Re: [15/23] recog: Add a validate_change_xveclen function

[Jeff Law via Gcc-patches]

On 11/13/20 1:19 AM, Richard Sandiford via Gcc-patches wrote:
> A later patch wants to be able to use the validate_change machinery
> to reduce the XVECLEN of a PARALLEL.  This should be more efficient
> than allocating a separate PARALLEL at a possibly distant memory
> location, especially since the new PARALLEL would be garbage rtl if
> the new pattern turns out not to match.  Combine already pulls this
> trick with SUBST_INT.
>
> This patch adds a general helper for doing that.
>
> gcc/
>       * recog.h (validate_change_xveclen): Declare.
>       * recog.c (change_t::old_len): New field.
>       (validate_change_1): Add a new_len parameter.  Conditionally
>       replace the XVECLEN of an rtx, avoiding single-element PARALLELs.
>       (validate_change_xveclen): New function.
>       (cancel_changes): Undo changes made by validate_change_xveclen.
OK.

Presumably it's better to reduce the length of the existing vector,
possibly losing a bit of memory off the end rather than releasing the
existing vector and allocating a new one of the desired length (as you
say it'd likely have no good locality).

And note that when I say lose, I mean lose the ability to use those
slots after we shrink the vector (say if we later wanted to lengthen
it).  The memory doesn't actually leak as it'll be released if we ever
release the vector.


Jeff