[PATCH] __assume side-effects warning

[hfinkel at anl.gov]

Hi rsmith,

In MS-compatibility mode, we support the __assume builtin. The __assume builtin does not evaluate its arguments, and we should issue a warning if __assume is provided with an argument with side effects because these effects will be discarded.

This is similar in spirit to the warnings issued by other compilers (Intel Diagnostic 2261, MS Compiler Warning C4557).

The result currently looks like this:
```
<stdin>:3:12: warning: the argument to __assume has side effects that will be discarded
  __assume(++i > 2);
           ^~~~~~~
1 warning generated.

```

http://reviews.llvm.org/D4552

Files:
  include/clang/Basic/DiagnosticSemaKinds.td
  include/clang/Sema/Sema.h
  lib/Sema/SemaChecking.cpp
  test/Sema/builtin-assume.c

Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -444,6 +444,10 @@
   "change the argument to be the free space in the destination buffer minus " 
   "the terminating null byte">;
 
+def warn_assume_side_effects : Warning<
+  "the argument to __assume has side effects that will be discarded">,
+  InGroup<DiagGroup<"assume">>;
+
 /// main()
 // static main() is not an error in C, just in C++.
 def warn_static_main : Warning<"'main' should not be declared static">,
Index: include/clang/Sema/Sema.h
===================================================================
--- include/clang/Sema/Sema.h
+++ include/clang/Sema/Sema.h
@@ -8244,6 +8244,7 @@
 
 private:
   bool SemaBuiltinPrefetch(CallExpr *TheCall);
+  bool SemaBuiltinAssume(CallExpr *TheCall);
   bool SemaBuiltinLongjmp(CallExpr *TheCall);
   ExprResult SemaBuiltinAtomicOverloaded(ExprResult TheCallResult);
   ExprResult SemaAtomicOpsOverloaded(ExprResult TheCallResult,
Index: lib/Sema/SemaChecking.cpp
===================================================================
--- lib/Sema/SemaChecking.cpp
+++ lib/Sema/SemaChecking.cpp
@@ -175,6 +175,10 @@
     if (SemaBuiltinPrefetch(TheCall))
       return ExprError();
     break;
+  case Builtin::BI__assume:
+    if (SemaBuiltinAssume(TheCall))
+      return ExprError();
+    break;
   case Builtin::BI__builtin_object_size:
     if (SemaBuiltinConstantArgRange(TheCall, 1, 0, 3))
       return ExprError();
@@ -1942,6 +1946,20 @@
   return false;
 }
 
+/// SemaBuiltinAssume - Handle __assume (MS Extension).
+// __assume does not evaluate its arguments, and should warn if its argument
+// has side effects.
+bool Sema::SemaBuiltinAssume(CallExpr *TheCall) {
+  Expr *Arg = TheCall->getArg(0);
+  if (Arg->isTypeDependent() || Arg->isValueDependent()) return false;
+
+  if (Arg->HasSideEffects(Context))
+    return Diag(Arg->getLocStart(), diag::warn_assume_side_effects)
+      << Arg->getSourceRange();
+
+  return false;
+}
+
 /// SemaBuiltinConstantArg - Handle a check if argument ArgNum of CallExpr
 /// TheCall is a constant expression.
 bool Sema::SemaBuiltinConstantArg(CallExpr *TheCall, int ArgNum,
Index: test/Sema/builtin-assume.c
===================================================================
--- /dev/null
+++ test/Sema/builtin-assume.c
@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -triple i386-mingw32 -fms-extensions -fsyntax-only -verify %s
+
+int foo(int *a, int i) {
+  __assume(i != 4);
+  __assume(++i > 2); //expected-warning {{the argument to __assume has side effects that will be discarded}}
+  return a[i];
+}
+
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