[cfe-dev] r347339 - [clang][Parse] Diagnose useless null statements / empty init-statements

Aaron Ballman via cfe-dev cfe-dev at lists.llvm.org
Tue Dec 4 16:47:46 PST 2018


On Tue, Dec 4, 2018 at 7:35 PM George Karpenkov <ekarpenkov at apple.com> wrote:
>
> Hi Roman,
>
> I’m against this new warning.
>
> A very common idiom is defining a “DEBUG” macro to be a no-op in release, and a logging function otherwise.
> The new introduced warning warns on all such cases (e.g. 'DEBUG(“mystring”);')

Yeah, that's not good.

> As noted in the review, Clang warnings should generally not be used to diagnose style issues — and an extra semicolon does not seem to be a common bug source.
>
> It is a problem in practice for us since we have projects which enable all available warnings and also enable “-Werror”.

Would you be okay with the warning if it was only diagnosed when the
source location of the semi-colon is not immediately preceded by a
macro expansion location? e.g.,

EMPTY_EXPANSION(12); // Not diagnosed
EMPTY_EXPANSION; // Not diagnosed
; // Diagnosed

~Aaron

>
> Regards,
> George
>
>
> > On Nov 20, 2018, at 10:59 AM, Roman Lebedev via cfe-commits <cfe-commits at lists.llvm.org> wrote:
> >
> > Author: lebedevri
> > Date: Tue Nov 20 10:59:05 2018
> > New Revision: 347339
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=347339&view=rev
> > Log:
> > [clang][Parse] Diagnose useless null statements / empty init-statements
> >
> > Summary:
> > clang has `-Wextra-semi` (D43162), which is not dictated by the currently selected standard.
> > While that is great, there is at least one more source of need-less semis - 'null statements'.
> > Sometimes, they are needed:
> > ```
> > for(int x = 0; continueToDoWork(x); x++)
> >  ; // Ugly code, but the semi is needed here.
> > ```
> >
> > But sometimes they are just there for no reason:
> > ```
> > switch(X) {
> > case 0:
> >  return -2345;
> > case 5:
> >  return 0;
> > default:
> >  return 42;
> > }; // <- oops
> >
> > ;;;;;;;;;;; <- OOOOPS, still not diagnosed. Clearly this is junk.
> > ```
> >
> > Additionally:
> > ```
> > if(; // <- empty init-statement
> >   true)
> >  ;
> >
> > switch (; // empty init-statement
> >        x) {
> >  ...
> > }
> >
> > for (; // <- empty init-statement
> >     int y : S())
> >  ;
> > }
> >
> > As usual, things may or may not go sideways in the presence of macros.
> > While evaluating this diag on my codebase of interest, it was unsurprisingly
> > discovered that Google Test macros are *very* prone to this.
> > And it seems many issues are deep within the GTest itself, not
> > in the snippets passed from the codebase that uses GTest.
> >
> > So after some thought, i decided not do issue a diagnostic if the semi
> > is within *any* macro, be it either from the normal header, or system header.
> >
> > Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=39111 | PR39111 ]]
> >
> > Reviewers: rsmith, aaron.ballman, efriedma
> >
> > Reviewed By: aaron.ballman
> >
> > Subscribers: cfe-commits
> >
> > Differential Revision: https://reviews.llvm.org/D52695
> >
> > Added:
> >    cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt-in-init-statement.cpp
> >    cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt.cpp
> > Modified:
> >    cfe/trunk/docs/ReleaseNotes.rst
> >    cfe/trunk/include/clang/Basic/DiagnosticGroups.td
> >    cfe/trunk/include/clang/Basic/DiagnosticParseKinds.td
> >    cfe/trunk/include/clang/Parse/Parser.h
> >    cfe/trunk/lib/Parse/ParseExprCXX.cpp
> >    cfe/trunk/lib/Parse/ParseStmt.cpp
> >
> > Modified: cfe/trunk/docs/ReleaseNotes.rst
> > URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/docs/ReleaseNotes.rst?rev=347339&r1=347338&r2=347339&view=diff
> > ==============================================================================
> > --- cfe/trunk/docs/ReleaseNotes.rst (original)
> > +++ cfe/trunk/docs/ReleaseNotes.rst Tue Nov 20 10:59:05 2018
