Richard Henderson <richard.henderson@linaro.org> writes:
On 9/10/24 14:23, Richard Henderson wrote:
With tcg_last_op(), we always get the last op of the stream.
With TCGContext.emit_before_op, the most recently emitted op
is no longer the last op.
Instead, pass the op being emitted back from the allocator so
that we can link it to the label without needing to look it up.
Oh, I meant to point out from whence this comes.
The plugin uses a conditional
size_t n_insns = qemu_plugin_tb_n_insns(tb);
qemu_plugin_u64 quantum_insn =
qemu_plugin_scoreboard_u64_in_struct(vcpus, vCPUTime, quantum_insn);
/* count (and eventually trap) once per tb */
qemu_plugin_register_vcpu_tb_exec_inline_per_vcpu(
tb, QEMU_PLUGIN_INLINE_ADD_U64, quantum_insn, n_insns);
ld_i32 tmp18,env,$0xffffffffffffdb10
mul_i32 tmp18,tmp18,$0x18
ext_i32_i64 tmp17,tmp18
add_i64 tmp17,tmp17,$0x575410edadc8
qemu_plugin_register_vcpu_tb_exec_cond_cb(
tb, every_quantum_insn,
QEMU_PLUGIN_CB_NO_REGS, QEMU_PLUGIN_COND_GE,
quantum_insn, max_insn_per_quantum, NULL);
?
ld_i64 tmp21,tmp17,$0x0
brcond_i64 tmp21,$0x0,ltu,$L1
ld_i32 tmp18,env,$0xffffffffffffdb10
call plugin(0x79a2abfde66a),$0x1,$0,tmp18,$0x0
set_label $L1
Note that the branch is X < 0 (unsigned), which is always false, and
thus the branch is optimized away.
I'm obviously missing something reading this. How can TCG know the state
of the scoreboard variables and optimise away the branch?