8000 YJIT: Stop padding nops at jump instructions on Arm by k0kubun · Pull Request #6755 · ruby/ruby · GitHub
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YJIT: Stop padding nops at jump instructions on Arm #6755

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YJIT: Stop padding nops at jump instructions on Arm #6755

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23 changes: 1 addition & 22 deletions yjit/src/backend/arm64/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -621,18 +621,14 @@ impl Assembler
let dst_addr = dst_ptr.into_i64();
let src_addr = cb.get_write_ptr().into_i64();

let num_insns = if bcond_offset_fits_bits((dst_addr - src_addr) / 4) {
if bcond_offset_fits_bits((dst_addr - src_addr) / 4) {
// If the jump offset fits into the conditional jump as
// an immediate value and it's properly aligned, then we
// can use the b.cond instruction directly. We're safe
// to use as i32 here since we already checked that it
// fits.
let bytes = (dst_addr - src_addr) as i32;
b 8000 cond(cb, CONDITION, InstructionOffset::from_bytes(bytes));

// Here we're going to return 1 because we've only
// written out 1 instruction.
1
} else {
// Otherwise, we need to load the address into a
// register and use the branch register instruction.
Expand All @@ -645,17 +641,7 @@ impl Assembler
bcond(cb, Condition::inverse(CONDITION), (load_insns + 2).into());
emit_load_value(cb, Assembler::SCRATCH0, dst_addr);
br(cb, Assembler::SCRATCH0);

// Here we'll return the number of instructions that it
// took to write out the destination address + 1 for the
// b.cond and 1 for the br.
load_insns + 2
};

// We need to make sure we have at least 6 instructions for
// every kind of jump for invalidation purposes, so we're
// going to write out padding nop instructions here.
for _ in num_insns..6 { nop(cb); }
},
Target::Label(label_idx) => {
// Here we're going to save enough space for ourselves and
Expand Down Expand Up @@ -699,13 +685,6 @@ impl Assembler
br(cb, Assembler::SCRATCH0);
num_insns + 1
};

// Make sure it's always a consistent number of
// instructions in case it gets patched and has to
// use the other branch.
for _ in num_insns..(JMP_PTR_BYTES / 4) {
nop(cb);
}
}

/// Call emit_jmp_ptr and immediately invalidate the written range.
Expand Down
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