[Clang] Don't warn if deferencing void pointers in unevaluated context

[junaire]

Summary

After https://reviews.llvm.org/D134461, Clang will diagnose a warning if
trying to deference void pointers in C mode. However, this causes a lot
of noises when compiling a 5.19.11 Linux kernel.

This patch reduces the warning by marking deferencing void pointers in
unevaluated context OK, like sizeof(*void_ptr), typeof(*void_ptr)
and etc.

Fixes https://github.com/ClangBuiltLinux/linux/issues/1720

Signed-off-by: Jun Zhang <[e-mail]>

[nathanchance]

Thanks a lot for fixing this! I took it for a spin against the Linux kernel and all instances of -Wvoid-ptr-dereference disappeared :)

[junaire]

In D134702#3818274, @nathanchance wrote:

Thanks a lot for fixing this! I took it for a spin against the Linux kernel and all instances of -Wvoid-ptr-dereference disappeared :)

Awesome! Glad to hear this fixed your problem!

[nickdesaulniers]

Thanks for the quick fix!

[aaron.ballman]

LGTM!

[nickdesaulniers]

clang/lib/Sema/SemaExpr.cpp

14539

Sorry, I should have asked more about this subexpresssion.

Our CI is still red for older branches of the Linux kernel ("stable" branches) because the kernel only recently started using -std=gnu11 from -std=gnu89.

Why is this considered an error for -std=gnu89?

[nickdesaulniers]

clang/lib/Sema/SemaExpr.cpp

14539

https://reviews.llvm.org/D135090

clang/lib/Sema/SemaExpr.cpp

if (Result.isNull()) {
     return QualType();
   }
 
   if (Result->isVoidType()) {
     // C++ [expr.unary.op]p1:
     //   [...] the expression to which [the unary * operator] is applied shall
     //   be a pointer to an object type, or a pointer to a function type
     LangOptions LO = S.getLangOpts();
-    if (LO.CPlusPlus || !(LO.C99 && IsAfterAmp))
+    if (LO.CPlusPlus || !(LO.C99 && (IsAfterAmp || S.isUnevaluatedContext())))
       S.Diag(OpLoc, diag::ext_typecheck_indirection_through_void_pointer)
           << LO.CPlusPlus << OpTy << Op->getSourceRange();
   }
 
   // Dereferences are usually l-values...
   VK = VK_LValue;
 
   // ...except that certain expressions are never l-values in C.

clang/test/Analysis/misc-ps.m

if (sizeof(void) == 1)
       return;
     // Infeasible.
     *p = 1; // no-warning
   }
   
   void* q;
   
   if (!flag) {
-    if (sizeof(*q) == 1) // expected-warning {{ISO C does not allow indirection on operand of type 'void *'}}
+    if (sizeof(*q) == 1)
       return;
     // Infeasibe.
     *p = 1; // no-warning
   }
     
   // Infeasible.
   *p = 1; // no-warning
 }

clang/test/Sema/no-warn-void-ptr-uneval.c

+// RUN: %clang_cc1 -fsyntax-only -Wno-unused-value -verify %s
+
+// expected-no-diagnostics
+void foo(void *vp) {
+  sizeof(*vp);
+  sizeof(*(vp));
+  void inner(typeof(*vp));
+}