new I guess). Typical case: if (enum < 0 || enum > MAX)
> The warning strongly suggests that the enum < 0 check has no effect (for
> enums with nonnegative ranges).
> Clang doesn't seem to optimize such checks out though, and they seem likely
> to catch bugs in some cases. Yes, only if there's UB elsewhere, but I assume
> not optimizing out these checks indicates a deliberate decision to stay
> somewhat compatible with a technically-incorrect mental model.
> If this is the case, should we move these to a -Wtautological-compare-enum
> subcategory?
Just another data point: we're seeing this a lot in Chromium, e.g.
enum MediaDeviceType {
MEDIA_DEVICE_TYPE_AUDIO_INPUT,
MEDIA_DEVICE_TYPE_VIDEO_INPUT,
MEDIA_DEVICE_TYPE_AUDIO_OUTPUT,
NUM_MEDIA_DEVICE_TYPES,
};
inline bool IsValidMediaDeviceType(MediaDeviceType type) {
return type >= 0 && type < NUM_MEDIA_DEVICE_TYPES;
}
../../content/common/media/media_devices.h:53:15: warning: comparison
of unsigned enum expression >= 0 is always true
[-Wtautological-unsigned-zero-compare]
return type >= 0 && type < NUM_MEDIA_DEVICE_TYPES;
~~~~ ^ ~
The problem is that the signedness of the enum varies per platform.
For example, when targeting Windows the underlying type is 'int' by
default, and then the comparison isn't tautological.
> On Fri, Sep 8, 2017 at 12:14 AM, Roman Lebedev via cfe-commits
> <cfe-commits@lists.llvm.org> wrote:
>>
>> Author: lebedevri
>> Date: Thu Sep 7 15:14:25 2017
>> New Revision: 312750
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=312750&view=rev
>> Log:
>> [Sema] -Wtautological-compare: handle comparison of unsigned with 0S.
>>
>> Summary:
>> This is a first half(?) of a fix for the following bug:
>> https://bugs.llvm.org/show_bug.cgi?id=34147 (gcc -Wtype-limits)
>>
>> GCC's -Wtype-limits does warn on comparison of unsigned value
>> with signed zero (as in, with 0), but clang only warns if the
>> zero is unsigned (i.e. 0U).
>>
>> Also, be careful not to double-warn, or falsely warn on
>> comparison of signed/fp variable and signed 0.
>>
>> Yes, all these testcases are needed.
>>
>> Testing: $ ninja check-clang-sema check-clang-semacxx
>> Also, no new warnings for clang stage-2 build.
>>
>> Reviewers: rjmccall, rsmith, aaron.ballman
>>
>> Reviewed By: rjmccall
>>
>> Subscribers: cfe-commits
>>
>> Tags: #clang
>>
>> Differential Revision: https://reviews.llvm.org/D37565
>>
>> Modified:
>> cfe/trunk/docs/ReleaseNotes.rst
>> cfe/trunk/lib/Sema/SemaChecking.cpp
>> cfe/trunk/test/Sema/compare.c
>> cfe/trunk/test/Sema/outof-range-constant-compare.c
>> cfe/trunk/test/SemaCXX/compare.cpp
>>
>> Modified: cfe/trunk/docs/ReleaseNotes.rst
>> URL:
>> http://llvm.org/viewvc/llvm-project/cfe/trunk/docs/ReleaseNotes.rst?rev=312750&r1=312749&r2=312750&view=diff
>>
>> ==============================================================================
>> --- cfe/trunk/docs/ReleaseNotes.rst (original)
>> +++ cfe/trunk/docs/ReleaseNotes.rst Thu Sep 7 15:14:25 2017
>> @@ -71,6 +71,13 @@ Improvements to Clang's diagnostics
>> errors/warnings, as the system frameworks might add a method with the
>> same
>> selector which could make the message send to ``id`` ambiguous.
>>
>> +- ``-Wtautological-compare`` now warns when comparing an unsigned integer
>> and 0
>> + regardless of whether the constant is signed or unsigned."
>> +
>> +- ``-Wtautological-compare`` now warns about comparing a signed integer
>> and 0
>> + when the signed integer is coerced to an unsigned type for the
>> comparison.
>> + ``-Wsign-compare`` was adjusted not to warn in this case.
