[Ada] More precise setting of Do_Overflow_Check flag for division

[Arnaud Charlet <charlet at adacore dot com>]

The Do_Overflow_Check flag was being set on division operators in
many cases where it was not needed. Now the flag will be set only
if there is a possibility of the (largest neg number) / (-1) case
and this only if code is not being generated (-gnatc mode), since
if code is generated, the check is explicitly generated. The test
program:

     1. procedure JunkDCheck
     2.   (Y : Positive;
     3.    X, Z : in out Integer) is
     4. begin
     5.    Z := X / Z;
     6.    X := 10 / Y;
     7. end;

if compiled with -gnatc -gnatG generates:

procedure junkdcheck
 (y : positive; x : in out integer;
  z : in out integer) is
begin
   z := x {/} z;
   x := 10 / y;
end junkdcheck;

(previously the second assignment had {/}

and if compiled with -gnatc -gnatG:

procedure junkdcheck
 (y : positive; x : in out integer;
  z : in out integer) is
begin
   [constraint_error when
     z = 0
     "divide by zero"]
   [constraint_error when
     x = -16#8000_0000# and then z = -1
     "overflow check failed"]
   z := x / z;
   x := 10 / y;
   return;
end junkdcheck;

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

2014-07-31  Robert Dewar  <dewar@adacore.com>

	* checks.adb (Enable_Overflow_Check): More precise setting of
	Do_Overflow_Check flag for division.

Attachment: difs
Description: Text document