r259271 - Improve -Wconstant-conversion
Nico Weber via cfe-commits
cfe-commits at lists.llvm.org
Fri Feb 5 15:13:09 PST 2016
Thanks!
Maybe it's interesting if I list the instances where this fired in Chromium.
* libwebp calls _mm_set1_epi16(33050) (but then is careful to only use
unsigned intrinsics with this). There's no version of this function that
takes unsigned short, so the only thing to change here was to add a cast.
* Some function taking int was passed 2651229008. I don't understand why;
it was some machine_id thing. The parameter probably wanted to be unsigned.
* In two cases, a char array contained a 255 (this no longer fires after
your change)
* some file had `volatile short lineno = (__LINE__ << 8) + __COUNTER__;
(void)lineno;` in a macro to prevent the compiler from combining functions
(in dynamic_annotations.c) and that number became larget than 32k (but not
larger than 64k, so we changed that to unsigned short).
* finally we had the line `const int16_t kReservedManufacturerID = 1 << 15;`
None of these were bugs, but it seems like a good change anyhow. I can see
how this could catch bugs.
On Fri, Feb 5, 2016 at 6:07 PM, Richard Trieu <rtrieu at google.com> wrote:
> r259947 will stop this warning on char arrays.
>
> On Mon, Feb 1, 2016 at 1:40 PM, Richard Trieu <rtrieu at google.com> wrote:
>
>> C++11 narrowing only happens during certain conversions while this
>> warning checks all conversions.
>>
>> We might be able to avoid char array initialization, but it would be a
>> little tricky. These warning usually have little information about where
>> the conversion is coming from, so distinguishing array initialization
>> versus variable initialization could be hard.
>>
>> On Sat, Jan 30, 2016 at 6:41 AM, Nico Weber <thakis at chromium.org> wrote:
>>
>>> Shouldn't the new case be in -Wc++11-narrowing instead? This is warning
>>> in similar places where that warning used to warn.
>>>
>>> In practice, char arrays seem to be often used for storing binary blobs
>>> in header files, and those are usually initialized with numbers 0...255
>>> instead of -128...127 (why not make the array uint8_t then? Because maybe
>>> the function you want to pass the data from wants a char* array, and having
>>> to reinterpret_cast from uint8_t to char is annoying). Maybe this shouldn't
>>> fire for char arrays?
>>>
>>> On Fri, Jan 29, 2016 at 6:51 PM, Richard Trieu via cfe-commits <
>>> cfe-commits at lists.llvm.org> wrote:
>>>
>>>> Author: rtrieu
>>>> Date: Fri Jan 29 17:51:16 2016
>>>> New Revision: 259271
>>>>
>>>> URL: http://llvm.org/viewvc/llvm-project?rev=259271&view=rev
>>>> Log:
>>>> Improve -Wconstant-conversion
>>>>
>>>> Switch the evaluation from isIntegerConstantExpr to EvaluateAsInt.
>>>> EvaluateAsInt will evaluate more types of expressions than
>>>> isIntegerConstantExpr.
>>>>
>>>> Move one case from -Wsign-conversion to -Wconstant-conversion. The
>>>> case is:
>>>> 1) Source and target types are signed
>>>> 2) Source type is wider than the target type
>>>> 3) The source constant value is positive
>>>> 4) The conversion will store the value as negative in the target.
>>>>
>>>> Modified:
>>>> cfe/trunk/lib/Sema/SemaChecking.cpp
>>>> cfe/trunk/test/CXX/expr/expr.const/p2-0x.cpp
>>>> cfe/trunk/test/Sema/constant-conversion.c
>>>>
>>>> Modified: cfe/trunk/lib/Sema/SemaChecking.cpp
>>>> URL:
>>>> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaChecking.cpp?rev=259271&r1=259270&r2=259271&view=diff
>>>>
>>>> ==============================================================================
>>>> --- cfe/trunk/lib/Sema/SemaChecking.cpp (original)
>>>> +++ cfe/trunk/lib/Sema/SemaChecking.cpp Fri Jan 29 17:51:16 2016
>>>> @@ -7578,7 +7578,7 @@ void CheckImplicitConversion(Sema &S, Ex
>>>> // If the source is a constant, use a default-on diagnostic.
>>>> // TODO: this should happen for bitfield stores, too.
>>>> llvm::APSInt Value(32);
>>>> - if (E->isIntegerConstantExpr(Value, S.Context)) {
>>>> + if (E->EvaluateAsInt(Value, S.Context, Expr::SE_AllowSideEffects))
>>>> {
>>>> if (S.SourceMgr.isInSystemMacro(CC))
>>>> return;
>>>>
>>>> @@ -7603,6 +7603,42 @@ void CheckImplicitConversion(Sema &S, Ex
>>>> return DiagnoseImpCast(S, E, T, CC,
>>>> diag::warn_impcast_integer_precision);
>>>> }
>>>>
>>>> + if (TargetRange.Width == SourceRange.Width &&
>>>> !TargetRange.NonNegative &&
>>>> + SourceRange.NonNegative && Source->isSignedIntegerType()) {
>>>> + // Warn when doing a signed to signed conversion, warn if the
>>>> positive
>>>> + // source value is exactly the width of the target type, which will
>>>> + // cause a negative value to be stored.
>>>> +
>>>> + llvm::APSInt Value;
>>>> + if (E->EvaluateAsInt(Value, S.Context, Expr::SE_AllowSideEffects))
>>>> {
>>>> + if (!S.SourceMgr.isInSystemMacro(CC)) {
>>>> +
>>>> + IntegerLiteral *IntLit =
>>>> + dyn_cast<IntegerLiteral>(E->IgnoreParenImpCasts());
>>>> +
>>>> + // If initializing from a constant, and the constant starts
>>>> with '0',
>>>> + // then it is a binary, octal, or hexadecimal. Allow these
>>>> constants
>>>> + // to fill all the bits, even if there is a sign change.
