Tcl Source Code

Donal Fellows has authored bytecode improvements in the branch https://core.tcl-lang.org/tcl/timeline?r=no-variable-width-instruction-issue&c=2025-05-04+16%3A43%3A44.

The posted descriptions are recorded in this ticket by the cronicler Harald Oehlmann:

The branch deprecates all operations that have a single byte for a jump offset, a variable index or an argument count (except for string concatenation). These are replaced with versions that use four-byte offsets/indices/counts. The affected instructions are:

• INST_PUSH1
• INST_INVOKE_STK1
• INST_LOAD_SCALAR1
• INST_LOAD_SCALAR_STK (wasn't issues by our compiler anyway)
• INST_LOAD_ARRAY1
• INST_STORE_SCALAR1
• INST_STORE_SCALAR_STK (another never-generated one)
• INST_STORE_ARRAY1
• INST_INCR_SCALAR1
• INST_INCR_ARRAY1
• INST_INCR_SCALAR1_IMM
• INST_INCR_ARRAY1_IMM
• INST_APPEND_SCALAR1
• INST_APPEND_ARRAY1
• INST_LAPPEND_SCALAR1
• INST_LAPPEND_ARRAY1
• INST_RETURN_CODE_BRANCH
• INST_TAILCALL
• INST_TCLOO_NEXT
• INST_TCLOO_NEXT_CLASS

Some of these are replaced with their existing 4-byte versions. Some have new versions (now with 4-byte args):

• INST_INCR_SCALAR
• INST_INCR_ARRAY
• INST_INCR_SCALAR_IMM
• INST_INCR_ARRAY_IMM
• INST_TAILCALL
• INST_TCLOO_NEXT
• INST_TCLOO_NEXT_CLASS

The replacement for INST_RETURN_CODE_BRANCH is INST_JUMP_TABLE_NUM that is a general numeric-keyed jump table. (Take note for the Tcl Compiler and TBCLoad: there's a new AUX type).

I also add these new opcodes:

• INST_SWAP - Swap the top two items on the stack.
• INST_ERROR_PREFIX_EQ - Special equality for [try/trap]
• INST_TCLOO_ID - Get the creation ID of an object; not common, but easy
• INST_DICT_PUT - [dict replace] as an opcode, simplifying [try]
• INST_DICT_REMOVE - [dict remove] as an opcode, to fill out the suite available
• INST_IS_EMPTY - access to Tcl_IsEmpty (and enhancements to bytecode optimiser to use it)

Some of the operations are made available to [tcl::unsupported::assemble]. Only the error prefix comparator isn't; that's got safety requirements on its argument that I'm not bothering to make the assembler understand.

As far as I can tell, the test suite is passing. At least on Windows and excluding some tests that don't run on this laptop.

A side note: the INST_JUMP_TABLE_NUM instruction will enable the addition of a new mode of operation for [switch] that I've been thinking about for a while: switching on integer equality, which I've seen the need for in handling things like switching on the number of arguments passed to a procedure. (It can't be safely done now because we use the wrong sort of equality testing.) I'll TIP that up once the instruction to make it practical hits the trunk.