[Ada] Fix floating-point exponentiation with Integer'First exponent

Message ID 20210616084356.GA96108@adacore.com
State New
Headers show
Series
  • [Ada] Fix floating-point exponentiation with Integer'First exponent
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Commit Message

Pierre-Marie de Rodat June 16, 2021, 8:43 a.m.
It works neither at compile time nor at run time because of the classical
issue that -Integer'First is not a valid Integer value.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* urealp.adb (Scale): Change first paramter to Uint and adjust.
	(Equivalent_Decimal_Exponent): Pass U.Den directly to Scale.
	* libgnat/s-exponr.adb (Negative): Rename to...
	(Safe_Negative): ...this and change its lower bound.
	(Exponr): Adjust to above renaming and deal with Integer'First.

Patch

diff --git a/gcc/ada/libgnat/s-exponr.adb b/gcc/ada/libgnat/s-exponr.adb
--- a/gcc/ada/libgnat/s-exponr.adb
+++ b/gcc/ada/libgnat/s-exponr.adb
@@ -57,8 +57,8 @@  function System.Exponr (Left : Num; Right : Integer) return Num is
    subtype Double_T is Double_Real.Double_T;
    --  The double floating-point type
 
-   subtype Negative is Integer range Integer'First .. -1;
-   --  The range of negative exponents
+   subtype Safe_Negative is Integer range Integer'First + 1 .. -1;
+   --  The range of safe negative exponents
 
    function Expon (Left : Num; Right : Natural) return Num;
    --  Routine used if Right is greater than 4
@@ -113,9 +113,12 @@  begin
             return Num'Machine (Sqr * Sqr);
          end;
 
-      when Negative =>
+      when Safe_Negative =>
          return Num'Machine (1.0 / Exponr (Left, -Right));
 
+      when Integer'First =>
+         return Num'Machine (1.0 / (Exponr (Left, Integer'Last) * Left));
+
       when others =>
          return Num'Machine (Expon (Left, Right));
    end case;


diff --git a/gcc/ada/urealp.adb b/gcc/ada/urealp.adb
--- a/gcc/ada/urealp.adb
+++ b/gcc/ada/urealp.adb
@@ -270,23 +270,21 @@  package body Urealp is
         15 => (Num =>  53_385_559, Den =>   45_392_361),  -- 1.176091259055681
         16 => (Num =>  78_897_839, Den =>   65_523_237)); -- 1.204119982655924
 
-      function Scale (X : Int; R : Ratio) return Int;
+      function Scale (X : Uint; R : Ratio) return Int;
       --  Compute the value of X scaled by R
 
       -----------
       -- Scale --
       -----------
 
-      function Scale (X : Int; R : Ratio) return Int is
-         type Wide_Int is range -2**63 .. 2**63 - 1;
-
+      function Scale (X : Uint; R : Ratio) return Int is
       begin
-         return Int (Wide_Int (X) * Wide_Int (R.Num) / Wide_Int (R.Den));
+         return UI_To_Int (X * R.Num / R.Den);
       end Scale;
 
    begin
       pragma Assert (U.Rbase /= 0);
-      return Scale (UI_To_Int (U.Den), Logs (U.Rbase));
+      return Scale (U.Den, Logs (U.Rbase));
    end Equivalent_Decimal_Exponent;
 
    ----------------