> > @@ -55,6 +55,63 @@ Major New Features
> > Improvements to Clang's diagnostics
> > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >
> > +- ``-Wextra-semi-stmt`` is a new diagnostic that diagnoses extra semicolons,
> > +  much like ``-Wextra-semi``. This new diagnostic diagnoses all *unnecessary*
> > +  null statements (expression statements without an expression), unless: the
> > +  semicolon directly follows a macro that was expanded to nothing or if the
> > +  semicolon is within the macro itself. This applies to macros defined in system
> > +  headers as well as user-defined macros.
> > +
> > +  .. code-block:: c++
> > +
> > +      #define MACRO(x) int x;
> > +      #define NULLMACRO(varname)
> > +
> > +      void test() {
> > +        ; // <- warning: ';' with no preceding expression is a null statement
> > +
> > +        while (true)
> > +          ; // OK, it is needed.
> > +
> > +        switch (my_enum) {
> > +        case E1:
> > +          // stuff
> > +          break;
> > +        case E2:
> > +          ; // OK, it is needed.
> > +        }
> > +
> > +        MACRO(v0;) // Extra semicolon, but within macro, so ignored.
> > +
> > +        MACRO(v1); // <- warning: ';' with no preceding expression is a null statement
> > +
> > +        NULLMACRO(v2); // ignored, NULLMACRO expanded to nothing.
> > +      }
> > +
> > +- ``-Wempty-init-stmt`` is a new diagnostic that diagnoses empty init-statements
> > +  of ``if``, ``switch``, ``range-based for``, unless: the semicolon directly
> > +  follows a macro that was expanded to nothing or if the semicolon is within the
> > +  macro itself (both macros from system headers, and normal macros). This
> > +  diagnostic is in the ``-Wextra-semi-stmt`` group and is enabled in
> > +  ``-Wextra``.
> > +
> > +  .. code-block:: c++
> > +
> > +      void test() {
> > +        if(; // <- warning: init-statement of 'if' is a null statement
> > +           true)
> > +          ;
> > +
> > +        switch (; // <- warning: init-statement of 'switch' is a null statement
> > +                x) {
> > +          ...
> > +        }
> > +
> > +        for (; // <- warning: init-statement of 'range-based for' is a null statement
> > +             int y : S())
> > +          ;
> > +      }
> > +
> >
> > Non-comprehensive list of changes in this release
> > -------------------------------------------------
> >
> > Modified: cfe/trunk/include/clang/Basic/DiagnosticGroups.td
> > URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticGroups.td?rev=347339&r1=347338&r2=347339&view=diff
> > ==============================================================================
> > --- cfe/trunk/include/clang/Basic/DiagnosticGroups.td (original)
> > +++ cfe/trunk/include/clang/Basic/DiagnosticGroups.td Tue Nov 20 10:59:05 2018
> > @@ -162,6 +162,8 @@ def GNUEmptyStruct : DiagGroup<"gnu-empt
> > def ExtraTokens : DiagGroup<"extra-tokens">;
> > def CXX98CompatExtraSemi : DiagGroup<"c++98-compat-extra-semi">;
> > def CXX11ExtraSemi : DiagGroup<"c++11-extra-semi">;
> > +def EmptyInitStatement : DiagGroup<"empty-init-stmt">;
> > +def ExtraSemiStmt : DiagGroup<"extra-semi-stmt", [EmptyInitStatement]>;
> > def ExtraSemi : DiagGroup<"extra-semi", [CXX98CompatExtraSemi,
> >                                          CXX11ExtraSemi]>;
> >
> > @@ -768,7 +770,8 @@ def Extra : DiagGroup<"extra", [
> >     MissingMethodReturnType,
> >     SignCompare,
> >     UnusedParameter,
> > -    NullPointerArithmetic
> > +    NullPointerArithmetic,
> > +    EmptyInitStatement
> >   ]>;
> >
> > def Most : DiagGroup<"most", [
> >
> > Modified: cfe/trunk/include/clang/Basic/DiagnosticParseKinds.td
> > URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticParseKinds.td?rev=347339&r1=347338&r2=347339&view=diff
> > ==============================================================================
