Index: gcc/c-family/c-common.c
===================================================================
--- gcc/c-family/c-common.c	(revision 203640)
+++ gcc/c-family/c-common.c	(working copy)
@@ -2518,7 +2518,7 @@ shorten_binary_op (tree result_type, tre
 }
 
 /* Checks if expression EXPR of real/integer type cannot be converted 
-   to the real/integer type TYPE. Function returns true when:
+   to the real/integer type TYPE. Function returns non-zero when:
 	* EXPR is a constant which cannot be exactly converted to TYPE 
 	* EXPR is not a constant and size of EXPR's type > than size of TYPE, 
 	  for EXPR type and TYPE being both integers or both real.
@@ -2526,12 +2526,12 @@ shorten_binary_op (tree result_type, tre
 	* EXPR is not a constant of integer type which cannot be 
 	  exactly converted to real type.  
    Function allows conversions between types of different signedness and
-   does not return true in that case.  Function can produce signedness
-   warnings if PRODUCE_WARNS is true.  */
-bool
+   can return SAFE_CONVERSION (zero) in that case.  Function can produce
+   signedness warnings if PRODUCE_WARNS is true.  */
+enum conversion_safety
 unsafe_conversion_p (tree type, tree expr, bool produce_warns)
 {
-  bool give_warning = false;
+  enum conversion_safety give_warning = SAFE_CONVERSION; /* is 0 or false */
   tree expr_type = TREE_TYPE (expr);
   location_t loc = EXPR_LOC_OR_HERE (expr);
 
@@ -2543,7 +2543,7 @@ unsafe_conversion_p (tree type, tree exp
 	  && TREE_CODE (type) == INTEGER_TYPE)
 	{
 	  if (!real_isinteger (TREE_REAL_CST_PTR (expr), TYPE_MODE (expr_type)))
-	    give_warning = true;
+	    give_warning = UNSAFE_REAL;
 	}
       /* Warn for an integer constant that does not fit into integer type.  */
       else if (TREE_CODE (expr_type) == INTEGER_TYPE
@@ -2564,7 +2564,7 @@ unsafe_conversion_p (tree type, tree exp
 			    " constant value to negative integer");
 	    }
 	  else
-	    give_warning = true;
+	    give_warning = UNSAFE_OTHER;
 	}
       else if (TREE_CODE (type) == REAL_TYPE)
 	{
@@ -2573,7 +2573,7 @@ unsafe_conversion_p (tree type, tree exp
 	    {
 	      REAL_VALUE_TYPE a = real_value_from_int_cst (0, expr);
 	      if (!exact_real_truncate (TYPE_MODE (type), &a))
-		give_warning = true;
+		give_warning = UNSAFE_REAL;
 	    }
 	  /* Warn for a real constant that does not fit into a smaller
 	     real type.  */
@@ -2582,7 +2582,7 @@ unsafe_conversion_p (tree type, tree exp
 	    {
 	      REAL_VALUE_TYPE a = TREE_REAL_CST (expr);
 	      if (!exact_real_truncate (TYPE_MODE (type), &a))
-		give_warning = true;
+		give_warning = UNSAFE_REAL;
 	    }
 	}
     }
@@ -2591,7 +2591,7 @@ unsafe_conversion_p (tree type, tree exp
       /* Warn for real types converted to integer types.  */
       if (TREE_CODE (expr_type) == REAL_TYPE
 	  && TREE_CODE (type) == INTEGER_TYPE)
-	give_warning = true;
+	give_warning = UNSAFE_REAL;
 
       else if (TREE_CODE (expr_type) == INTEGER_TYPE
 	       && TREE_CODE (type) == INTEGER_TYPE)
@@ -2629,7 +2629,7 @@ unsafe_conversion_p (tree type, tree exp
 			  && int_fits_type_p (op1, c_common_signed_type (type))
 			  && int_fits_type_p (op1,
 					      c_common_unsigned_type (type))))
-		    return false;
+		    return SAFE_CONVERSION;
 		  /* If constant is unsigned and fits in the target
 		     type, then the result will also fit.  */
 		  else if ((TREE_CODE (op0) == INTEGER_CST
@@ -2638,12 +2638,12 @@ unsafe_conversion_p (tree type, tree exp
 			   || (TREE_CODE (op1) == INTEGER_CST
 			       && unsigned1
 			       && int_fits_type_p (op1, type)))
-		    return false;
+		    return SAFE_CONVERSION;
 		}
 	    }
 	  /* Warn for integer types converted to smaller integer types.  */
 	  if (TYPE_PRECISION (type) < TYPE_PRECISION (expr_type))
-	    give_warning = true;
+	    give_warning = UNSAFE_OTHER;
 
 	  /* When they are the same width but different signedness,
 	     then the value may change.  */
@@ -2679,14 +2679,14 @@ unsafe_conversion_p (tree type, tree exp
 
 	  if (!exact_real_truncate (TYPE_MODE (type), &real_low_bound)
 	      || !exact_real_truncate (TYPE_MODE (type), &real_high_bound))
-	    give_warning = true;
+	    give_warning = UNSAFE_OTHER;
 	}
 