>> +
>> Non-comprehensive list of changes in this release
>> -------------------------------------------------
>>
>>
>> Modified: cfe/trunk/lib/Sema/SemaChecking.cpp
>> URL:
>> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaChecking.cpp?rev=312750&r1=312749&r2=312750&view=diff
>>
>> ==============================================================================
>> --- cfe/trunk/lib/Sema/SemaChecking.cpp (original)
>> +++ cfe/trunk/lib/Sema/SemaChecking.cpp Thu Sep 7 15:14:25 2017
>> @@ -8567,32 +8567,51 @@ bool HasEnumType(Expr *E) {
>> return E->getType()->isEnumeralType();
>> }
>>
>> -void CheckTrivialUnsignedComparison(Sema &S, BinaryOperator *E) {
>> +bool isNonBooleanUnsignedValue(Expr *E) {
>> + // We are checking that the expression is not known to have boolean
>> value,
>> + // is an integer type; and is either unsigned after implicit casts,
>> + // or was unsigned before implicit casts.
>> + return !E->isKnownToHaveBooleanValue() && E->getType()->isIntegerType()
>> &&
>> + (!E->getType()->isSignedIntegerType() ||
>> + !E->IgnoreParenImpCasts()->getType()->isSignedIntegerType());
>> +}
>> +
>> +bool CheckTautologicalComparisonWithZero(Sema &S, BinaryOperator *E) {
>> // Disable warning in template instantiations.
>> if (S.inTemplateInstantiation())
>> - return;
>> + return false;
>> +
>> + // bool values are handled by DiagnoseOutOfRangeComparison().
>>
>> BinaryOperatorKind op = E->getOpcode();
>> if (E->isValueDependent())
>> - return;
>> + return false;
>>
>> - if (op == BO_LT && IsZero(S, E->getRHS())) {
>> + Expr *LHS = E->getLHS();
>> + Expr *RHS = E->getRHS();
>> +
>> + bool Match = true;
>> +
>> + if (op == BO_LT && isNonBooleanUnsignedValue(LHS) && IsZero(S, RHS)) {
>> S.Diag(E->getOperatorLoc(),
>> diag::warn_lunsigned_always_true_comparison)
>> - << "< 0" << "false" << HasEnumType(E->getLHS())
>> - << E->getLHS()->getSourceRange() << E->getRHS()->getSourceRange();
>> - } else if (op == BO_GE && IsZero(S, E->getRHS())) {
>> + << "< 0" << "false" << HasEnumType(LHS)
>> + << LHS->getSourceRange() << RHS->getSourceRange();
>> + } else if (op == BO_GE && isNonBooleanUnsignedValue(LHS) && IsZero(S,
>> RHS)) {
>> S.Diag(E->getOperatorLoc(),
>> diag::warn_lunsigned_always_true_comparison)
>> - << ">= 0" << "true" << HasEnumType(E->getLHS())
>> - << E->getLHS()->getSourceRange() << E->getRHS()->getSourceRange();
>> - } else if (op == BO_GT && IsZero(S, E->getLHS())) {
>> + << ">= 0" << "true" << HasEnumType(LHS)
>> + << LHS->getSourceRange() << RHS->getSourceRange();
>> + } else if (op == BO_GT && isNonBooleanUnsignedValue(RHS) && IsZero(S,
>> LHS)) {
>> S.Diag(E->getOperatorLoc(),
>> diag::warn_runsigned_always_true_comparison)
>> - << "0 >" << "false" << HasEnumType(E->getRHS())
>> - << E->getLHS()->getSourceRange() << E->getRHS()->getSourceRange();
>> - } else if (op == BO_LE && IsZero(S, E->getLHS())) {
>> + << "0 >" << "false" << HasEnumType(RHS)
>> + << LHS->getSourceRange() << RHS->getSourceRange();
>> + } else if (op == BO_LE && isNonBooleanUnsignedValue(RHS) && IsZero(S,
>> LHS)) {
>> S.Diag(E->getOperatorLoc(),
>> diag::warn_runsigned_always_true_comparison)
>> - << "0 <=" << "true" << HasEnumType(E->getRHS())
>> - << E->getLHS()->getSourceRange() << E->getRHS()->getSourceRange();
>> - }
>> + << "0 <=" << "true" << HasEnumType(RHS)
>> + << LHS->getSourceRange() << RHS->getSourceRange();
>> + } else
>> + Match = false;
>> +
>> + return Match;
>> }
>>
>> void DiagnoseOutOfRangeComparison(Sema &S, BinaryOperator *E, Expr
>> *Constant,
>> @@ -8612,7 +8631,7 @@ void DiagnoseOutOfRangeComparison(Sema &
>>
>> bool OtherIsBooleanType = Other->isKnownToHaveBooleanValue();
>>
>> - // 0 values are handled later by CheckTrivialUnsignedComparison().
>> + // 0 values are handled later by CheckTautologicalComparisonWithZero().