> > --- cfe/trunk/include/clang/Basic/DiagnosticParseKinds.td (original)
> > +++ cfe/trunk/include/clang/Basic/DiagnosticParseKinds.td Tue Nov 20 10:59:05 2018
> > @@ -53,6 +53,10 @@ def ext_extra_semi_cxx11 : Extension<
> > def warn_extra_semi_after_mem_fn_def : Warning<
> >   "extra ';' after member function definition">,
> >   InGroup<ExtraSemi>, DefaultIgnore;
> > +def warn_null_statement : Warning<
> > +  "empty expression statement has no effect; "
> > +  "remove unnecessary ';' to silence this warning">,
> > +  InGroup<ExtraSemiStmt>, DefaultIgnore;
> >
> > def ext_thread_before : Extension<"'__thread' before '%0'">;
> > def ext_keyword_as_ident : ExtWarn<
> > @@ -554,6 +558,9 @@ def ext_for_range_init_stmt : ExtWarn<
> > def warn_cxx17_compat_for_range_init_stmt : Warning<
> >   "range-based for loop initialization statements are incompatible with "
> >   "C++ standards before C++2a">, DefaultIgnore, InGroup<CXXPre2aCompat>;
> > +def warn_empty_init_statement : Warning<
> > +  "empty initialization statement of '%select{if|switch|range-based for}0' "
> > +  "has no effect">, InGroup<EmptyInitStatement>, DefaultIgnore;
> >
> > // C++ derived classes
> > def err_dup_virtual : Error<"duplicate 'virtual' in base specifier">;
> >
> > Modified: cfe/trunk/include/clang/Parse/Parser.h
> > URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Parse/Parser.h?rev=347339&r1=347338&r2=347339&view=diff
> > ==============================================================================
> > --- cfe/trunk/include/clang/Parse/Parser.h (original)
> > +++ cfe/trunk/include/clang/Parse/Parser.h Tue Nov 20 10:59:05 2018
> > @@ -1888,6 +1888,7 @@ private:
> >   StmtResult ParseCompoundStatement(bool isStmtExpr,
> >                                     unsigned ScopeFlags);
> >   void ParseCompoundStatementLeadingPragmas();
> > +  bool ConsumeNullStmt(StmtVector &Stmts);
> >   StmtResult ParseCompoundStatementBody(bool isStmtExpr = false);
> >   bool ParseParenExprOrCondition(StmtResult *InitStmt,
> >                                  Sema::ConditionResult &CondResult,
> >
> > Modified: cfe/trunk/lib/Parse/ParseExprCXX.cpp
> > URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Parse/ParseExprCXX.cpp?rev=347339&r1=347338&r2=347339&view=diff
> > ==============================================================================
> > --- cfe/trunk/lib/Parse/ParseExprCXX.cpp (original)
> > +++ cfe/trunk/lib/Parse/ParseExprCXX.cpp Tue Nov 20 10:59:05 2018
> > @@ -1773,7 +1773,13 @@ Sema::ConditionResult Parser::ParseCXXCo
> >     //   if (; true);
> >     if (InitStmt && Tok.is(tok::semi)) {
> >       WarnOnInit();
> > -      SourceLocation SemiLoc = ConsumeToken();
> > +      SourceLocation SemiLoc = Tok.getLocation();
> > +      if (!Tok.hasLeadingEmptyMacro() && !SemiLoc.isMacroID()) {
> > +        Diag(SemiLoc, diag::warn_empty_init_statement)
> > +            << (CK == Sema::ConditionKind::Switch)
> > +            << FixItHint::CreateRemoval(SemiLoc);
> > +      }
> > +      ConsumeToken();
> >       *InitStmt = Actions.ActOnNullStmt(SemiLoc);
> >       return ParseCXXCondition(nullptr, Loc, CK);
> >     }
> >
> > Modified: cfe/trunk/lib/Parse/ParseStmt.cpp
> > URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Parse/ParseStmt.cpp?rev=347339&r1=347338&r2=347339&view=diff
> > ==============================================================================
> > --- cfe/trunk/lib/Parse/ParseStmt.cpp (original)
> > +++ cfe/trunk/lib/Parse/ParseStmt.cpp Tue Nov 20 10:59:05 2018
> > @@ -930,6 +930,34 @@ void Parser::ParseCompoundStatementLeadi
> >
> > }
> >
> > +/// Consume any extra semi-colons resulting in null statements,
> > +/// returning true if any tok::semi were consumed.
> > +bool Parser::ConsumeNullStmt(StmtVector &Stmts) {
> > +  if (!Tok.is(tok::semi))
> > +    return false;
> > +
> > +  SourceLocation StartLoc = Tok.getLocation();
