diff --git a/core/rexcode/ir/spirv/decoder.odin b/core/rexcode/ir/spirv/decoder.odin
index c75aa5252..7dcfd27ba 100644
--- a/core/rexcode/ir/spirv/decoder.odin
+++ b/core/rexcode/ir/spirv/decoder.odin
@@ -51,13 +51,16 @@ Decoder :: struct {
 	function_ids: [dynamic]Id,
 
 	// in-flight function / block
-	in_fn:     bool,
-	fn_sig:    Type_Ref,
-	fn_id:     Id,
-	fn_blocks: [dynamic]Block,
-	have_blk:  bool,
-	blk_id:    Id,
-	blk_ops:   [dynamic]Operation,
+	in_fn:       bool,
+	fn_sig:      Type_Ref,
+	fn_id:       Id,
+	fn_blocks:   [dynamic]Block,
+	fn_params:   [dynamic]Result,   // OpFunctionParameters, attached to the entry block
+	first_block: bool,
+	have_blk:    bool,
+	blk_id:      Id,
+	blk_params:  []Result,
+	blk_ops:     [dynamic]Operation,
 }
 
 @(private="file")
@@ -140,6 +143,13 @@ decode_operation :: proc(d: ^Decoder, opcode: Opcode, w: []u32) -> Operation {
 			wi += 1
 		}
 	}
+	// Trailing words beyond the fixed layout: the parameter operands an enum
+	// value/bit pulls in (MemoryAccess Aligned's alignment, etc.). Captured as
+	// literals -- enough to re-encode byte-exact (semantic typing is a refinement).
+	for wi < len(w) {
+		append(&ops, op_int(i64(w[wi])))
+		wi += 1
+	}
 	op.operands = ops[:]
 	return op
 }
@@ -147,8 +157,9 @@ decode_operation :: proc(d: ^Decoder, opcode: Opcode, w: []u32) -> Operation {
 @(private="file")
 finish_block :: proc(d: ^Decoder) {
 	if d.have_blk {
-		append(&d.fn_blocks, Block{id = d.blk_id, ops = d.blk_ops[:]})
+		append(&d.fn_blocks, Block{id = d.blk_id, ops = d.blk_ops[:], params = d.blk_params})
 		d.blk_ops = nil
+		d.blk_params = nil
 		d.have_blk = false
 	}
 }
@@ -240,10 +251,13 @@ lower :: proc(d: ^Decoder, opcode: Opcode, w: []u32) {
 	case .OpFunction:
 		d.in_fn = true
 		d.fn_id = Id(w[1]); d.fn_sig = tref(d, w[3])
-		d.fn_blocks = nil
+		d.fn_blocks = nil; d.fn_params = nil; d.first_block = true
+	case .OpFunctionParameter:
+		append(&d.fn_params, Result{id = Id(w[1]), type = tref(d, w[0])})
 	case .OpLabel:
 		finish_block(d)
 		d.have_blk = true; d.blk_id = Id(w[0]); d.blk_ops = nil
+		if d.first_block { d.blk_params = d.fn_params[:]; d.first_block = false }
 	case .OpFunctionEnd:
 		finish_function(d)
 
diff --git a/core/rexcode/ir/spirv/encoder.odin b/core/rexcode/ir/spirv/encoder.odin
index 530d0dd82..10ec5c3b6 100644
--- a/core/rexcode/ir/spirv/encoder.odin
+++ b/core/rexcode/ir/spirv/encoder.odin
@@ -274,7 +274,13 @@ emit_functions :: proc "contextless" (w: ^Writer, m: ^Module) {
 		w_word(w, 0)                                                    // FunctionControl (none)
 		w_id(w, tid(m, fn.signature))
 		inst_end(w, s, .OpFunction)
-		// (OpFunctionParameter not yet modelled in ir.Function)
+		// Function parameters: the entry block's params are the OpFunctionParameters,
+		// emitted between OpFunction and the entry OpLabel.
+		if len(fn.blocks) > 0 {
+			for p in fn.blocks[0].params {
+				sp := inst_begin(w); w_id(w, tid(m, p.type)); w_id(w, p.id); inst_end(w, sp, .OpFunctionParameter)
+			}
+		}
 		for blk in fn.blocks {
 			sl := inst_begin(w); w_id(w, blk.id); inst_end(w, sl, .OpLabel)
 			for &op in blk.ops { emit_operation(w, m, &op) }
@@ -287,10 +293,37 @@ emit_functions :: proc "contextless" (w: ^Writer, m: ^Module) {
 // Entry point
 // -----------------------------------------------------------------------------
 
