[PATCH] D121715: [Clang] Fix an unused-but-set-variable warning with volatile variable

[Yonghong Song via Phabricator via cfe-commits]

yonghong-song updated this revision to Diff 415665.
yonghong-song retitled this revision from "[Clang] Fix an unused-but-set-variable warning with compond assignment" to "[Clang] Fix an unused-but-set-variable warning with volatile variable".
yonghong-song edited the summary of this revision.
yonghong-song added a comment.

- only handle volatile variables with component assignment.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D121715/new/

https://reviews.llvm.org/D121715

Files:
  clang/lib/Sema/SemaDecl.cpp
  clang/lib/Sema/SemaExprCXX.cpp
  clang/test/Sema/warn-unused-but-set-variables.c


Index: clang/test/Sema/warn-unused-but-set-variables.c
===================================================================
--- clang/test/Sema/warn-unused-but-set-variables.c
+++ clang/test/Sema/warn-unused-but-set-variables.c
@@ -23,10 +23,19 @@
   int a;
   w = (a = 0);
 
+  int j = 0; // expected-warning{{variable 'j' set but not used}}
+  for (int i = 0; i < 1000; i++)
+    j += 1;
+
   // Following gcc, warn for a volatile variable.
   volatile int b; // expected-warning{{variable 'b' set but not used}}
   b = 0;
 
+  // volatile variable k is used, no warning.
+  volatile int k = 0;
+  for (int i = 0; i < 1000; i++)
+    k += 1;
+
   int x;
   x = 0;
   return x;
Index: clang/lib/Sema/SemaExprCXX.cpp
===================================================================
--- clang/lib/Sema/SemaExprCXX.cpp
+++ clang/lib/Sema/SemaExprCXX.cpp
@@ -7922,6 +7922,7 @@
 static void MaybeDecrementCount(
     Expr *E, llvm::DenseMap<const VarDecl *, int> &RefsMinusAssignments) {
   DeclRefExpr *LHS = nullptr;
+  bool IsComponentAssign = false;
   if (BinaryOperator *BO = dyn_cast<BinaryOperator>(E)) {
     if (BO->getLHS()->getType()->isDependentType() ||
         BO->getRHS()->getType()->isDependentType()) {
@@ -7929,6 +7930,8 @@
         return;
     } else if (!BO->isAssignmentOp())
       return;
+    else
+      IsComponentAssign = BO->isCompoundAssignmentOp();
     LHS = dyn_cast<DeclRefExpr>(BO->getLHS());
   } else if (CXXOperatorCallExpr *COCE = dyn_cast<CXXOperatorCallExpr>(E)) {
     if (COCE->getOperator() != OO_Equal)
@@ -7940,6 +7943,10 @@
   VarDecl *VD = dyn_cast<VarDecl>(LHS->getDecl());
   if (!VD)
     return;
+  // Don't decrement RefsMinusAssignments if volatile variable with component
+  // assignment (+=, ...) to avoid potential unused-but-set-variable warning.
+  if (IsComponentAssign && VD->getType().isVolatileQualified())
+    return;
   auto iter = RefsMinusAssignments.find(VD);
   if (iter == RefsMinusAssignments.end())
     return;
Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -2030,6 +2030,7 @@
          "Found a negative number of references to a VarDecl");
   if (iter->getSecond() != 0)
     return;
+
   unsigned DiagID = isa<ParmVarDecl>(VD) ? diag::warn_unused_but_set_parameter
                                          : diag::warn_unused_but_set_variable;
   Diag(VD->getLocation(), DiagID) << VD;


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