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docs/devel: Document decodetree no-overlap groups

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Fixes: 067e8b0f45
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200803205708.315829-1-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
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Richard Henderson 2020-08-03 13:57:08 -07:00 committed by Peter Maydell
parent d15532d91b
commit ffdfca6fac
1 changed files with 18 additions and 11 deletions
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29
docs/devel/decodetree.rst
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@ -173,18 +173,25 @@ Pattern Groups
Syntax::
group := '{' ( pat_def | group )+ '}'
group := overlap_group | no_overlap_group
overlap_group := '{' ( pat_def | group )+ '}'
no_overlap_group := '[' ( pat_def | group )+ ']'
A *group* begins with a lone open-brace, with all subsequent lines
indented two spaces, and ending with a lone close-brace. Groups
may be nested, increasing the required indentation of the lines
within the nested group to two spaces per nesting level.
A *group* begins with a lone open-brace or open-bracket, with all
subsequent lines indented two spaces, and ending with a lone
close-brace or close-bracket. Groups may be nested, increasing the
required indentation of the lines within the nested group to two
spaces per nesting level.
Unlike ungrouped patterns, grouped patterns are allowed to overlap.
Conflicts are resolved by selecting the patterns in order. If all
of the fixedbits for a pattern match, its translate function will
be called. If the translate function returns false, then subsequent
patterns within the group will be matched.
Patterns within overlap groups are allowed to overlap. Conflicts are
resolved by selecting the patterns in order. If all of the fixedbits
for a pattern match, its translate function will be called. If the
translate function returns false, then subsequent patterns within the
group will be matched.
Patterns within no-overlap groups are not allowed to overlap, just
the same as ungrouped patterns. Thus no-overlap groups are intended
to be nested inside overlap groups.
The following example from PA-RISC shows specialization of the *or*
instruction::
@ -200,7 +207,7 @@ instruction::
When the *cf* field is zero, the instruction has no side effects,
and may be specialized. When the *rt* field is zero, the output
is discarded and so the instruction has no effect. When the *rt2*
field is zero, the operation is ``reg[rt] | 0`` and so encodes
field is zero, the operation is ``reg[r1] | 0`` and so encodes
the canonical register copy operation.
The output from the generator might look like::