Tracing help.

docs/instruction_tracing.md

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+In x64_tracers.cc:
+
+Enable tracing:
+```
+#define ITRACE 1  <--- for only ppc instructions
+#define DTRACE 1  <--- add HIR data
+```
+
+If tracing data, run with the following flags:
+```
+--store_all_context_values
+```
+
+By default, tracing will start at the beginning and only for the specified
+thread.
+
+Change traced thread by thread creation ID:
+```
+#define TARGET_THREAD 4
+```
+
+To only trace at a certain point, change default trace flag to false:
+```
+bool trace_enabled = true;
+```
+Add a breakpoint:
+```
+--break_on_instruction=0x821009A4
+```
+On break, add the following to the Watch window and set it to true:
+```
+xe::cpu::backend::x64::trace_enabled
+```
+Continue, and watch stuff appear in the log.