>> if ((Value == 0) && (!OtherIsBooleanType))
>> return;
>>
>> @@ -8849,16 +8868,22 @@ void AnalyzeComparison(Sema &S, BinaryOp
>> (IsRHSIntegralLiteral && IsLHSIntegralLiteral);
>> } else if (!T->hasUnsignedIntegerRepresentation())
>> IsComparisonConstant = E->isIntegerConstantExpr(S.Context);
>> -
>> +
>> + // We don't care about value-dependent expressions or expressions
>> + // whose result is a constant.
>> + if (IsComparisonConstant)
>> + return AnalyzeImpConvsInComparison(S, E);
>> +
>> + // If this is a tautological comparison, suppress -Wsign-compare.
>> + if (CheckTautologicalComparisonWithZero(S, E))
>> + return AnalyzeImpConvsInComparison(S, E);
>> +
>> // We don't do anything special if this isn't an unsigned integral
>> // comparison: we're only interested in integral comparisons, and
>> // signed comparisons only happen in cases we don't care to warn about.
>> - //
>> - // We also don't care about value-dependent expressions or expressions
>> - // whose result is a constant.
>> - if (!T->hasUnsignedIntegerRepresentation() || IsComparisonConstant)
>> + if (!T->hasUnsignedIntegerRepresentation())
>> return AnalyzeImpConvsInComparison(S, E);
>> -
>> +
>> // Check to see if one of the (unmodified) operands is of different
>> // signedness.
>> Expr *signedOperand, *unsignedOperand;
>> @@ -8871,7 +8896,6 @@ void AnalyzeComparison(Sema &S, BinaryOp
>> signedOperand = RHS;
>> unsignedOperand = LHS;
>> } else {
>> - CheckTrivialUnsignedComparison(S, E);
>> return AnalyzeImpConvsInComparison(S, E);
>> }
>>
>> @@ -8883,11 +8907,9 @@ void AnalyzeComparison(Sema &S, BinaryOp
>> AnalyzeImplicitConversions(S, LHS, E->getOperatorLoc());
>> AnalyzeImplicitConversions(S, RHS, E->getOperatorLoc());
>>
>> - // If the signed range is non-negative, -Wsign-compare won't fire,
>> - // but we should still check for comparisons which are always true
>> - // or false.
>> + // If the signed range is non-negative, -Wsign-compare won't fire.
>> if (signedRange.NonNegative)
>> - return CheckTrivialUnsignedComparison(S, E);
>> + return;
>>
>> // For (in)equality comparisons, if the unsigned operand is a
>> // constant which cannot collide with a overflowed signed operand,
>>
>> Modified: cfe/trunk/test/Sema/compare.c
>> URL:
>> http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Sema/compare.c?rev=312750&r1=312749&r2=312750&view=diff
>>
>> ==============================================================================
>> --- cfe/trunk/test/Sema/compare.c (original)
>> +++ cfe/trunk/test/Sema/compare.c Thu Sep 7 15:14:25 2017
>> @@ -77,7 +77,7 @@ int ints(long a, unsigned long b) {
>> ((int) a == (unsigned int) B) +
>> ((short) a == (unsigned short) B) +
>> ((signed char) a == (unsigned char) B) +
>> - (a < (unsigned long) B) + // expected-warning {{comparison of
>> integers of different signs}}
>> + (a < (unsigned long) B) + // expected-warning {{comparison of
>> unsigned expression < 0 is always false}}
>> (a < (unsigned int) B) +
>> (a < (unsigned short) B) +
>> (a < (unsigned char) B) +
>> @@ -85,8 +85,8 @@ int ints(long a, unsigned long b) {
>> ((int) a < B) +
>> ((short) a < B) +
>> ((signed char) a < B) +
>> - ((long) a < (unsigned long) B) + // expected-warning
>> {{comparison of integers of different signs}}
>> - ((int) a < (unsigned int) B) + // expected-warning {{comparison
>> of integers of different signs}}
>> + ((long) a < (unsigned long) B) + // expected-warning
>> {{comparison of unsigned expression < 0 is always false}}
>> + ((int) a < (unsigned int) B) + // expected-warning {{comparison
>> of unsigned expression < 0 is always false}}
>> ((short) a < (unsigned short) B) +
>> ((signed char) a < (unsigned char) B) +
>>
>>
>> Modified: cfe/trunk/test/Sema/outof-range-constant-compare.c
>> URL:
>> http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Sema/outof-range-constant-compare.c?rev=312750&r1=312749&r2=312750&view=diff
