[clang-tidy] fix false positive that floating point variable only use…

…d in increment expr in cert-flp30-c (llvm#108706)

Fixes: llvm#108049
cert-flp30-c only provides non-compliant example with normal loop.
Essentially it wants to avoid that floating point variables are used as
loop counters which are checked in condition expr and modified in
increment expr.
This patch wants to give more precise matcheres to identify this cases.

clang-tools-extra/clang-tidy/cert/FloatLoopCounter.cpp

@@ -9,22 +9,38 @@
 #include "FloatLoopCounter.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
 
 using namespace clang::ast_matchers;
 
 namespace clang::tidy::cert {
 
 void FloatLoopCounter::registerMatchers(MatchFinder *Finder) {
   Finder->addMatcher(
-      forStmt(hasIncrement(expr(hasType(realFloatingPointType())))).bind("for"),
+      forStmt(hasIncrement(forEachDescendant(
+                  declRefExpr(hasType(realFloatingPointType()),
+                              to(varDecl().bind("var")))
+                      .bind("inc"))),
+              hasCondition(forEachDescendant(
+                  declRefExpr(hasType(realFloatingPointType()),
+                              to(varDecl(equalsBoundNode("var"))))
+                      .bind("cond"))))
+          .bind("for"),
       this);
 }
 
 void FloatLoopCounter::check(const MatchFinder::MatchResult &Result) {
   const auto *FS = Result.Nodes.getNodeAs<ForStmt>("for");
 
-  diag(FS->getInc()->getExprLoc(), "loop induction expression should not have "
-                                   "floating-point type");
+  diag(FS->getInc()->getBeginLoc(), "loop induction expression should not have "
+                                    "floating-point type")
+      << Result.Nodes.getNodeAs<DeclRefExpr>("inc")->getSourceRange()
+      << Result.Nodes.getNodeAs<DeclRefExpr>("cond")->getSourceRange();
+
+  if (!FS->getInc()->getType()->isRealFloatingType())
+    if (const auto *V = Result.Nodes.getNodeAs<VarDecl>("var"))
+      diag(V->getBeginLoc(), "floating-point type loop induction variable",
+           DiagnosticIDs::Note);
 }
 
 } // namespace clang::tidy::cert

clang-tools-extra/docs/ReleaseNotes.rst

@@ -111,6 +111,10 @@ Changes in existing checks
   <clang-tidy/checks/bugprone/casting-through-void>` check to suggest replacing
   the offending code with ``reinterpret_cast``, to more clearly express intent.
 
+- Improved :doc:`cert-flp30-c<clang-tidy/checks/cert/flp30-c>` check to 
+  fix false positive that floating point variable is only used in increment
+  expression.
+
 - Improved :doc:`cppcoreguidelines-prefer-member-initializer
   <clang-tidy/checks/cppcoreguidelines/prefer-member-initializer>` check to avoid
   false positive when member initialization depends on a structured binging

clang-tools-extra/test/clang-tidy/checkers/cert/flp30-c.c

@@ -1,19 +1,28 @@
 // RUN: %check_clang_tidy %s cert-flp30-c %t
 
 float g(void);
+int c(float);
+float f = 1.0f;
+
+void match(void) {
 
-void func(void) {
   for (float x = 0.1f; x <= 1.0f; x += 0.1f) {}
-  // CHECK-MESSAGES: :[[@LINE-1]]:37: warning: loop induction expression should not have floating-point type [cert-flp30-c]
+  // CHECK-MESSAGES: :[[@LINE-1]]:35: warning: loop induction expression should not have floating-point type [cert-flp30-c]
 
-  float f = 1.0f;
   for (; f > 0; --f) {}
-  // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: loop induction expression
+  // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: loop induction expression should not have floating-point type [cert-flp30-c]
 
-  for (;;g()) {}
-  // CHECK-MESSAGES: :[[@LINE-1]]:10: warning: loop induction expression
+  for (float x = 0.0f; c(x); x = g()) {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:30: warning: loop induction expression should not have floating-point type [cert-flp30-c]
 
-  for (int i = 0; i < 10; i += 1.0f) {}
+  for (int i=0; i < 10 && f < 2.0f; f++, i++) {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:37: warning: loop induction expression should not have floating-point type [cert-flp30-c]
+  // CHECK-MESSAGES: :5:1: note: floating-point type loop induction variable
+}
 
+void not_match(void) {
+  for (int i = 0; i < 10; i += 1.0f) {}
   for (int i = 0; i < 10; ++i) {}
+  for (int i = 0; i < 10; ++i, f++) {}
+  for (int i = 0; f < 10.f; ++i) {}
 }