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

George Karpenkov via cfe-dev cfe-dev at lists.llvm.org
Tue Dec 4 16:35:07 PST 2018


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”);')

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”.

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;
> +  }
> +}
> +
> +[[]];
> 
> 
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