> From: Hans-Peter Nilsson <h...@axis.com>
> CC: <gcc-patches@gcc.gnu.org>, <hubi...@ucw.cz>
> Date: Tue, 14 Mar 2023 17:04:43 +0100
Ping on contents (formatting is approved):
> I needed to check what was allowed in a define_split, but
> had problems understanding what was meant by "Splitting of
> jump instruction into sequence that over by another jump
> instruction".
>
> * doc/md.texi (Insn Splitting): Tweak wording for readability.
>
> Co-Authored-By: Sandra Loosemore <san...@codesourcery.com>
> ---
> gcc/doc/md.texi | 30 +++++++++++++++---------------
> 1 file changed, 15 insertions(+), 15 deletions(-)
>
> diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi
> index 8e3113599fdc..134b227b9a93 100644
> --- a/gcc/doc/md.texi
> +++ b/gcc/doc/md.texi
> @@ -8756,21 +8756,21 @@ insns that don't. Instead, write two separate
> @code{define_split}
> definitions, one for the insns that are valid and one for the insns that
> are not valid.
>
> -The splitter is allowed to split jump instructions into sequence of
> -jumps or create new jumps in while splitting non-jump instructions. As
> -the control flow graph and branch prediction information needs to be updated,
> -several restriction apply.
> -
> -Splitting of jump instruction into sequence that over by another jump
> -instruction is always valid, as compiler expect identical behavior of new
> -jump. When new sequence contains multiple jump instructions or new labels,
> -more assistance is needed. Splitter is required to create only unconditional
> -jumps, or simple conditional jump instructions. Additionally it must attach
> a
> -@code{REG_BR_PROB} note to each conditional jump. A global variable
> -@code{split_branch_probability} holds the probability of the original branch
> in case
> -it was a simple conditional jump, @minus{}1 otherwise. To simplify
> -recomputing of edge frequencies, the new sequence is required to have only
> -forward jumps to the newly created labels.
> +The splitter is allowed to split jump instructions into a sequence of jumps
> or
> +create new jumps while splitting non-jump instructions. As the control flow
> +graph and branch prediction information needs to be updated after the
> splitter
> +runs, several restrictions apply.
> +
> +Splitting of a jump instruction into a sequence that has another jump
> +instruction to the same label is always valid, as the compiler expects
> +identical behavior of the new jump. When the new sequence contains multiple
> +jump instructions or new labels, more assistance is needed. The splitter is
> +permitted to create only unconditional jumps, or simple conditional jump
> +instructions. Additionally it must attach a @code{REG_BR_PROB} note to each
> +conditional jump. A global variable @code{split_branch_probability} holds
> the
> +probability of the original branch in case it was a simple conditional jump,
> +@minus{}1 otherwise. To simplify recomputing of edge frequencies, the new
> +sequence is permitted to have only forward jumps to the newly-created labels.
>
> @findex define_insn_and_split
> For the common case where the pattern of a define_split exactly matches the
> --
> 2.30.2
>
> brgds, H-P
>