       /* Warn for real types converted to smaller real types.  */
       else if (TREE_CODE (expr_type) == REAL_TYPE
 	       && TREE_CODE (type) == REAL_TYPE
 	       && TYPE_PRECISION (type) < TYPE_PRECISION (expr_type))
-	give_warning = true;
+	give_warning = UNSAFE_REAL;
     }
 
   return give_warning;
@@ -2700,8 +2700,9 @@ conversion_warning (tree type, tree expr
 {
   tree expr_type = TREE_TYPE (expr);
   location_t loc = EXPR_LOC_OR_HERE (expr);
+  enum conversion_safety conversion_kind;
 
-  if (!warn_conversion && !warn_sign_conversion)
+  if (!warn_conversion && !warn_sign_conversion && !warn_float_conversion)
     return;
 
   switch (TREE_CODE (expr))
@@ -2728,7 +2729,12 @@ conversion_warning (tree type, tree expr
 
     case REAL_CST:
     case INTEGER_CST:
-      if (unsafe_conversion_p (type, expr, true))
+      conversion_kind = unsafe_conversion_p (type, expr, true);
+      if (conversion_kind == UNSAFE_REAL)
+	warning_at (loc, OPT_Wfloat_conversion,
+		    "conversion to %qT alters %qT constant value",
+		    type, expr_type);
+      else if (conversion_kind)
 	warning_at (loc, OPT_Wconversion,
 		    "conversion to %qT alters %qT constant value",
 		    type, expr_type);
@@ -2747,7 +2753,12 @@ conversion_warning (tree type, tree expr
       }
 
     default: /* 'expr' is not a constant.  */
-      if (unsafe_conversion_p (type, expr, true))
+      conversion_kind = unsafe_conversion_p (type, expr, true);
+      if (conversion_kind == UNSAFE_REAL)
+	warning_at (loc, OPT_Wfloat_conversion,
+		    "conversion to %qT from %qT may alter its value",
+		    type, expr_type);
+      else if (conversion_kind)
 	warning_at (loc, OPT_Wconversion,
 		    "conversion to %qT from %qT may alter its value",
 		    type, expr_type);
Index: gcc/c-family/c-common.h
===================================================================
--- gcc/c-family/c-common.h	(revision 203640)
+++ gcc/c-family/c-common.h	(working copy)
@@ -685,6 +685,16 @@ struct visibility_flags
   unsigned inlines_hidden : 1;	/* True when -finlineshidden in effect.  */
 };
 
+/* These enumerators are possible types of unsafe conversions.
+   SAFE_CONVERSION The conversion is safe
+   UNSAFE_OTHER Another type of conversion with problems
+   UNSAFE_SIGN Conversion between signed and unsigned integers
+    which are all warned about immediately, so this is unused
+   UNSAFE_REAL Conversions that reduce the precision of reals
+    including conversions from reals to integers
+ */
+enum conversion_safety { SAFE_CONVERSION = 0, UNSAFE_OTHER, UNSAFE_SIGN, UNSAFE_REAL };
+
 /* Global visibility options.  */
 extern struct visibility_flags visibility_options;
 
@@ -738,7 +748,7 @@ extern tree c_common_signed_type (tree);
 extern tree c_common_signed_or_unsigned_type (int, tree);
 extern void c_common_init_ts (void);
 extern tree c_build_bitfield_integer_type (unsigned HOST_WIDE_INT, int);
-extern bool unsafe_conversion_p (tree, tree, bool);
+extern enum conversion_safety unsafe_conversion_p (tree, tree, bool);
 extern bool decl_with_nonnull_addr_p (const_tree);
 extern tree c_fully_fold (tree, bool, bool *);
 extern tree decl_constant_value_for_optimization (tree);
Index: gcc/c-family/c.opt
===================================================================
--- gcc/c-family/c.opt	(revision 203640)
+++ gcc/c-family/c.opt	(working copy)
@@ -387,6 +387,10 @@ Werror-implicit-function-declaration
 C ObjC RejectNegative Warning Alias(Werror=, implicit-function-declaration)
 This switch is deprecated; use -Werror=implicit-function-declaration instead
 
+Wfloat-conversion
+C ObjC C++ ObjC++ Var(warn_float_conversion) LangEnabledBy(C ObjC C++ ObjC++,Wconversion)
+Warn for implicit type conversions that cause loss of floating point precision
+
 Wfloat-equal
 C ObjC C++ ObjC++ Var(warn_float_equal) Warning
 Warn if testing floating point numbers for equality
Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi	(revision 203640)
+++ gcc/doc/invoke.texi	(working copy)
@@ -263,7 +263,8 @@ Objective-C and Objective-C++ Dialects}.
 -Wpointer-arith  -Wno-pointer-to-int-cast @gol
 -Wredundant-decls  -Wno-return-local-addr @gol
 -Wreturn-type  -Wsequence-point  -Wshadow @gol
--Wsign-compare  -Wsign-conversion  -Wsizeof-pointer-memaccess @gol
+-Wsign-compare  -Wsign-conversion -Wfloat-conversion @gol
+-Wsizeof-pointer-memaccess @gol
 -Wstack-protector -Wstack-usage=@var{len} -Wstrict-aliasing @gol
 -Wstrict-aliasing=n @gol -Wstrict-overflow -Wstrict-overflow=@var{n} @gol
 -Wsuggest-attribute=@r{[}pure@r{|}const@r{|}noreturn@r{|}format@r{]} @gol
@@ -4570,6 +4571,14 @@ value, like assigning a signed integer e
 integer variable. An explicit cast silences the warning. In C, this
 option is enabled also by @option{-Wconversion}.
 
