[Ada] Raise Constraint_Error in overflow case involving rounding

Function Scaled_Divide in s-arith runtime unit computes the combined
multiplication and division of its arguments ((X*Y) / Z). In a very
special case where the quotient computed before rounding is 2**64-1,
then rounding may lead to undesirable wrap-around, leading to a computed
quotient of 0 instead of raising Constraint_Error as expected.

This function is only called in the runtime for arithmetic operations
involving fixed-point types. Rounding is performed only when the target
type is of a decimal fixed-point type, and the attribute 'Round of the
type is used to round the result of the arithmetic operation.

This fix correctly raises Constraint_Error in this special case.

2019-09-17  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* libgnat/s-arit64.adb (Scaled_Divide): Add protection against
	undesirable wrap-around.

gcc/testsuite/

	* gnat.dg/multfixed.adb: New testcase.

From-SVN: r275791

gcc/ada/ChangeLog

 2019-09-17  Yannick Moy  <moy@adacore.com>
 
+	* libgnat/s-arit64.adb (Scaled_Divide): Add protection against
+	undesirable wrap-around.
+
+2019-09-17  Yannick Moy  <moy@adacore.com>
+
 	* libgnat/s-arit64.adb (Double_Divide): Fix two possible
 	overflows.
 

gcc/ada/libgnat/s-arit64.adb

@@ -511,6 +511,14 @@ package body System.Arith_64 is
       --  Deal with rounding case
 
       if Round and then Ru > (Zu - Uns64'(1)) / Uns64'(2) then
+
+         --  Protect against wrapping around when rounding, by signaling
+         --  an overflow when the quotient is too large.
+
+         if Qu = Uns64'Last then
+            Raise_Error;
+         end if;
+
          Qu := Qu + Uns64 (1);
       end if;
 

gcc/testsuite/ChangeLog

+2019-09-17  Yannick Moy  <moy@adacore.com>
+
+	* gnat.dg/multfixed.adb: New testcase.
+
 2019-09-17  Vadim Godunko  <godunko@adacore.com>
 
 	* gnat.dg/expect3.adb: New testcase.

gcc/testsuite/gnat.dg/multfixed.adb

+--  { dg-do run }
+
+with Ada.Exceptions; use Ada.Exceptions;
+
+procedure Multfixed is
+   Z : constant := 4387648782261400837.0;
+   type F1 is delta 1.0 / Z range 0.0 .. (2.0**63-1.0) / Z
+     with Small => 1.0 / Z;
+   type F2 is delta 1.0 range 0.0 .. (2.0**63-1.0)
+     with Small => 1.0;
+   type D is delta 1.0 digits 18;
+
+   X : F1 := 8914588002054909637.0 / Z;
+   Y : F2 := 9079256848778919936.0;
+   U : D;
+begin
+   U := D'Round(X * Y);
+   raise Program_Error;
+exception
+   when Exc : Constraint_Error =>
+      if Exception_Message (Exc) /= "System.Arith_64.Raise_Error: 64-bit arithmetic overflow" then
+         raise Program_Error;
+      end if;
+end Multfixed;
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