Re: [PATCH] doc clarification: DONE and FAIL in define_split and define_peephole2

[Paul Koning <paulkoning at comcast dot net>]

Thanks Richard.  Some comments, and an updated proposed patch.

	paul


> On Jul 6, 2018, at 9:04 AM, Richard Sandiford <richard.sandiford@arm.com> wrote:
> 
> ...
>> @@ -8232,6 +8256,15 @@ functionality as two separate @code{define_insn} a
>> patterns.  It exists for compactness, and as a maintenance tool to prevent
>> having to ensure the two patterns' templates match.
>> 
>> +In @code{define_insn_and_split}, the output template is usually simply
>> +@samp{#} since the assembly output is done by @code{define_insn}
>> +statements matching the generated insns, not by this
>> +@code{define_insn_and_split} statement.  But if @code{FAIL} is used in
>> +the preparation statements for certain input insns, those will not be
>> +split and during assembly output will again match this
>> +@code{define_insn_and_split}.  In that case, the appropriate assembly
>> +output statements are needed in the output template.
>> +
> 
> I agree "#" on its own is relatively common, but it's also not that
> unusual to have a define_insn_and_split in which the define_insn part
> handles simple alternatives directly and leaves more complex ones to
> be split.  Maybe that's more common on RISC-like targets.
> 
> Also, the define_split matches the template independently of the
> define_insn, so it can sometimes split insns that match an earlier
> define_insn rather than the one in the define_insn_and_split.
> (That might be bad practice.)  So using "#" and FAIL together is valid
> if the FAIL only happens for cases that match earlier define_insns.
> 
> Another case is when the define_split condition doesn't start with
> "&&" and is less strict than the define_insn condition.  This can
> be useful if the define_split is supposed to match patterns created
> by combine.
> 
> So maybe we should instead expand the FAIL documentation to say that
> a define_split must not FAIL when splitting an instruction whose
> output template is "#".

I played with this a bit and couldn't come up with a good wording.  The case is already covered; I was trying to make it clearer but your comments indicate the picture is bigger than I thought.

>> @@ -8615,6 +8648,31 @@ so here's a silly made-up example:
>>   "")
>> @end smallexample
>> 
>> +There are two special macros defined for use in the preparation statements:
>> +@code{DONE} and @code{FAIL}.  Use them with a following semicolon,
>> +as a statement.
>> +
>> +@table @code
>> +
>> +@findex DONE
>> +@item DONE
>> +Use the @code{DONE} macro to end RTL generation for the peephole.  The
>> +only RTL insns generated as replacement for the matched input insn will
>> +be those already emitted by explicit calls to @code{emit_insn} within
>> +the preparation statements; the replacement pattern is not used.
>> +
>> +@findex FAIL
>> +@item FAIL
>> +Make the @code{define_peephole2} fail on this occasion.  When a @code{define_peephole2}
>> +fails, it means that the replacement was not truly available for the
>> +particular inputs it was given, and the input insns are left unchanged.
> 
> If it FAILs, GCC will try to apply later define_peehole2s instead.
> (This is in contrast to define_split, so it's a bit inconsistent.
> Would be easy to make define_split behave the same way if there was a
> motivating case.)

Interesting.  I added words to the FAIL description of both define_split and define_peephole2 to state the behavior and highlight the difference.

ChangeLog:

2018-07-05  Paul Koning  <ni1d@arrl.net>

	* doc/md.texi (define_split): Document DONE and FAIL.  Describe
	interaction with usual "#" output template in
	define_insn_and_split.
	(define_peephole2): Document DONE and FAIL.

Index: doc/md.texi
===================================================================
--- doc/md.texi	(revision 262455)
+++ doc/md.texi	(working copy)
@@ -8060,6 +8060,30 @@ those in @code{define_expand}, however, these stat
 generate any new pseudo-registers.  Once reload has completed, they also
 must not allocate any space in the stack frame.
 
+There are two special macros defined for use in the preparation statements:
+@code{DONE} and @code{FAIL}.  Use them with a following semicolon,
+as a statement.
+
+@table @code
+
+@findex DONE
+@item DONE
+Use the @code{DONE} macro to end RTL generation for the splitter.  The
+only RTL insns generated as replacement for the matched input insn will
+be those already emitted by explicit calls to @code{emit_insn} within
+the preparation statements; the replacement pattern is not used.
+
+@findex FAIL
+@item FAIL
+Make the @code{define_split} fail on this occasion.  When a @code{define_split}
+fails, it means that the splitter was not truly available for the inputs
+it was given, and the input insn will not be split.
+@end table
+
+If the preparation falls through (invokes neither @code{DONE} nor
+@code{FAIL}), then the @code{define_split} uses the replacement
+template.
+
 Patterns are matched against @var{insn-pattern} in two different
 circumstances.  If an insn needs to be split for delay slot scheduling
 or insn scheduling, the insn is already known to be valid, which means
@@ -8615,6 +8639,34 @@ so here's a silly made-up example:
   "")
 @end smallexample
 
+There are two special macros defined for use in the preparation statements:
+@code{DONE} and @code{FAIL}.  Use them with a following semicolon,
+as a statement.
+
+@table @code
+
+@findex DONE
+@item DONE
+Use the @code{DONE} macro to end RTL generation for the peephole.  The
+only RTL insns generated as replacement for the matched input insn will
+be those already emitted by explicit calls to @code{emit_insn} within
+the preparation statements; the replacement pattern is not used.
+
+@findex FAIL
+@item FAIL
+Make the @code{define_peephole2} fail on this occasion.  When a @code{define_peephole2}
+fails, it means that the replacement was not truly available for the
+particular inputs it was given.  In that case, GCC may still apply a
+later @code{define_peephole2} that also matches the given insn pattern.
+(Note that this is different from @code{define_split}, where @code{FAIL}
+prevents the input insn from being split at all.)
+@end table
+
+If the preparation falls through (invokes neither @code{DONE} nor
+@code{FAIL}), then the @code{define_peephole2} uses the replacement
+template.
+
+
 @noindent
 If we had not added the @code{(match_dup 4)} in the middle of the input
 sequence, it might have been the case that the register we chose at the