From: Eli Friedman <eli.friedman@gmail.com>
Date: Thu, 26 Jan 2012 23:11:39 +0000 (+0000)
Subject: Make the bitfield implicit truncation warning slightly more aggressive, and make... 
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=3a643afa402d51b5fc4faccb4329e9c4392d4551;p=clang

Make the bitfield implicit truncation warning slightly more aggressive, and make the printed warning a bit more accurate.  The new behavior matches gcc's -Wconversion.  <rdar://problem/10238797>.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@149089 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Sema/SemaChecking.cpp b/lib/Sema/SemaChecking.cpp
index 950a575891..8d01352fd2 100644
--- a/lib/Sema/SemaChecking.cpp
+++ b/lib/Sema/SemaChecking.cpp
@@ -3655,19 +3655,20 @@ bool AnalyzeBitFieldAssignment(Sema &S, FieldDecl *Bitfield, Expr *Init,
   if (OriginalWidth <= FieldWidth)
     return false;
 
+  // Compute the value which the bitfield will contain.
   llvm::APSInt TruncatedValue = Value.trunc(FieldWidth);
+  TruncatedValue.setIsSigned(Bitfield->getType()->isSignedIntegerType());
 
-  // It's fairly common to write values into signed bitfields
-  // that, if sign-extended, would end up becoming a different
-  // value.  We don't want to warn about that.
-  if (Value.isSigned() && Value.isNegative())
-    TruncatedValue = TruncatedValue.sext(OriginalWidth);
-  else
-    TruncatedValue = TruncatedValue.zext(OriginalWidth);
-
+  // Check whether the stored value is equal to the original value.
+  TruncatedValue = TruncatedValue.extend(OriginalWidth);
   if (Value == TruncatedValue)
     return false;
 
+  // Special-case bitfields of width 1: booleans are naturally 0/1, and
+  // therefore don't strictly fit into a bitfield of width 1.
+  if (FieldWidth == 1 && Value.getBoolValue() == TruncatedValue.getBoolValue())
+    return false;
+
   std::string PrettyValue = Value.toString(10);
   std::string PrettyTrunc = TruncatedValue.toString(10);
 
diff --git a/test/Sema/constant-conversion.c b/test/Sema/constant-conversion.c
index a2c48d3549..c97eb05cd1 100644
--- a/test/Sema/constant-conversion.c
+++ b/test/Sema/constant-conversion.c
@@ -31,8 +31,8 @@ void test3() {
     int bar : 2;
   };
 
-  struct A a = { 0, 10 };            // expected-warning {{implicit truncation from 'int' to bitfield changes value from 10 to 2}}
-  struct A b[] = { 0, 10, 0, 0 };    // expected-warning {{implicit truncation from 'int' to bitfield changes value from 10 to 2}}
+  struct A a = { 0, 10 };            // expected-warning {{implicit truncation from 'int' to bitfield changes value from 10 to -2}}
+  struct A b[] = { 0, 10, 0, 0 };    // expected-warning {{implicit truncation from 'int' to bitfield changes value from 10 to -2}}
   struct A c[] = {{10, 0}};          // expected-warning {{implicit truncation from 'int' to bitfield changes value from 10 to 2}}
   struct A d = (struct A) { 10, 0 }; // expected-warning {{implicit truncation from 'int' to bitfield changes value from 10 to 2}}
   struct A e = { .foo = 10 };        // expected-warning {{implicit truncation from 'int' to bitfield changes value from 10 to 2}}
@@ -62,3 +62,13 @@ void test6() {
   unsigned char y = 1 ? 65535 : 1; // expected-warning {{changes value}}
 }
 
+void test7() {
+	struct {
+		unsigned int twoBits1:2;
+		unsigned int twoBits2:2;
+		unsigned int reserved:28;
+	} f;
+
+	f.twoBits1 = ~1; // expected-warning {{implicit truncation from 'int' to bitfield changes value from -2 to 2}}
+	f.twoBits2 = ~2; // expected-warning {{implicit truncation from 'int' to bitfield changes value from -3 to 1}}
+}