[PATCH] D13969: Tweak how -Wunused-value interacts with macros

Nico Weber via cfe-commits cfe-commits at lists.llvm.org
Wed Oct 21 19:47:09 PDT 2015

thakis created this revision.
thakis added a reviewer: rsmith.
thakis added a subscriber: cfe-commits.

1. Make the warning more strict in C mode. r172696 added code to suppress warnings from macro expansions in system headers, which checks `SourceMgr.isMacroBodyExpansion(E->IgnoreParens()->getExprLoc())`. Consider this snippet:

        #define FOO(x) (x)
        void f(int a) {

   In C, the line `FOO(a)` is an `ImplicitCastExpr(ParenExpr(DeclRefExpr))`, while it's just a `ParenExpr(DeclRefExpr)` in C++. So in C++, `E->IgnoreParens()` returns the `DeclRefExpr` and the check tests the SourceLoc of `a`. In C, the `ImplicitCastExpr` has the effect of checking the SourceLoc of `FOO`, which is a macro body expansion, which causes the diagnostic to be skipped. It looks unintentional that clang does different things for C and C++ here, so use `IgnoreParenImpCasts` instead of `IgnoreParens` here. This has the effect of the warning firing more often than previously in C code – it now fires as often as it fires in C++ code.

  2. In MS mode, suppress the warning if it would warn on `UNREFERENCED_PARAMETER`. `UNREFERENCED_PARAMETER` is a commonly used macro on Windows and it happens to uselessly trigger -Wunused-value. As discussed in the thread "rfc: winnt.h's UNREFERENCED_PARAMETER() vs clang's -Wunused-value" on cfe-dev, fix this by special-casing this specific macro. (This costs a string comparison and some fast-path lexing per warning, but the warning is emitted rarely. It fires once in Windows.h itself, so this code runs at least once per TU including Windows.h, but it doesn't run hundreds of times.)



Index: test/Sema/unused-expr.c
--- test/Sema/unused-expr.c
+++ test/Sema/unused-expr.c
@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s -Wno-unreachable-code
+// RUN: %clang_cc1 -DMS_COMPAT -fms-compatibility -fsyntax-only -verify %s -Wno-unreachable-code
 int foo(int X, int Y);
@@ -156,3 +157,13 @@
 #undef M5
 #undef M6
 #undef M7
+void unused_parm(int a) {
+#ifdef MS_COMPAT
+  UNREFERENCED_PARAMETER(a); // expected-warning {{expression result unused}}
Index: lib/Sema/SemaStmt.cpp
--- lib/Sema/SemaStmt.cpp
+++ lib/Sema/SemaStmt.cpp
@@ -195,7 +195,7 @@
   if (isUnevaluatedContext())
-  SourceLocation ExprLoc = E->IgnoreParens()->getExprLoc();
+  SourceLocation ExprLoc = E->IgnoreParenImpCasts()->getExprLoc();
   // In most cases, we don't want to warn if the expression is written in a
   // macro body, or if the macro comes from a system header. If the offending
   // expression is a call to a function with the warn_unused_result attribute,
@@ -218,6 +218,16 @@
   if (isa<StmtExpr>(E) && Loc.isMacroID())
+  // Check if this is the UNREFERENCED_PARAMETER from the Microsoft headers.
+  // That macro is frequently used to suppress "unused parameter" warnings,
+  // but its implementation makes clang's -Wunused-value fire.  Prevent this.
+  if (getLangOpts().MSVCCompat && isa<ParenExpr>(E->IgnoreImpCasts()) &&
+      Loc.isMacroID()) {
+    SourceLocation SpellLoc = Loc;
+    if (findMacroSpelling(SpellLoc, "UNREFERENCED_PARAMETER"))
+      return;
+  }
   // Okay, we have an unused result.  Depending on what the base expression is,
   // we might want to make a more specific diagnostic.  Check for one of these
   // cases now.

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