s390x: Improved TrapIf implementation

Following up on the discussion in
bytecodealliance#6011
this adds an improved implementation of TrapIf for s390x
using a single conditional branch instruction.

If the trap conditions is true, we branch into the middle of
the branch instruction - those middle two bytes are zero,
which matches the encoding of the trap instruction.

cranelift/codegen/src/isa/s390x/inst/emit.rs

@@ -682,6 +682,7 @@ fn enc_ril_b(opcode: u16, r1: Reg, ri2: u32) -> [u8; 6] {
  let opcode1 = ((opcode >> 4) & 0xff) as u8;
  let opcode2 = (opcode & 0xf) as u8;
  let r1 = machreg_to_gpr(r1) & 0x0f;
+ let ri2 = ri2 >> 1;
 
  enc[0] = opcode1;
  enc[1] = r1 << 4 | opcode2;
@@ -700,6 +701,7 @@ fn enc_ril_c(opcode: u16, m1: u8, ri2: u32) -> [u8; 6] {
  let opcode1 = ((opcode >> 4) & 0xff) as u8;
  let opcode2 = (opcode & 0xf) as u8;
  let m1 = m1 & 0x0f;
+ let ri2 = ri2 >> 1;
 
  enc[0] = opcode1;
  enc[1] = m1 << 4 | opcode2;
@@ -3590,14 +3592,18 @@ impl Inst {
  put_with_trap(sink, &enc_e(0x0000), trap_code);
  }
  &Inst::TrapIf { cond, trap_code } => {
- // Branch over trap if condition is false.
- let opcode = 0xa74; // BCR
- put(sink, &enc_ri_c(opcode, cond.invert().bits(), 4 + 2));
- // Now emit the actual trap.
  if let Some(s) = state.take_stack_map() {
- sink.add_stack_map(StackMapExtent::UpcomingBytes(2), s);
+ sink.add_stack_map(StackMapExtent::UpcomingBytes(6), s);
  }
- put_with_trap(sink, &enc_e(0x0000), trap_code);
+ // We implement a TrapIf as a conditional branch into the middle
+ // of the branch (BRCL) instruction itself - those middle two bytes
+ // are zero, which matches the trap instruction itself.
+ let opcode = 0xc04; // BCRL
+ let enc = &enc_ril_c(opcode, cond.bits(), 2);
+ // The trap must be placed on the last byte of the embedded trap
+ // instruction, so we need to emit the encoding in two parts.
+ put_with_trap(sink, &enc[0..4], trap_code);
+ put(sink, &enc[4..6]);
  }
  &Inst::JTSequence { ridx, ref targets } => {
  let ridx = allocs.next(ridx);

cranelift/codegen/src/isa/s390x/inst/emit_tests.rs

@@ -7041,8 +7041,8 @@ fn test_s390x_binemit() {
  cond: Cond::from_mask(1),
  trap_code: TrapCode::StackOverflow,
  },
- "A7E400030000",
- "jno 6 ; trap",
+ "C01400000001",
+ "jgo .+2",
  ));
 
  insns.push((

cranelift/codegen/src/isa/s390x/inst/mod.rs

@@ -3188,8 +3188,8 @@ impl Inst {
  &Inst::Debugtrap => "debugtrap".to_string(),
  &Inst::Trap { .. } => "trap".to_string(),
  &Inst::TrapIf { cond, .. } => {
- let cond = cond.invert().pretty_print_default();
- format!("j{} 6 ; trap", cond)
+ let cond = cond.pretty_print_default();
+ format!("jg{} .+2", cond)
  }
  &Inst::JTSequence { ridx, ref targets } => {
  let ridx = pretty_print_reg(ridx, allocs);