diff options
Diffstat (limited to 'docs/devel/decodetree.rst')
| -rw-r--r-- | docs/devel/decodetree.rst | 33 |
1 files changed, 20 insertions, 13 deletions
diff --git a/docs/devel/decodetree.rst b/docs/devel/decodetree.rst index ce7f52308f..74f66bf46e 100644 --- a/docs/devel/decodetree.rst +++ b/docs/devel/decodetree.rst @@ -173,18 +173,25 @@ Pattern Groups Syntax:: - 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. - -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. + 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 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. + +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:: |