+@(private="file")
+max_id :: #force_inline proc "contextless" (cur: u32, id: Id) -> u32 {
+	return id != ID_NONE ? max(cur, u32(id)) : cur
+}
+
+// The exclusive upper bound on every <id> in the module (the header's `bound`).
+@(private="file")
+compute_bound :: proc "contextless" (m: ^Module) -> u32 {
+	hi := u32(0)
+	for id in m.type_ids     { hi = max_id(hi, id) }
+	for id in m.global_ids   { hi = max_id(hi, id) }
+	for id in m.function_ids { hi = max_id(hi, id) }
+	for ei in m.ext_imports  { hi = max_id(hi, ei.result) }
+	for s in m.debug.strings { hi = max_id(hi, s.result) }
+	for c in m.constants     { hi = max_id(hi, c.result.id) }
+	for fn in m.functions {
+		for blk in fn.blocks {
+			hi = max_id(hi, blk.id)
+			for p in blk.params { hi = max_id(hi, p.id) }
+			for op in blk.ops   { hi = max_id(hi, op.result.id) }
+		}
+	}
+	return hi + 1
+}
+
 // encode: serialize `m` into `code` in spec layout order, returning the byte
-// count written. `m.bound` must be the exclusive upper bound on all <id>s.
+// count written. `bound` is computed when left 0 (else taken as-is, so a decoded
+// module re-encodes with its original bound).
 encode :: proc(m: Module, code: []u8, relocs: ^[dynamic]Relocation, errors: ^[dynamic]Error) -> (byte_count: u32, ok: bool) {
 	m := m
+	if m.bound == 0 { m.bound = compute_bound(&m) }
 	w := Writer{code = code, ok = true}
 	emit_header(&w, &m)
 	emit_preamble(&w, &m)
diff --git a/core/rexcode/ir/spirv/tests/roundtrip_test.odin b/core/rexcode/ir/spirv/tests/roundtrip_test.odin
index b937e6942..6e7c73644 100644
--- a/core/rexcode/ir/spirv/tests/roundtrip_test.odin
+++ b/core/rexcode/ir/spirv/tests/roundtrip_test.odin
@@ -125,6 +125,64 @@ main :: proc() {
 		roundtrip("iadd_function", m)
 	}
 
+	// (4) a function with a parameter, and bound left 0 so encode computes it:
+	//     i32 @f(i32 %4) { %5: OpReturnValue %4 }
+	{
+		m := spirv.make_module()
+		m.capabilities = {.Shader}
+		m.types = {
+			{kind = .INT, bits = 32, aux = 1},
+			{kind = .FUNCTION, fields = {spirv.Type_Ref(0), spirv.Type_Ref(0)}, count = 1},
+		}
+		m.type_ids = {spirv.Id(1), spirv.Id(2)}
+		ret := spirv.Operation{
+			opcode   = u16(spirv.Opcode.OpReturnValue),
+			result   = {id = spirv.ID_NONE},
+			operands = {spirv.op_value(spirv.Id(4))},
+		}
+		m.functions = {
+			{signature = spirv.Type_Ref(1), blocks = {
+				{
+					id     = spirv.Id(5),
+					params = {{id = spirv.Id(4), type = spirv.Type_Ref(0)}},   // OpFunctionParameter
+					ops    = {ret},
+				},
+			}},
+		}
+		m.function_ids = {spirv.Id(3)}
+		// m.bound left 0 -> computed
+		roundtrip("param_function", m)
+	}
+
+	// (5) an enum-parameter operand: OpLoad ... MemoryAccess Aligned 16, where the
+	//     alignment (16) is a trailing operand pulled in by the Aligned bit.
+	{
+		m := spirv.make_module()
+		m.capabilities = {.Shader}
+		m.types = {
+			{kind = .VOID},
+			{kind = .FLOAT, bits = 32},
+			{kind = .POINTER, aux = 7, elem = spirv.Type_Ref(1)},   // Function* float
+			{kind = .FUNCTION, fields = {spirv.Type_Ref(0)}, count = 0},
+		}
+		m.type_ids = {spirv.Id(1), spirv.Id(2), spirv.Id(3), spirv.Id(4)}
+		var := spirv.Operation{   // %7 = OpVariable %ptr Function
+			opcode = u16(spirv.Opcode.OpVariable), result = {spirv.Id(7), spirv.Type_Ref(2)},
+			operands = {spirv.op_int(7)},
+		}
+		load := spirv.Operation{  // %8 = OpLoad %float %7 Aligned 16
+			opcode = u16(spirv.Opcode.OpLoad), result = {spirv.Id(8), spirv.Type_Ref(1)},
+			operands = {spirv.op_value(spirv.Id(7)), spirv.op_int(0x2), spirv.op_int(16)},
+		}
+		m.functions = {
+			{signature = spirv.Type_Ref(3), blocks = {
+				{id = spirv.Id(6), ops = {var, load, {opcode = u16(spirv.Opcode.OpReturn)}}},
+			}},
+		}
+		m.function_ids = {spirv.Id(5)}
+		roundtrip("load_aligned", m)
+	}
+
 	fmt.printf("\n%d passed, %d failed\n", ok_count, fail_count)
 	if fail_count > 0 { os.exit(1) }
 }