> > +  SourceLocation EndLoc;
> > +
> > +  while (Tok.is(tok::semi) && !Tok.hasLeadingEmptyMacro() &&
> > +         Tok.getLocation().isValid() && !Tok.getLocation().isMacroID()) {
> > +    EndLoc = Tok.getLocation();
> > +
> > +    // Don't just ConsumeToken() this tok::semi, do store it in AST.
> > +    StmtResult R = ParseStatementOrDeclaration(Stmts, ACK_Any);
> > +    if (R.isUsable())
> > +      Stmts.push_back(R.get());
> > +  }
> > +
> > +  // Did not consume any extra semi.
> > +  if (EndLoc.isInvalid())
> > +    return false;
> > +
> > +  Diag(StartLoc, diag::warn_null_statement)
> > +      << FixItHint::CreateRemoval(SourceRange(StartLoc, EndLoc));
> > +  return true;
> > +}
> > +
> > /// ParseCompoundStatementBody - Parse a sequence of statements and invoke the
> > /// ActOnCompoundStmt action.  This expects the '{' to be the current token, and
> > /// consume the '}' at the end of the block.  It does not manipulate the scope
> > @@ -992,6 +1020,9 @@ StmtResult Parser::ParseCompoundStatemen
> >       continue;
> >     }
> >
> > +    if (ConsumeNullStmt(Stmts))
> > +      continue;
> > +
> >     StmtResult R;
> >     if (Tok.isNot(tok::kw___extension__)) {
> >       R = ParseStatementOrDeclaration(Stmts, ACK_Any);
> > @@ -1588,10 +1619,15 @@ StmtResult Parser::ParseForStatement(Sou
> >   ParsedAttributesWithRange attrs(AttrFactory);
> >   MaybeParseCXX11Attributes(attrs);
> >
> > +  SourceLocation EmptyInitStmtSemiLoc;
> > +
> >   // Parse the first part of the for specifier.
> >   if (Tok.is(tok::semi)) {  // for (;
> >     ProhibitAttributes(attrs);
> >     // no first part, eat the ';'.
> > +    SourceLocation SemiLoc = Tok.getLocation();
> > +    if (!Tok.hasLeadingEmptyMacro() && !SemiLoc.isMacroID())
> > +      EmptyInitStmtSemiLoc = SemiLoc;
> >     ConsumeToken();
> >   } else if (getLangOpts().CPlusPlus && Tok.is(tok::identifier) &&
> >              isForRangeIdentifier()) {
> > @@ -1723,6 +1759,11 @@ StmtResult Parser::ParseForStatement(Sou
> >                    : diag::ext_for_range_init_stmt)
> >               << (FirstPart.get() ? FirstPart.get()->getSourceRange()
> >                                   : SourceRange());
> > +          if (EmptyInitStmtSemiLoc.isValid()) {
> > +            Diag(EmptyInitStmtSemiLoc, diag::warn_empty_init_statement)
> > +                << /*for-loop*/ 2
> > +                << FixItHint::CreateRemoval(EmptyInitStmtSemiLoc);
> > +          }
> >         }
> >       } else {
> >         ExprResult SecondExpr = ParseExpression();
> >
> > Added: cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt-in-init-statement.cpp
> > URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt-in-init-statement.cpp?rev=347339&view=auto
> > ==============================================================================
> > --- cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt-in-init-statement.cpp (added)
> > +++ cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt-in-init-statement.cpp Tue Nov 20 10:59:05 2018
> > @@ -0,0 +1,64 @@
> > +// RUN: %clang_cc1 -fsyntax-only -Wextra -std=c++2a -verify %s
> > +// RUN: %clang_cc1 -fsyntax-only -Wextra-semi-stmt -std=c++2a -verify %s
> > +// RUN: %clang_cc1 -fsyntax-only -Wempty-init-stmt -std=c++2a -verify %s
> > +// RUN: cp %s %t
> > +// RUN: %clang_cc1 -x c++ -Wempty-init-stmt -std=c++2a -fixit %t
> > +// RUN: %clang_cc1 -x c++ -Wempty-init-stmt -std=c++2a -Werror %t
> > +
> > +struct S {
> > +  int *begin();
> > +  int *end();
> > +};
> > +
> > +void naive(int x) {
> > +  if (; true) // expected-warning {{empty initialization statement of 'if' has no effect}}
> > +    ;
> > +
> > +  switch (; x) { // expected-warning {{empty initialization statement of 'switch' has no effect}}
> > +  }
> > +
> > +  for (; int y : S()) // expected-warning {{empty initialization statement of 'range-based for' has no effect}}
> > +    ;