+@item -Wfloat-conversion
+@opindex Wfloat-conversion
+@opindex Wno-float-conversion
+Warn for implicit conversions that reduce the precision of a real value.
+This includes conversions from real to integer, and from higher precision
+real to lower precision real values.  This option is also enabled by
+@option{-Wconversion}.
+
 @item -Wsizeof-pointer-memaccess
 @opindex Wsizeof-pointer-memaccess
 @opindex Wno-sizeof-pointer-memaccess
Index: gcc/testsuite/c-c++-common/Wfloat-conversion.c
===================================================================
--- gcc/testsuite/c-c++-common/Wfloat-conversion.c	(revision 0)
+++ gcc/testsuite/c-c++-common/Wfloat-conversion.c	(working copy)
@@ -0,0 +1,58 @@
+/* Test for diagnostics for Wconversion for floating-point.  */
+
+/* { dg-do compile } */
+/* { dg-options "-std=c99 -Wfloat-conversion" { target c } } */
+/* { dg-options "-Wfloat-conversion" { target c++ } } */
+/* { dg-require-effective-target large_double } */
+/* { dg-require-effective-target int32plus } */
+/* { dg-require-effective-target double64plus } */
+#include <limits.h>
+
+float  vfloat;
+double vdouble;
+long double vlongdouble;
+int bar;
+
+void fsi (signed int x);
+void fui (unsigned int x);
+void ffloat (float f);
+void fdouble (double d);
+void flongdouble (long double ld);
+
+void h (void)
+{
+  unsigned int ui = 3;
+  int   si = 3;
+  unsigned char uc = 3;
+  signed char sc = 3;
+  float f = 0;
+  double d = 0;
+  long double ld = 0;
+
+  ffloat (3.1); /* { dg-warning "conversion to 'float' alters 'double' constant value" } */
+  vfloat = 3.1; /* { dg-warning "conversion to 'float' alters 'double' constant value" } */
+  ffloat (3.1L); /* { dg-warning "conversion to 'float' alters 'long double' constant value" } */
+  vfloat = 3.1L;  /* { dg-warning "conversion to 'float' alters 'long double' constant value" } */
+  fdouble (3.1L); /* { dg-warning "conversion to 'double' alters 'long double' constant value" "" { target large_long_double } } */
+  vdouble = 3.1L; /* { dg-warning "conversion to 'double' alters 'long double' constant value" "" { target large_long_double } } */
+  ffloat (vdouble); /* { dg-warning "conversion to 'float' from 'double' may alter its value" } */
+  vfloat = vdouble; /* { dg-warning "conversion to 'float' from 'double' may alter its value" } */
+  ffloat (vlongdouble); /* { dg-warning "conversion to 'float' from 'long double' may alter its value" } */
+  vfloat = vlongdouble; /* { dg-warning "conversion to 'float' from 'long double' may alter its value" } */
+  fdouble (vlongdouble); /* { dg-warning "conversion to 'double' from 'long double' may alter its value" "" { target large_long_double } } */
+  vdouble = vlongdouble; /* { dg-warning "conversion to 'double' from 'long double' may alter its value" "" { target large_long_double } } */
+
+  fsi (3.1f); /* { dg-warning "conversion to 'int' alters 'float' constant value" } */
+  si = 3.1f; /* { dg-warning "conversion to 'int' alters 'float' constant value" } */
+  fsi (3.1);  /* { dg-warning "conversion to 'int' alters 'double' constant value" } */
+  si = 3.1;  /* { dg-warning "conversion to 'int' alters 'double' constant value" } */
+  fsi (d);    /* { dg-warning "conversion to 'int' from 'double' may alter its value" } */
+  si = d;    /* { dg-warning "conversion to 'int' from 'double' may alter its value" } */
+  ffloat (INT_MAX);  /* { dg-warning "conversion to 'float' alters 'int' constant value" } */
+  vfloat = INT_MAX;  /* { dg-warning "conversion to 'float' alters 'int' constant value" } */
+  ffloat (16777217); /* { dg-warning "conversion to 'float' alters 'int' constant value" } */
+  vfloat = 16777217; /* { dg-warning "conversion to 'float' alters 'int' constant value" } */
+
+  sc = bar != 0 ? 2.1 : 10; /* { dg-warning "conversion to 'signed char' alters 'double' constant value" } */
+  uc = bar != 0 ? 2.1 : 10; /* { dg-warning "conversion to 'unsigned char' alters 'double' constant value" } */
+}
