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SSA optimizer enhancements #242

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Merged
merged 9 commits into from
Aug 19, 2025
Merged

SSA optimizer enhancements #242

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f59d38f
Improve code readability
jserv Aug 14, 2025
f7fc6ae
Enhance constant folding optimization
jserv Aug 14, 2025
171d597
Add SCCP optimization
jserv Aug 14, 2025
c3b10cf
Add bounds checking to solve_phi_insertion
jserv Aug 14, 2025
61c709f
Extend CSE to support more binary operations
jserv Aug 14, 2025
06ecbd7
Enhance DCE optimization
jserv Aug 14, 2025
6527865
Add constant cast optimization to SCCP
jserv Aug 17, 2025
9091e24
Add constant sign extension optimization to SCCP
jserv Aug 17, 2025
f386319
Merge binary/comparison operations folding in SCCP
jserv Aug 17, 2025
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286 changes: 286 additions & 0 deletions src/opt-sccp.c
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Original file line number Diff line number Diff line change
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/*
* shecc - Self-Hosting and Educational C Compiler.
*
* shecc is freely redistributable under the BSD 2 clause license. See the
* file "LICENSE" for information on usage and redistribution of this file.
*/

/* SCCP (Sparse Conditional Constant Propagation) Optimization Pass
*
* This optimization pass performs:
* - Constant propagation through assignments
* - Constant folding for arithmetic and comparison operations
* - Branch folding when conditions are compile-time constants
* - Dead code elimination through unreachable branch removal
*/

/* Simple constant propagation within basic blocks */
bool simple_sccp(func_t *func)
{
if (!func || !func->bbs)
return false;

bool changed = false;

/* Iterate through basic blocks */
for (basic_block_t *bb = func->bbs; bb; bb = bb->rpo_next) {
/* Process instructions in the block */
for (insn_t *insn = bb->insn_list.head; insn; insn = insn->next) {
/* Skip if no destination */
if (!insn->rd)
continue;

/* Handle simple constant propagation */
switch (insn->opcode) {
case OP_assign:
/* Propagate constants through assignments */
if (insn->rs1 && insn->rs1->is_const && !insn->rd->is_const) {
insn->rd->is_const = true;
insn->rd->init_val = insn->rs1->init_val;
insn->opcode = OP_load_constant;
insn->rs1 = NULL;
changed = true;
}
break;

case OP_trunc:
/* Constant truncation optimization integrated into SCCP */
if (insn->rs1 && insn->rs1->is_const && !insn->rd->is_global &&
insn->sz > 0) {
int value = insn->rs1->init_val;
int result = value;

/* Perform truncation based on size */
if (insn->sz == 1) {
/* Truncate to 8 bits */
result = value & 0xFF;
} else if (insn->sz == 2) {
/* Truncate to 16 bits */
result = value & 0xFFFF;
} else if (insn->sz == 4) {
/* No truncation needed for 32-bit */
result = value;
} else {
/* Invalid size, skip */
break;
}

/* Convert to constant load */
insn->opcode = OP_load_constant;
insn->rd->is_const = true;
insn->rd->init_val = result;
insn->rs1 = NULL;
insn->sz = 0;
changed = true;
}
break;

case OP_sign_ext:
/* Constant sign extension optimization integrated into SCCP */
if (insn->rs1 && insn->rs1->is_const && !insn->rd->is_global &&
insn->sz > 0) {
int value = insn->rs1->init_val;
int result = value;

/* Perform sign extension based on source size */
if (insn->sz == 1) {
/* Sign extend from 8 bits */
result = (value & 0x80) ? (value | 0xFFFFFF00)
: (value & 0xFF);
} else if (insn->sz == 2) {
/* Sign extend from 16 bits */
result = (value & 0x8000) ? (value | 0xFFFF0000)
: (value & 0xFFFF);
} else if (insn->sz == 4) {
/* No sign extension needed for 32-bit */
result = value;
} else {
/* Invalid size, skip */
break;
}

/* Convert to constant load */
insn->opcode = OP_load_constant;
insn->rd->is_const = true;
insn->rd->init_val = result;
insn->rs1 = NULL;
insn->sz = 0;
changed = true;
}
break;

case OP_add:
case OP_sub:
case OP_mul:
/* Fold binary operations with constants */
if (insn->rs1 && insn->rs1->is_const && insn->rs2 &&
insn->rs2->is_const &&
!insn->rd->is_global) { /* Don't modify globals */
int result = 0;
int l = insn->rs1->init_val, r = insn->rs2->init_val;

switch (insn->opcode) {
case OP_add:
result = l + r;
break;
case OP_sub:
result = l - r;
break;
case OP_mul:
result = l * r;
break;
default:
continue;
}

