doc: document -ftree-scev-cprop

PR other/86726
	* invoke.texi (Optimization Options): List -ftree-scev-cprop.
	(-O): Ditto.
	(-ftree-scev-cprop): Document.

From-SVN: r263955

gcc/ChangeLog

@@ -1,3 +1,10 @@
+2018-08-29  Alexander Monakov  <amonakov@ispras.ru>
+
+	PR other/86726
+	* invoke.texi (Optimization Options): List -ftree-scev-cprop.
+	(-O): Ditto.
+	(-ftree-scev-cprop): Document.
+
 2018-08-29  Jan Hubicka  <jh@suse.cz>
 
 	* sreal.h (normalize, normalize_up, normalize_down): Add new_sig/new_exp

gcc/doc/invoke.texi

@@ -464,7 +464,7 @@ Objective-C and Objective-C++ Dialects}.
 -ftree-loop-ivcanon  -ftree-loop-linear  -ftree-loop-optimize @gol
 -ftree-loop-vectorize @gol
 -ftree-parallelize-loops=@var{n}  -ftree-pre  -ftree-partial-pre  -ftree-pta @gol
--ftree-reassoc  -ftree-sink  -ftree-slsr  -ftree-sra @gol
+-ftree-reassoc  -ftree-scev-cprop  -ftree-sink  -ftree-slsr  -ftree-sra @gol
 -ftree-switch-conversion  -ftree-tail-merge @gol
 -ftree-ter  -ftree-vectorize  -ftree-vrp  -funconstrained-commons @gol
 -funit-at-a-time  -funroll-all-loops  -funroll-loops @gol
@@ -7813,6 +7813,7 @@ compilation time.
 -ftree-forwprop @gol
 -ftree-fre @gol
 -ftree-phiprop @gol
+-ftree-scev-cprop @gol
 -ftree-sink @gol
 -ftree-slsr @gol
 -ftree-sra @gol
@@ -9120,6 +9121,15 @@ determining number of iterations requires complicated analysis.  Later
 optimizations then may determine the number easily.  Useful especially
 in connection with unrolling.
 
+@item -ftree-scev-cprop
+@opindex ftree-scev-cprop
+Perform final value replacement.  If a variable is modified in a loop
+in such a way that its value when exiting the loop can be determined using
+only its initial value and the number of loop iterations, replace uses of
+the final value by such a computation, provided it is sufficiently cheap.
+This reduces data dependencies and may allow further simplifications.
+Enabled by default at @option{-O} and higher.
+
 @item -fivopts
 @opindex fivopts
 Perform induction variable optimizations (strength reduction, induction