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Clarify define_insn documentation 

This patch tries to clarify some of the restrictions on define_insn
conditions, and also on the use of "#".

2017-06-06  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* doc/md.texi: Clarify the restrictions on a define_insn condition.
	Say that # requires an associated define_split to exist, and that
	the define_split must be suitable for use after register allocation.

From-SVN: r248946
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Richard Sandiford 2017-06-07 06:45:36 +00:00 committed by Richard Sandiford
parent 6fcdf71413
commit 49e478afe3
2 changed files with 27 additions and 4 deletions
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6
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@ -1,3 +1,9 @@
2017-06-07 Richard Sandiford <richard.sandiford@linaro.org>
* doc/md.texi: Clarify the restrictions on a define_insn condition.
Say that # requires an associated define_split to exist, and that
the define_split must be suitable for use after register allocation.
2017-06-06 Jan Hubicka <hubicka@ucw.cz>
* cfgbuild.c (find_bb_boundaries): Initialize profile of split blocks.

25
gcc/doc/md.texi
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@ -115,7 +115,7 @@ A @code{define_insn} is an RTL expression containing four or five operands:
@enumerate
@item
An optional name. The presence of a name indicate that this instruction
An optional name. The presence of a name indicates that this instruction
pattern can perform a certain standard job for the RTL-generation
pass of the compiler. This pass knows certain names and will use
the instruction patterns with those names, if the names are defined
@ -166,9 +166,21 @@ individual insn, and only after the insn has matched the pattern's
recognition template. The insn's operands may be found in the vector
@code{operands}.
For an insn where the condition has once matched, it
cannot later be used to control register allocation by excluding
certain register or value combinations.
An instruction condition cannot become more restrictive as compilation
progresses. If the condition accepts a particular RTL instruction at
one stage of compilation, it must continue to accept that instruction
until the final pass. For example, @samp{!reload_completed} and
@samp{can_create_pseudo_p ()} are both invalid instruction conditions,
because they are true during the earlier RTL passes and false during
the later ones. For the same reason, if a condition accepts an
instruction before register allocation, it cannot later try to control
register allocation by excluding certain register or value combinations.
Although a condition cannot become more restrictive as compilation
progresses, the condition for a nameless pattern @emph{can} become
more permissive. For example, a nameless instruction can require
@samp{reload_completed} to be true, in which case it only matches
after register allocation.
@item
The @dfn{output template} or @dfn{output statement}: This is either
@ -561,6 +573,11 @@ already defined, then you can simply use @code{#} as the output template
instead of writing an output template that emits the multiple assembler
instructions.
Note that @code{#} only has an effect while generating assembly code;
it does not affect whether a split occurs earlier. An associated
@code{define_split} must exist and it must be suitable for use after
register allocation.
If the macro @code{ASSEMBLER_DIALECT} is defined, you can use construct
of the form @samp{@{option0|option1|option2@}} in the templates. These
describe multiple variants of assembler language syntax.