[Ada] Optization of predicate checks

[Arnaud Charlet <charlet@adacore.com>]

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This patch marks the generated predicate functions as Pure, so that the
back-end can optimize redundant calls to these functions when inlining and
high level of optimization are requested.

This is a performance enhancement, no change in behavior.

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

2015-10-16  Ed Schonberg  <schonberg@adacore.com>

	* sem_ch13.adb (Build_Predicate_Functions): If expression for
	predicate is side-effect free, indicate that the predicate
	function is pure, to allow for optimization of redundant
	predicate checks.


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Index: sem_ch13.adb
===================================================================
--- sem_ch13.adb	(revision 228874)
+++ sem_ch13.adb	(working copy)
@@ -8702,6 +8702,16 @@
 
             Insert_Before_And_Analyze (N, FDecl);
             Insert_After_And_Analyze  (N, FBody);
+
+            --  Static predicate functions are always side-effect free, and
+            --  in most cases dynamic predicate functions are as well. Mark
+            --  them as such whenever possible, so redundant predicate checks
+            --  can be optimized.
+
+            if Expander_Active then
+               Set_Is_Pure (SId, Side_Effect_Free (Expr));
+               Set_Is_Inlined (SId);
+            end if;
          end;
 
          --  Test for raise expressions present and if so build M version