[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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