As mentioned in the comment inside the code, the Intel documentation states that the internal CPU buffer is forcefully flushed out to RAM right away when tracing is disabled. Otherwise, the buffer on RAM might be stale, as the CPU's automatic flushing happens only from time to time from time and the CPU decides when.
This diff disables tracing when the trace buffer is going to be read. This is a quite safe operation, as the reading is done when the inferior is paused at a breakpoint, so we are not losing any packets because there's no code being executed.
After the reading is finished, tracing is enabled back.
I run the following manual tests
(lldb) b main Breakpoint 1: where = a.out`main + 15 at main.cpp:4:7, address = 0x000000000040050f (lldb) r Process 3078226 stopped * thread #1, name = 'a.out', stop reason = breakpoint 1.1 frame #0: 0x000000000040050f a.out`main at main.cpp:4:7 (lldb) processor-trace start (lldb) b 5 Breakpoint 2: where = a.out`main + 22 at main.cpp:5:12, address = 0x0000000000400516 (lldb) c Process 3078226 resuming Process 3078226 stopped * thread #1, name = 'a.out', stop reason = breakpoint 2.1 frame #0: 0x0000000000400516 a.out`main at main.cpp:5:12 (lldb) processor-trace show-instr-log thread #1: tid=3078226 0x40050f <+15>: movl $0x0, -0x8(%rbp) >>> Before, some runs of the script up to this point lead to empty traces (lldb) b 6 Breakpoint 3: where = a.out`main + 42 at main.cpp:6:14, address = 0x000000000040052a (lldb) c Process 3092991 resuming Process 3092991 stopped * thread #1, name = 'a.out', stop reason = breakpoint 3.1 frame #0: 0x000000000040052a a.out`main at main.cpp:6:14 (lldb) processor-trace show-instr-log thread #1: tid=3092991 0x40050f <+15>: movl $0x0, -0x8(%rbp) 0x400516 <+22>: movl $0x0, -0xc(%rbp) 0x40051d <+29>: cmpl $0x2710, -0xc(%rbp) ; imm = 0x2710 0x400524 <+36>: jge 0x400546 ; <+70> at main.cpp 0x400524 <+36>: jge 0x400546 ; <+70> at main.cpp >>> The trace was re-enabled correctly and includes the instruction of the first reading.
Those instructions correspond to these lines
3 int main() { 4 int z = 0; 5 for (int i = 0; i < 10000; i++) { 6 z += fun(z) ...