isaspec: Add support for "absolute" branches

afuc has branches which use an absolute offset in instructions from the
microcode base. Add another type to support them.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23949>

docs/drivers/freedreno/isaspec.rst

@@ -244,6 +244,38 @@ In the case of ``<override>`` elements, the override applies if the expression
 evaluates to non-zero.  In the case of ``<derived>`` fields, the expression
 evaluates to the value of the derived field.
 
+Branching
+---------
+
+isaspec supports a few special field types for printing branch destinations. If
+``isaspec_decode_options::branch_labels`` is true, a pre-pass over the program
+to be disassembled determines which instructions are branch destinations and
+then they are printed when disassembling, in addition to printing the name of
+the destination when printing the field itself.
+
+There are two different types, which affect how the destination is computed. If
+the field type is ``branch``, then the field is interpreted as a signed offset
+from the current instruction. If the type is ``absbranch``, then it is
+interpreted as an offset from the first instruction to be disassembled. In
+either case, the offset is multiplied by the instruction size.
+
+For example, here is what a signed-offset unconditional jump instruction might
+look like:
+
+.. code-block:: xml
+
+   <bitset name="jump" extends="#instruction">
+      <display>
+         jump #{OFFSET}
+      </display>
+      <pattern low="26" high="31">110010</pattern> <!-- opcode goes here -->
+      <field name="OFFSET" low="0" high="25" type="branch"/>
+   </bitset>
+
+This would produce a disassembly like ``jump #l42`` if the destination is 42
+instructions after the start of the disassembly. The destination would be
+preceded by a line with just ``l42:``.
+
 Encoding
 --------
 

src/compiler/isaspec/isa.py

@@ -492,7 +492,7 @@ class ISA(object):
 
         # Validate that all bitset fields have valid types, and in
         # the case of bitset type, the sizes match:
-        builtin_types = ['branch', 'int', 'uint', 'hex', 'offset', 'uoffset', 'float', 'bool', 'enum']
+        builtin_types = ['branch', 'absbranch', 'int', 'uint', 'hex', 'offset', 'uoffset', 'float', 'bool', 'enum']
         for bitset_name, bitset in self.bitsets.items():
             if bitset.extends is not None:
                 assert bitset.extends in self.bitsets, "{} extends invalid type: {}".format(

src/compiler/isaspec/isaspec_decode_decl.h

@@ -79,7 +79,8 @@ struct isa_field {
 	unsigned high;
 	enum {
 		/* Basic types: */
-		TYPE_BRANCH,   /* branch target, like INT but optional labeling*/
+		TYPE_BRANCH,   /* relative branch target, like INT but optional labeling*/
+		TYPE_ABSBRANCH,   /* absolute branch target */
 		TYPE_INT,
 		TYPE_UINT,
 		TYPE_HEX,

src/compiler/isaspec/isaspec_decode_impl.c

@@ -610,8 +610,14 @@ display_field(struct decode_scope *scope, const char *field_name)
 	switch (field->type) {
 	/* Basic types: */
 	case TYPE_BRANCH:
+        case TYPE_ABSBRANCH:
 		if (scope->state->options->branch_labels) {
-			int offset = util_sign_extend(val, width) + scope->state->n;
+			int offset;
+			if (field->type == TYPE_BRANCH) {
+				offset = util_sign_extend(val, width) + scope->state->n;
+			} else {
+				offset = val;
+			}
 			if (offset < scope->state->num_instr) {
 				print(scope->state, "l%d", offset);
 				BITSET_SET(scope->state->branch_targets, offset);