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Add Sparse Conditional Constant Propagation optimization
Implement basic SCCP optimization pass that performs: - Constant propagation through assignments - Constant folding for arithmetic operations (+, -, *) - Constant folding for comparison operations (==, \!=, <, <=, >, >=) - Branch folding when conditions are compile-time constants - Dead code elimination through unreachable branch removal
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‎src/opt-sccp.c

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/*
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* shecc - Self-Hosting and Educational C Compiler.
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*
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* shecc is freely redistributable under the BSD 2 clause license. See the
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* file "LICENSE" for information on usage and redistribution of this file.
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*/
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/* SCCP (Sparse Conditional Constant Propagation) Optimization Pass
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*
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* This optimization pass performs:
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* - Constant propagation through assignments
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* - Constant folding for arithmetic and comparison operations
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* - Branch folding when conditions are compile-time constants
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* - Dead code elimination through unreachable branch removal
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*
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* The implementation is designed to work within shecc's constraints,
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* avoiding dynamic memory allocation and complex data structures.
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*/
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/* Simple constant propagation within basic blocks */
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bool simple_sccp(func_t *func)
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{
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if (!func || !func->bbs)
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return false;
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bool changed = false;
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/* Iterate through basic blocks */
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for (basic_block_t *bb = func->bbs; bb; bb = bb->rpo_next) {
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/* Process instructions in the block */
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for (insn_t *insn = bb->insn_list.head; insn; insn = insn->next) {
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/* Skip if no destination */
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if (!insn->rd)
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continue;
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/* Handle simple constant propagation */
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switch (insn->opcode) {
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case OP_assign:
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/* Propagate constants through assignments */
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if (insn->rs1 && insn->rs1->is_const && !insn->rd->is_const) {
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insn->rd->is_const = true;
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insn->rd->init_val = insn->rs1->init_val;
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insn->opcode = OP_load_constant;
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insn->rs1 = NULL;
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changed = true;
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}
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break;
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case OP_add:
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case OP_sub:
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case OP_mul:
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/* Fold binary operations with constants */
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if (insn->rs1 && insn->rs1->is_const && insn->rs2 &&
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insn->rs2->is_const &&
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!insn->rd->is_global) { /* Don't modify globals */
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int result = 0;
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int l = insn->rs1->init_val;
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int r = insn->rs2->init_val;
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switch (insn->opcode) {
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case OP_add:
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result = l + r;
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break;
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case OP_sub:
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result = l - r;
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break;
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case OP_mul:
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result = l * r;
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break;
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default:
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continue;
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}
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/* Convert to constant load */
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insn->opcode = OP_load_constant;
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insn->rd->is_const = true;
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insn->rd->init_val = result;
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insn->rs1 = NULL;
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insn->rs2 = NULL;
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changed = true;
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}
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break;
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case OP_eq:
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case OP_neq:
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case OP_lt:
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case OP_leq:
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case OP_gt:
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case OP_geq:
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/* Fold comparison operations */
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if (insn->rs1 && insn->rs1->is_const && insn->rs2 &&
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insn->rs2->is_const &&
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!insn->rd->is_global) { /* Don't modify globals */
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int result = 0;
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int l = insn->rs1->init_val;
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int r = insn->rs2->init_val;
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switch (insn->opcode) {
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case OP_eq:
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result = (l == r);
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break;
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case OP_neq:
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result = (l != r);
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break;
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case OP_lt:
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result = (l < r);
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break;
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case OP_leq:
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result = (l <= r);
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break;
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case OP_gt:
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result = (l > r);
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break;
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case OP_geq:
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result = (l >= r);
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break;
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default:
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continue;
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}
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/* Convert to constant load */
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insn->opcode = OP_load_constant;
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insn->rd->is_const = true;
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insn->rd->init_val = result;
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insn->rs1 = NULL;
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insn->rs2 = NULL;
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changed = true;
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}
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break;
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default:
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/* Other opcodes - no optimization */
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break;
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}
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}
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/* Simple constant branch folding */
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insn_t *last = bb->insn_list.tail;
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if (last && last->opcode == OP_branch) {
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if (last->rs1 && last->rs1->is_const) {
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/* Convert to unconditional jump */
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last->opcode = OP_jump;
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if (last->rs1->init_val != 0) {
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/* Take then branch */
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bb->else_ = NULL;
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} else {
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/* Take else branch */
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bb->then_ = bb->else_;
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bb->else_ = NULL;
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}
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last->rs1 = NULL;
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changed = true;
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}
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}
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}
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return changed;
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}

‎src/ssa.c

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#include "defs.h"
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#include "globals.c"
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/* SCCP (Sparse Conditional Constant Propagation) optimization */
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#include "opt-sccp.c"
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/* cfront does not accept structure as an argument, pass pointer */
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void bb_forward_traversal(bb_traversal_args_t *args)
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{
@@ -1886,6 +1889,11 @@ void optimize(void)
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use_chain_build();
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/* Run simple SCCP optimization first */
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for (func_t *func = FUNC_LIST.head; func; func = func->next) {
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simple_sccp(func);
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}
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for (func_t *func = FUNC_LIST.head; func; func = func->next) {
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/* basic block level (control flow) optimizations */
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