Better b.cond usage on AArch64 (#6305)

* Better b.cond usage on AArch64

When we're lowering a conditional jump, we previously had a bit of
a complicated setup where we could emit a conditional jump to skip
over a jump that was the next instruction, and then write out the
destination and use a branch register.

Now instead we use the b.cond instruction if our offset fits (not
common, but not unused either) and if it doesn't we write out an
inverse condition to jump past loading the destination and
branching directly.

* Added an inverse fn for Condition (#443)

Prevents the need to pass two params and potentially reduces errors.

Co-authored-by: Jimmy Miller <jimmyhmiller@jimmys-mbp.lan>

Co-authored-by: Maxime Chevalier-Boisvert <maximechevalierb@gmail.com>
Co-authored-by: Jimmy Miller <jimmyhmiller@jimmys-mbp.lan>

1 files changed, 58 insertions, 64 deletions

diff --git a/yjit/src/backend/arm64/mod.rs b/yjit/src/backend/arm64/mod.rs
index 69524be611..4f07bf8062 100644
--- a/yjit/src/backend/arm64/mod.rs
+++ b/yjit/src/backend/arm64/mod.rs
@@ -565,64 +565,42 @@ impl Assembler
         fn emit_conditional_jump<const CONDITION: u8>(cb: &mut CodeBlock, target: Target) {
             match target {
                 Target::CodePtr(dst_ptr) => {
-                    let dst_addr = dst_ptr.into_u64();
-                    //let src_addr = cb.get_write_ptr().into_i64() + 4;
-                    //let offset = dst_addr - src_addr;
-
-                    // If the condition is met, then we'll skip past the
-                    // next instruction, put the address in a register, and
-                    // jump to it.
-                    bcond(cb, CONDITION, A64Opnd::new_imm(8));
-
-                    // If we get to this instruction, then the condition
-                    // wasn't met, in which case we'll jump past the
-                    // next instruction that perform the direct jump.
-
-                    b(cb, A64Opnd::new_imm(2i64 + emit_load_size(dst_addr) as i64));
-                    let num_insns = emit_load_value(cb, Assembler::SCRATCH0, dst_addr);
-                    br(cb, Assembler::SCRATCH0);
-                    for _ in num_insns..4 {
-                        nop(cb);
-                    }
+                    let dst_addr = dst_ptr.into_i64();
+                    let src_addr = cb.get_write_ptr().into_i64();
+                    let offset = dst_addr - src_addr;
 
-                    /*
-                    // 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. Otherwise, we need
-                    // to load the address into a register and use the branch
-                    // register instruction.
-                    if bcond_offset_fits_bits(offset) {
-                        bcond(cb, CONDITION, A64Opnd::new_imm(dst_addr - src_addr));
+                    let num_insns = if bcond_offset_fits_bits(offset) {
+                        // 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.
+                        bcond(cb, CONDITION, A64Opnd::new_imm(offset));
+
+                        // Here we're going to return 1 because we've only
+                        // written out 1 instruction.
+                        1
                     } else {
-                        // If the condition is met, then we'll skip past the
-                        // next instruction, put the address in a register, and
-                        // jump to it.
-                        bcond(cb, CONDITION, A64Opnd::new_imm(8));
-
-                        // If the offset fits into a direct jump, then we'll use
-                        // that and the number of instructions will be shorter.
-                        // Otherwise we'll use the branch register instruction.
-                        if b_offset_fits_bits(offset) {
-                            // If we get to this instruction, then the condition
-                            // wasn't met, in which case we'll jump past the
-                            // next instruction that performs the direct jump.
-                            b(cb, A64Opnd::new_imm(1));
-
-                            // Here we'll perform the direct jump to the target.
-                            let offset = dst_addr - cb.get_write_ptr().into_i64() + 4;
-                            b(cb, A64Opnd::new_imm(offset / 4));
-                        } else {
-                            // If we get to this instruction, then the condition
-                            // wasn't met, in which case we'll jump past the
-                            // next instruction that perform the direct jump.
-                            let value = dst_addr as u64;
-
-                            b(cb, A64Opnd::new_imm(emit_load_size(value).into()));
-                            emit_load_value(cb, Assembler::SCRATCH0, value);
-                            br(cb, Assembler::SCRATCH0);
-                        }
-                    }
-                    */
+                        // Otherwise, we need to load the address into a
+                        // register and use the branch register instruction.
+                        let dst_addr = dst_ptr.into_u64();
+                        let load_insns: i64 = emit_load_size(dst_addr).into();
+
+                        // We're going to write out the inverse condition so
+                        // that if it doesn't match it will skip over the
+                        // instructions used for branching.
+                        bcond(cb, Condition::inverse(CONDITION), A64Opnd::new_imm((load_insns + 2) * 4));
+                        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
@@ -904,10 +882,10 @@ impl Assembler
                         _ => unreachable!()
                     };
                 },
-                Insn::Je(target) => {
+                Insn::Je(target) | Insn::Jz(target) => {
                     emit_conditional_jump::<{Condition::EQ}>(cb, *target);
                 },
-                Insn::Jne(target) => {
+                Insn::Jne(target) | Insn::Jnz(target) => {
                     emit_conditional_jump::<{Condition::NE}>(cb, *target);
                 },
                 Insn::Jl(target) => {
@@ -916,12 +894,6 @@ impl Assembler
                 Insn::Jbe(target) => {
                     emit_conditional_jump::<{Condition::LS}>(cb, *target);
                 },
-                Insn::Jz(target) => {
-                    emit_conditional_jump::<{Condition::EQ}>(cb, *target);
-                },
-                Insn::Jnz(target) => {
-                    emit_conditional_jump::<{Condition::NE}>(cb, *target);
-                },
                 Insn::Jo(target) => {
                     emit_conditional_jump::<{Condition::VS}>(cb, *target);
                 },
@@ -1054,6 +1026,28 @@ mod tests {
     }
 
     #[test]
+    fn test_emit_je_fits_into_bcond() {
+        let (mut asm, mut cb) = setup_asm();
+
+        let offset = 80;
+        let target: CodePtr = ((cb.get_write_ptr().into_u64() + offset) as *mut u8).into();
+
+        asm.je(Target::CodePtr(target));
+        asm.compile_with_num_regs(&mut cb, 0);
+    }
+
+    #[test]
+    fn test_emit_je_does_not_fit_into_bcond() {
+        let (mut asm, mut cb) = setup_asm();
+
+        let offset = 1 << 21;
+        let target: CodePtr = ((cb.get_write_ptr().into_u64() + offset) as *mut u8).into();
+
+        asm.je(Target::CodePtr(target));
+        asm.compile_with_num_regs(&mut cb, 0);
+    }
+
+    #[test]
     fn test_emit_lea_label() {
         let (mut asm, mut cb) = setup_asm();