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riscv64-asm: implement far branches via expansion

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For branch targets that are external/static symbols (where the
offset cannot be determined at assembly time), expand to:
  b<inverse> .+8       # skip if condition false
  auipc t0, %pcrel_hi  # load target upper bits
  jalr x0, %pcrel_lo(t0)  # jump

This handles the ±4 KiB B-type limitation without requiring
linker relaxation (R_RISCV_RELAX).  Local immediates within
range still use the compact B-type encoding.
This commit is contained in:
Meng Zhuo 2026-05-03 00:17:33 +08:00
parent 5c2240a896
commit 419b527657
1 changed files with 23 additions and 5 deletions
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28
riscv64-asm.c
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@ -247,11 +247,9 @@ static void parse_branch_offset_operand(TCCState *s1, Operand *op){
if ((int) op->e.v >= -0x1000 && (int) op->e.v < 0x1000)
op->type = OP_IM12S;
} else if (op->e.sym->type.t & (VT_EXTERN | VT_STATIC)) {
greloca(cur_text_section, op->e.sym, ind, R_RISCV_BRANCH, 0);
/* XXX: Implement far branches */
op->type = OP_IM12S;
/* For extern/static symbols, always use far-branch expansion
since linker relaxation (R_RISCV_RELAX) is not implemented. */
op->type = OP_IM32;
op->e.v = 0;
} else {
expect("operand");
@ -1384,6 +1382,26 @@ static void asm_emit_b(int token, uint32_t opcode, const Operand *rs1, const Ope
if (rs2->type != OP_REG) {
tcc_error("'%s': Expected destination operand that is a register", get_tok_str(token, NULL));
}
if (imm->type == OP_IM32 && imm->e.sym) {
/* far branch: expand to inverted short branch + auipc + jalr */
int b_ofs = ind;
uint32_t inv_func3 = ((opcode >> 12) & 7) ^ 1;
uint32_t inv_opcode = (opcode & ~(7 << 12)) | (inv_func3 << 12);
/* b<inverse> .+8 */
asm_emit_opcode(inv_opcode | ENCODE_RS1(rs1->reg)
| ENCODE_RS2(rs2->reg) | (1 << 8));
/* auipc t0, 0 */
greloca(cur_text_section, imm->e.sym, ind, R_RISCV_CALL, 0);
asm_emit_opcode(0x17 | ENCODE_RD(5));
/* jalr x0, 0(t0) */
write32le(cur_text_section->data + b_ofs,
read32le(cur_text_section->data + b_ofs)
| (((ind - b_ofs) >> 1) & 0xf) << 8
| (((ind - b_ofs) >> 5) & 0x3f) << 25
| (((ind - b_ofs) >> 11) & 1) << 7
| (((ind - b_ofs) >> 12) & 1) << 31);
return;
}
if (imm->type != OP_IM12S) {
tcc_error("'%s': Expected second source operand that is an immediate value between 0 and 8191", get_tok_str(token, NULL));
}