doc: document -ftree-scev-cprop

Hi,

PR 86726 is pointing out that -ftree-scev-cprop is not documented.

Here's documentation for the final-value-replacement aspect of the
option. It technically can also replace in-loop references by 
constants, but that doesn't seem very useful.

I'm not aware of any required order for option paragraphs; placing
new text between -floop-* and -fivopts.

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

Alexander Monakov <amonakov@ispras.ru> writes:
> @@ -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 a way that its value when exiting the loop can be determined using

"in such a way"

> +only its initial value and number of loop iterations, replace uses of

"and the number"

> +the final value by such computation, provided it is sufficiently cheap.

"by such a computation".

OK otherwise, thanks.

Richard

> +This reduces data dependencies and may allow further simplifications.
> +Enabled by default at @option{-O} and higher.

On Tue, Aug 28, 2018 at 4:22 PM Alexander Monakov <amonakov@ispras.ru> wrote:
>
> Hi,
>
> PR 86726 is pointing out that -ftree-scev-cprop is not documented.
>
> Here's documentation for the final-value-replacement aspect of the
> option. It technically can also replace in-loop references by
> constants, but that doesn't seem very useful.

Yeah, it's on my list of things to investigate whether we can remove
that...  (aka, look for testsuite fallout)

>
> I'm not aware of any required order for option paragraphs; placing
> new text between -floop-* and -fivopts.
>
>         PR other/86726
>         * invoke.texi (Optimization Options): List -ftree-scev-cprop.
>         (-O): Ditto.
>         (-ftree-scev-cprop): Document.
>
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index e37233d6ed4..de11ae27c95 100644
> --- a/gcc/doc/invoke.texi
> +++ b/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 a way that its value when exiting the loop can be determined using
> +only its initial value and number of loop iterations, replace uses of
> +the final value by such 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