/* Convert to constant load */
insn->opcode = OP_load_constant;
insn->rd->is_const = true;
insn->rd->init_val = result;
insn->rs1 = NULL;
insn->rs2 = NULL;
changed = true;
}
break;

case OP_eq:
case OP_neq:
case OP_lt:
case OP_leq:
case OP_gt:
case OP_geq:
/* Fold comparison operations */
if (insn->rs1 && insn->rs1->is_const && insn->rs2 &&
insn->rs2->is_const &&
!insn->rd->is_global) { /* Don't modify globals */
int result = 0;
int l = insn->rs1->init_val;
int r = insn->rs2->init_val;

switch (insn->opcode) {
case OP_eq:
result = (l == r);
break;
case OP_neq:
result = (l != r);
break;
case OP_lt:
result = (l < r);
break;
case OP_leq:
result = (l <= r);
break;
case OP_gt:
result = (l > r);
break;
case OP_geq:
result = (l >= r);
break;
default:
continue;
}

/* Convert to constant load */
insn->opcode = OP_load_constant;
insn->rd->is_const = true;
insn->rd->init_val = result;
insn->rs1 = NULL;
insn->rs2 = NULL;
changed = true;
}
break;

default:
/* Other opcodes - no optimization */
break;
}
}

/* Simple constant branch folding */
insn_t *last = bb->insn_list.tail;
if (last && last->opcode == OP_branch) {
if (last->rs1 && last->rs1->is_const) {
/* Convert to unconditional jump */
last->opcode = OP_jump;

if (last->rs1->init_val != 0) {
/* Take then branch */
bb->else_ = NULL;
} else {
/* Take else branch */
bb->then_ = bb->else_;
bb->else_ = NULL;
}

last->rs1 = NULL;
changed = true;
}
}
}

return changed;
}

/* Targeted constant truncation peephole optimization */
bool optimize_constant_casts(func_t *func)
{
if (!func || !func->bbs)
return false;

bool changed = false;

/* Simple peephole optimization: const + trunc pattern */
for (basic_block_t *bb = func->bbs; bb; bb = bb->rpo_next) {
if (!bb)
continue;

for (insn_t *insn = bb->insn_list.head; insn && insn->next;
insn = insn->next) {
insn_t *next_insn = insn->next;

/* Look for pattern: const %.tX, VALUE followed by
* %.tY = trunc %.tX, SIZE
*/
if (insn->opcode == OP_load_constant &&
next_insn->opcode == OP_trunc && insn->rd && next_insn->rs1 &&
insn->rd == next_insn->rs1 && next_insn->sz > 0 &&
!next_insn->rd->is_global) {
int value = insn->rd->init_val;
int result = value;

/* Perform truncation based on size */
if (next_insn->sz == 1) {
/* Truncate to 8 bits */
result = value & 0xFF;
} else if (next_insn->sz == 2) {
/* Truncate to 16 bits */
result = value & 0xFFFF;
} else if (next_insn->sz == 4) {
/* No truncation needed for 32-bit */
result = value;
} else {
/* Invalid size, skip */
continue;
}

/* Optimize: Replace both instructions with single const */
insn->rd = next_insn->rd; /* Update dest to final target */
insn->rd->is_const = true;
insn->rd->init_val = result;

/* Remove the truncation instruction by converting it to
* NOP-like
*/
next_insn->opcode = OP_load_constant;
next_insn->rd->is_const = true;
next_insn->rd->init_val = result;
next_insn->rs1 = NULL;
next_insn->sz = 0;
Comment on lines +248 to +255
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@DrXiao DrXiao Aug 17, 2025

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How about processing it as following?

insn->next = next_insn->next;
if (next_insn->next)
    next_insn->next->prev = insn;

Instead of converting it to NOP-like instruction, we can directly link the previous and next instructions to skip this unused instruction.

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@jserv jserv Aug 17, 2025

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Instead of converting it to NOP-like instruction, we can directly link the previous and next instructions to skip this unused instruction.

Direct instruction linking was investigated, but it causes bootstrap failures.


changed = true;
}
}
}

return changed;
}
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