[clang] Add -Wimplicit-fallthrough to -Wextra

[vegerot]

This patch adds -Wimplicit-fallthrough to -Wextra. GCC already includes it in -Wextra.

This patch also adds a test to check that -Wimplicit-fallthrough is included in -Wextra.

Note: This patch may regress performance when building with -Wextra.
This is because -Wextra requires forming a CFG for every function.

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[llvmbot]

@llvm/pr-subscribers-clang

Author: Max Coplan (vegerot)

Changes

This patch adds -Wimplicit-fallthrough to -Wextra. GCC already includes it in -Wextra.

This patch also adds a test to check that -Wimplicit-fallthrough is included in -Wextra.

Note: This patch may regress performance when building with -Wextra.
This is because -Wextra requires forming a CFG for every function.

---

Full diff: https://github.com/llvm/llvm-project/pull/97926.diff

3 Files Affected:

• (modified) clang/include/clang/Basic/DiagnosticGroups.td (+1)
• (modified) clang/test/Sema/fallthrough-attr.c (+1)
• (modified) clang/test/SemaCXX/switch-implicit-fallthrough-macro.cpp (+1)

diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
index 2241f8481484e..a875f665b10f9 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -1063,6 +1063,7 @@ def Extra : DiagGroup<"extra", [
     StringConcatation,
     FUseLdPath,
     CastFunctionTypeMismatch,
+    ImplicitFallthrough,
   ]>;
 
 def Most : DiagGroup<"most", [
diff --git a/clang/test/Sema/fallthrough-attr.c b/clang/test/Sema/fallthrough-attr.c
index de50ebf39d42f..6cc19136f30a7 100644
--- a/clang/test/Sema/fallthrough-attr.c
+++ b/clang/test/Sema/fallthrough-attr.c
@@ -2,6 +2,7 @@
 // RUN: %clang_cc1 -fsyntax-only -std=gnu99 -verify -Wimplicit-fallthrough %s
 // RUN: %clang_cc1 -fsyntax-only -std=c99 -verify -Wimplicit-fallthrough %s
 // RUN: %clang_cc1 -fsyntax-only -std=c11 -verify -Wimplicit-fallthrough %s
+// RUN: %clang_cc1 -fsyntax-only -std=c11 -verify -Wextra %s
 // RUN: %clang_cc1 -fsyntax-only -std=c2x -DC2X -verify -Wimplicit-fallthrough %s
 
 int fallthrough_attribute_spelling(int n) {
diff --git a/clang/test/SemaCXX/switch-implicit-fallthrough-macro.cpp b/clang/test/SemaCXX/switch-implicit-fallthrough-macro.cpp
index 11df2cbfb53f0..cbbff1f1793b8 100644
--- a/clang/test/SemaCXX/switch-implicit-fallthrough-macro.cpp
+++ b/clang/test/SemaCXX/switch-implicit-fallthrough-macro.cpp
@@ -3,6 +3,7 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++1z -Wimplicit-fallthrough -DCLANG_PREFIX -DCOMMAND_LINE_FALLTHROUGH=[[clang::fallthrough]] %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++1z -Wimplicit-fallthrough -DCOMMAND_LINE_FALLTHROUGH=[[clang::fallthrough]] %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++1z -Wimplicit-fallthrough -DCOMMAND_LINE_FALLTHROUGH=[[fallthrough]] -DUNCHOSEN=[[clang::fallthrough]] %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++1z -Wextra -DCOMMAND_LINE_FALLTHROUGH=[[fallthrough]] -DUNCHOSEN=[[clang::fallthrough]] %s
 
 int fallthrough_compatibility_macro_from_command_line(int n) {
   switch (n) {

[vegerot]

@AaronBallman please review

I don't know how to go about measuring if there even is a performance regression. How would I do that?

[AaronBallman]

@AaronBallman please review

I don't know how to go about measuring if there even is a performance regression. How would I do that?

I would put up a branch on http://llvm-compile-time-tracker.com (instructions in the About page) and see how this impacts compile times. We've always left diagnostics out of -Wextra that require a CFG, so getting performance measurements is important.