> > +
> > +  for (;;) // OK
> > +    ;
> > +}
> > +
> > +#define NULLMACRO
> > +
> > +void with_null_macro(int x) {
> > +  if (NULLMACRO; true)
> > +    ;
> > +
> > +  switch (NULLMACRO; x) {
> > +  }
> > +
> > +  for (NULLMACRO; int y : S())
> > +    ;
> > +}
> > +
> > +#define SEMIMACRO ;
> > +
> > +void with_semi_macro(int x) {
> > +  if (SEMIMACRO true)
> > +    ;
> > +
> > +  switch (SEMIMACRO x) {
> > +  }
> > +
> > +  for (SEMIMACRO int y : S())
> > +    ;
> > +}
> > +
> > +#define PASSTHROUGHMACRO(x) x
> > +
> > +void with_passthrough_macro(int x) {
> > +  if (PASSTHROUGHMACRO(;) true)
> > +    ;
> > +
> > +  switch (PASSTHROUGHMACRO(;) x) {
> > +  }
> > +
> > +  for (PASSTHROUGHMACRO(;) int y : S())
> > +    ;
> > +}
> >
> > Added: cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt.cpp
> > URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt.cpp?rev=347339&view=auto
> > ==============================================================================
> > --- cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt.cpp (added)
> > +++ cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt.cpp Tue Nov 20 10:59:05 2018
> > @@ -0,0 +1,96 @@
> > +// RUN: %clang_cc1 -fsyntax-only -Wextra-semi-stmt -verify %s
> > +// RUN: cp %s %t
> > +// RUN: %clang_cc1 -x c++ -Wextra-semi-stmt -fixit %t
> > +// RUN: %clang_cc1 -x c++ -Wextra-semi-stmt -Werror %t
> > +
> > +#define GOODMACRO(varname) int varname
> > +#define BETTERMACRO(varname) GOODMACRO(varname);
> > +#define NULLMACRO(varname)
> > +
> > +enum MyEnum {
> > +  E1,
> > +  E2
> > +};
> > +
> > +void test() {
> > +  ; // expected-warning {{empty expression statement has no effect; remove unnecessary ';' to silence this warning}}
> > +  ;
> > +
> > +  // This removal of extra semi also consumes all the comments.
> > +  // clang-format: off
> > +  ;;;
> > +  // clang-format: on
> > +
> > +  // clang-format: off
> > +  ;NULLMACRO(ZZ);
> > +  // clang-format: on
> > +
> > +  {}; // expected-warning {{empty expression statement has no effect; remove unnecessary ';' to silence this warning}}
> > +
> > +  {
> > +    ; // expected-warning {{empty expression statement has no effect; remove unnecessary ';' to silence this warning}}
> > +  }
> > +
> > +  if (true) {
> > +    ; // expected-warning {{empty expression statement has no effect; remove unnecessary ';' to silence this warning}}
> > +  }
> > +
> > +  GOODMACRO(v0); // OK
> > +
> > +  GOODMACRO(v1;) // OK
> > +
> > +  BETTERMACRO(v2) // OK
> > +
> > +  BETTERMACRO(v3;) // Extra ';', but within macro expansion, so ignored.
> > +
> > +  BETTERMACRO(v4); // expected-warning {{empty expression statement has no effect; remove unnecessary ';' to silence this warning}}
> > +
> > +  BETTERMACRO(v5;); // expected-warning {{empty expression statement has no effect; remove unnecessary ';' to silence this warning}}
> > +
> > +  NULLMACRO(v6) // OK
> > +
> > +  NULLMACRO(v7); // OK. This could be either GOODMACRO() or BETTERMACRO() situation, so we can't know we can drop it.
> > +
> > +  if (true)
> > +    ; // OK
> > +
> > +  while (true)
> > +    ; // OK
> > +
> > +  do
> > +    ; // OK
> > +  while (true);
> > +
> > +  for (;;) // OK
> > +    ;      // OK
> > +
> > +  MyEnum my_enum;
> > +  switch (my_enum) {
> > +  case E1:
> > +    // stuff
> > +    break;
> > +  case E2:; // OK
> > +  }
> > +
> > +  for (;;) {
> > +    for (;;) {
> > +      goto contin_outer;
> > +    }
> > +  contin_outer:; // OK
> > +  }
> > +}
> > +
> > +;
> > +
> > +namespace NS {};
> > +
> > +void foo(int x) {
> > +  switch (x) {
> > +  case 0:
> > +    [[fallthrough]];
> > +  case 1:
> > +    return;
> > +  }
> > +}
> > +
> > +[[]];
> >
> >
> > _______________________________________________
> > cfe-commits mailing list
> > cfe-commits at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
>


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