Predicated Instruction support in memval_simple

Created by: toshipiazza

Working on the ARM port for memval_simple (XREF #1551 (closed)) and, at least on my machine I see consistent crashes before main in the following basic block:

...
 +77  L3  95032730   str.ls %r2 -> -0x0730(%r3)[4byte]
...
 +84  m4  e5944000   ldr    (%r4)[4byte] -> %r4
...

The crash is at (+84), when we're trying to dereference %r4, which is computed via drutil_insert_get_mem_addr(), i.e. %r3 - 0x730. When I run through this in gdb without DR, it appears as though %r3 - 0x730 does not point to valid memory. The native app doesn't crash though, since the predicated instruction doesn't perform the store.

A (very temporary) fix would be the following patch, which I've confirmed works with no crash:

diff --git a/api/samples/memval_simple.c b/api/samples/memval_simple.c
index 20970f9..e8622cc 100644
--- a/api/samples/memval_simple.c
+++ b/api/samples/memval_simple.c
@@ -341,6 +341,8 @@ event_app_instruction(void *drcontext, void *tag, instrlist_t *bb,
         return DR_EMIT_DEFAULT;
     if (!instr_writes_memory(instr))
         return DR_EMIT_DEFAULT;
+    if (instr_is_predicated(instr))
+        return DR_EMIT_DEFAULT;

     /* XXX: See above, in handle_post_write(). To simplify the handling of registers, we
      * assume no instruction has multiple distinct memory destination operands.

though I would prefer a solution which allows us to still instrument predicated instructions, say something like an app2app drmgr routine like drutil_expand_predicated, analogous to the drutil_expand_rep_string routine we use in many samples.

For the record, I've never seen a crash caused by predicated instructions on x86.