>>
>> ==============================================================================
>> --- cfe/trunk/test/Sema/outof-range-constant-compare.c (original)
>> +++ cfe/trunk/test/Sema/outof-range-constant-compare.c Thu Sep 7 15:14:25
>> 2017
>> @@ -6,6 +6,59 @@ int value(void);
>> int main()
>> {
>> int a = value();
>> +
>> + if (a == 0x0000000000000000L)
>> + return 0;
>> + if (a != 0x0000000000000000L)
>> + return 0;
>> + if (a < 0x0000000000000000L)
>> + return 0;
>> + if (a <= 0x0000000000000000L)
>> + return 0;
>> + if (a > 0x0000000000000000L)
>> + return 0;
>> + if (a >= 0x0000000000000000L)
>> + return 0;
>> +
>> + if (0x0000000000000000L == a)
>> + return 0;
>> + if (0x0000000000000000L != a)
>> + return 0;
>> + if (0x0000000000000000L < a)
>> + return 0;
>> + if (0x0000000000000000L <= a)
>> + return 0;
>> + if (0x0000000000000000L > a)
>> + return 0;
>> + if (0x0000000000000000L >= a)
>> + return 0;
>> +
>> + if (a == 0x0000000000000000UL)
>> + return 0;
>> + if (a != 0x0000000000000000UL)
>> + return 0;
>> + if (a < 0x0000000000000000UL) // expected-warning {{comparison of
>> unsigned expression < 0 is always false}}
>> + return 0;
>> + if (a <= 0x0000000000000000UL)
>> + return 0;
>> + if (a > 0x0000000000000000UL)
>> + return 0;
>> + if (a >= 0x0000000000000000UL) // expected-warning {{comparison of
>> unsigned expression >= 0 is always true}}
>> + return 0;
>> +
>> + if (0x0000000000000000UL == a)
>> + return 0;
>> + if (0x0000000000000000UL != a)
>> + return 0;
>> + if (0x0000000000000000UL < a)
>> + return 0;
>> + if (0x0000000000000000UL <= a) // expected-warning {{comparison of 0
>> <= unsigned expression is always true}}
>> + return 0;
>> + if (0x0000000000000000UL > a) // expected-warning {{comparison of 0 >
>> unsigned expression is always false}}
>> + return 0;
>> + if (0x0000000000000000UL >= a)
>> + return 0;
>> +
>> if (a == 0x1234567812345678L) // expected-warning {{comparison of
>> constant 1311768465173141112 with expression of type 'int' is always false}}
>> return 0;
>> if (a != 0x1234567812345678L) // expected-warning {{comparison of
>> constant 1311768465173141112 with expression of type 'int' is always true}}
>> @@ -74,6 +127,7 @@ int main()
>> return 0;
>> if (0x1234567812345678L >= s) // expected-warning {{comparison of
>> constant 1311768465173141112 with expression of type 'short' is always
>> true}}
>> return 0;
>> +
>> long l = value();
>> if (l == 0x1234567812345678L)
>> return 0;
>> @@ -106,13 +160,13 @@ int main()
>> return 0;
>> if (un != 0x0000000000000000L)
>> return 0;
>> - if (un < 0x0000000000000000L)
>> + if (un < 0x0000000000000000L) // expected-warning {{comparison of
>> unsigned expression < 0 is always false}}
>> return 0;
>> if (un <= 0x0000000000000000L)
>> return 0;
>> if (un > 0x0000000000000000L)
>> return 0;
>> - if (un >= 0x0000000000000000L)
>> + if (un >= 0x0000000000000000L) // expected-warning {{comparison of
>> unsigned expression >= 0 is always true}}
>> return 0;
>>
>> if (0x0000000000000000L == un)
>> @@ -121,19 +175,92 @@ int main()
>> return 0;
>> if (0x0000000000000000L < un)
>> return 0;
>> - if (0x0000000000000000L <= un)
>> + if (0x0000000000000000L <= un) // expected-warning {{comparison of 0
>> <= unsigned expression is always true}}
>> return 0;
>> - if (0x0000000000000000L > un)
>> + if (0x0000000000000000L > un) // expected-warning {{comparison of 0 >
>> unsigned expression is always false}}
>> return 0;
>> if (0x0000000000000000L >= un)
>> return 0;
>> +
>> + if (un == 0x0000000000000000UL)
>> + return 0;
>> + if (un != 0x0000000000000000UL)
>> + return 0;
>> + if (un < 0x0000000000000000UL) // expected-warning {{comparison of
>> unsigned expression < 0 is always false}}
>> + return 0;
>> + if (un <= 0x0000000000000000UL)