>>>> + if (!IntLit ||
>>>> +
>>>> *(S.getSourceManager().getCharacterData(IntLit->getLocStart())) !=
>>>> + '0') {
>>>> +
>>>> + std::string PrettySourceValue = Value.toString(10);
>>>> + std::string PrettyTargetValue =
>>>> + PrettyPrintInRange(Value, TargetRange);
>>>> +
>>>> + S.DiagRuntimeBehavior(
>>>> + E->getExprLoc(), E,
>>>> + S.PDiag(diag::warn_impcast_integer_precision_constant)
>>>> + << PrettySourceValue << PrettyTargetValue <<
>>>> E->getType() << T
>>>> + << E->getSourceRange() << clang::SourceRange(CC));
>>>> + return;
>>>> + }
>>>> + }
>>>> + }
>>>> + // Fall through for non-constants to give a sign conversion
>>>> warning.
>>>> + }
>>>> +
>>>> if ((TargetRange.NonNegative && !SourceRange.NonNegative) ||
>>>> (!TargetRange.NonNegative && SourceRange.NonNegative &&
>>>> SourceRange.Width == TargetRange.Width)) {
>>>>
>>>> Modified: cfe/trunk/test/CXX/expr/expr.const/p2-0x.cpp
>>>> URL:
>>>> http://llvm.org/viewvc/llvm-project/cfe/trunk/test/CXX/expr/expr.const/p2-0x.cpp?rev=259271&r1=259270&r2=259271&view=diff
>>>>
>>>> ==============================================================================
>>>> --- cfe/trunk/test/CXX/expr/expr.const/p2-0x.cpp (original)
>>>> +++ cfe/trunk/test/CXX/expr/expr.const/p2-0x.cpp Fri Jan 29 17:51:16
>>>> 2016
>>>> @@ -242,8 +242,8 @@ namespace UndefinedBehavior {
>>>> constexpr int n13 = n5 + n5; // expected-error {{constant
>>>> expression}} expected-note {{value -4294967296 is outside the range of }}
>>>> constexpr int n14 = n3 - n5; // expected-error {{constant
>>>> expression}} expected-note {{value 4294967295 is outside the range of }}
>>>> constexpr int n15 = n5 * n5; // expected-error {{constant
>>>> expression}} expected-note {{value 4611686018427387904 is outside the range
>>>> of }}
>>>> - constexpr signed char c1 = 100 * 2; // ok
>>>> - constexpr signed char c2 = '\x64' * '\2'; // also ok
>>>> + constexpr signed char c1 = 100 * 2; // ok
>>>> expected-warning{{changes value}}
>>>> + constexpr signed char c2 = '\x64' * '\2'; // also ok
>>>> expected-warning{{changes value}}
>>>> constexpr long long ll1 = 0x7fffffffffffffff; // ok
>>>> constexpr long long ll2 = ll1 + 1; // expected-error {{constant}}
>>>> expected-note {{ 9223372036854775808 }}
>>>> constexpr long long ll3 = -ll1 - 1; // ok
>>>>
>>>> Modified: cfe/trunk/test/Sema/constant-conversion.c
>>>> URL:
>>>> http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Sema/constant-conversion.c?rev=259271&r1=259270&r2=259271&view=diff
>>>>
>>>> ==============================================================================
>>>> --- cfe/trunk/test/Sema/constant-conversion.c (original)
>>>> +++ cfe/trunk/test/Sema/constant-conversion.c Fri Jan 29 17:51:16 2016
>>>> @@ -80,3 +80,34 @@ void test8() {
>>>> struct { enum E x : 1; } f;
>>>> f.x = C; // expected-warning {{implicit truncation from 'int' to
>>>> bitfield changes value from 2 to 0}}
>>>> }
>>>> +
>>>> +void test9() {
>>>> + const char max_char = 0x7F;
>>>> + const short max_short = 0x7FFF;
>>>> + const int max_int = 0x7FFFFFFF;
>>>> +
>>>> + const short max_char_plus_one = (short)max_char + 1;
>>>> + const int max_short_plus_one = (int)max_short + 1;
>>>> + const long max_int_plus_one = (long)max_int + 1;
>>>> +
>>>> + char new_char = max_char_plus_one; // expected-warning {{implicit
>>>> conversion from 'const short' to 'char' changes value from 128 to -128}}
>>>> + short new_short = max_short_plus_one; // expected-warning
>>>> {{implicit conversion from 'const int' to 'short' changes value from 32768
>>>> to -32768}}
>>>> + int new_int = max_int_plus_one; // expected-warning {{implicit
>>>> conversion from 'const long' to 'int' changes value from 2147483648 to
>>>> -2147483648}}
>>>> +
>>>> + char hex_char = 0x80;
>>>> + short hex_short = 0x8000;
>>>> + int hex_int = 0x80000000;
>>>> +
>>>> + char oct_char = 0200;
>>>> + short oct_short = 0100000;
>>>> + int oct_int = 020000000000;
>>>> +
>>>> + char bin_char = 0b10000000;
>>>> + short bin_short = 0b1000000000000000;
>>>> + int bin_int = 0b10000000000000000000000000000000;
>>>> +
>>>> +#define CHAR_MACRO_HEX 0xff
>>>> + char macro_char_hex = CHAR_MACRO_HEX;
>>>> +#define CHAR_MACRO_DEC 255
>>>> + char macro_char_dec = CHAR_MACRO_DEC; // expected-warning
>>>> {{implicit conversion from 'int' to 'char' changes value from 255 to -1}}
>>>> +}
>>>>
>>>>
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>>>>
>>>
>>>
>>
>
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