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-rw-r--r--docs/devel/decodetree.rst58
1 files changed, 58 insertions, 0 deletions
diff --git a/docs/devel/decodetree.rst b/docs/devel/decodetree.rst
index 62cb7f687c..44ac621ea8 100644
--- a/docs/devel/decodetree.rst
+++ b/docs/devel/decodetree.rst
@@ -161,3 +161,61 @@ which will, in part, invoke::
and::
trans_addl_i(ctx, &arg_opi, insn)
+
+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.
+
+The following example from PA-RISC shows specialization of the *or*
+instruction::
+
+ {
+ {
+ nop 000010 ----- ----- 0000 001001 0 00000
+ copy 000010 00000 r1:5 0000 001001 0 rt:5
+ }
+ or 000010 rt2:5 r1:5 cf:4 001001 0 rt:5
+ }
+
+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
+the canonical register copy operation.
+
+The output from the generator might look like::
+
+ switch (insn & 0xfc000fe0) {
+ case 0x08000240:
+ /* 000010.. ........ ....0010 010..... */
+ if ((insn & 0x0000f000) == 0x00000000) {
+ /* 000010.. ........ 00000010 010..... */
+ if ((insn & 0x0000001f) == 0x00000000) {
+ /* 000010.. ........ 00000010 01000000 */
+ extract_decode_Fmt_0(&u.f_decode0, insn);
+ if (trans_nop(ctx, &u.f_decode0)) return true;
+ }
+ if ((insn & 0x03e00000) == 0x00000000) {
+ /* 00001000 000..... 00000010 010..... */
+ extract_decode_Fmt_1(&u.f_decode1, insn);
+ if (trans_copy(ctx, &u.f_decode1)) return true;
+ }
+ }
+ extract_decode_Fmt_2(&u.f_decode2, insn);
+ if (trans_or(ctx, &u.f_decode2)) return true;
+ return false;
+ }