>> + return 0;
>> + if (un > 0x0000000000000000UL)
>> + return 0;
>> + if (un >= 0x0000000000000000UL) // expected-warning {{comparison of
>> unsigned expression >= 0 is always true}}
>> + return 0;
>> +
>> + if (0x0000000000000000UL == un)
>> + return 0;
>> + if (0x0000000000000000UL != un)
>> + return 0;
>> + if (0x0000000000000000UL < un)
>> + return 0;
>> + if (0x0000000000000000UL <= un) // expected-warning {{comparison of 0
>> <= unsigned expression is always true}}
>> + return 0;
>> + if (0x0000000000000000UL > un) // expected-warning {{comparison of 0
>> > unsigned expression is always false}}
>> + return 0;
>> + if (0x0000000000000000UL >= un)
>> + return 0;
>> +
>> float fl = 0;
>> - if (fl == 0x0000000000000000L) // no warning
>> - return 0;
>> + if (fl == 0x0000000000000000L)
>> + return 0;
>> + if (fl != 0x0000000000000000L)
>> + return 0;
>> + if (fl < 0x0000000000000000L)
>> + return 0;
>> + if (fl <= 0x0000000000000000L)
>> + return 0;
>> + if (fl > 0x0000000000000000L)
>> + return 0;
>> + if (fl >= 0x0000000000000000L)
>> + return 0;
>>
>> - float dl = 0;
>> - if (dl == 0x0000000000000000L) // no warning
>> - return 0;
>> + if (0x0000000000000000L == fl)
>> + return 0;
>> + if (0x0000000000000000L != fl)
>> + return 0;
>> + if (0x0000000000000000L < fl)
>> + return 0;
>> + if (0x0000000000000000L <= fl)
>> + return 0;
>> + if (0x0000000000000000L > fl)
>> + return 0;
>> + if (0x0000000000000000L >= fl)
>> + return 0;
>> +
>> + double dl = 0;
>> + if (dl == 0x0000000000000000L)
>> + return 0;
>> + if (dl != 0x0000000000000000L)
>> + return 0;
>> + if (dl < 0x0000000000000000L)
>> + return 0;
>> + if (dl <= 0x0000000000000000L)
>> + return 0;
>> + if (dl > 0x0000000000000000L)
>> + return 0;
>> + if (dl >= 0x0000000000000000L)
>> + return 0;
>> +
>> + if (0x0000000000000000L == dl)
>> + return 0;
>> + if (0x0000000000000000L != dl)
>> + return 0;
>> + if (0x0000000000000000L < dl)
>> + return 0;
>> + if (0x0000000000000000L <= dl)
>> + return 0;
>> + if (0x0000000000000000L > dl)
>> + return 0;
>> + if (0x0000000000000000L >= dl)
>> + return 0;
>>
>> enum E {
>> yes,
>>
>> Modified: cfe/trunk/test/SemaCXX/compare.cpp
>> URL:
>> http://llvm.org/viewvc/llvm-project/cfe/trunk/test/SemaCXX/compare.cpp?rev=312750&r1=312749&r2=312750&view=diff
>>
>> ==============================================================================
>> --- cfe/trunk/test/SemaCXX/compare.cpp (original)
>> +++ cfe/trunk/test/SemaCXX/compare.cpp Thu Sep 7 15:14:25 2017
>> @@ -73,7 +73,7 @@ int test0(long a, unsigned long b) {
>> ((int) a == (unsigned int) B) +
>> ((short) a == (unsigned short) B) +
>> ((signed char) a == (unsigned char) B) +
>> - (a < (unsigned long) B) + // expected-warning {{comparison of
>> integers of different signs}}
>> + (a < (unsigned long) B) + // expected-warning {{comparison of
>> unsigned expression < 0 is always false}}
>> (a < (unsigned int) B) +
>> (a < (unsigned short) B) +
>> (a < (unsigned char) B) +
>> @@ -81,8 +81,8 @@ int test0(long a, unsigned long b) {
>> ((int) a < B) +
>> ((short) a < B) +
>> ((signed char) a < B) +
>> - ((long) a < (unsigned long) B) + // expected-warning
>> {{comparison of integers of different signs}}
>> - ((int) a < (unsigned int) B) + // expected-warning {{comparison
>> of integers of different signs}}
>> + ((long) a < (unsigned long) B) + // expected-warning
>> {{comparison of unsigned expression < 0 is always false}}
>> + ((int) a < (unsigned int) B) + // expected-warning {{comparison
>> of unsigned expression < 0 is always false}}
>> ((short) a < (unsigned short) B) +
>> ((signed char) a < (unsigned char) B) +
>>
>>
>>
>> _______________________________________________
>> cfe-commits mailing list
>> cfe-commits@lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
>
>
>
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