[MSAN RT] Use __sanitizer::mem_is_zero in __msan_test_shadow

[guiand]

Summary

The former function is particularly optimized for exactly the
use case we're interested in: an all-zero buffer.

This reduces the overhead of calling this function some 80% or
more. This is particularly for instrumenting code heavy with
string processing functions, like grep. An invocation of grep
with the pattern '[aeiou]k[aeiou]' has its runtime reduced by
~75% with this patch

[vitalybuka]

compiler-rt/lib/msan/msan.cpp

560

__builtin_clzll can do the trick, however you will need to check little/big endian
same for non-aligned parts
shadow is alligned, so you can read as 64bit

but I don't know if it will make a difference
if not, as is good enough

[guiand]

Modified to just use __sanitizer::mem_is_zero

[eugenis]

compiler-rt/lib/msan/msan.cpp

535

use early exit

[guiand]

Addressed comments

[eugenis]

LGTM

compiler-rt/lib/msan/msan.cpp

void __msan_dump_shadow(const void *x, uptr size) {
   for (uptr i = 0; i < size; i++)
     Printf("%x%x ", s[i] >> 4, s[i] & 0xf);
   Printf("\n");
 }
 
 sptr __msan_test_shadow(const void *x, uptr size) {
   if (!MEM_IS_APP(x)) return -1;
   unsigned char *s = (unsigned char *)MEM_TO_SHADOW((uptr)x);
+  if (__sanitizer::mem_is_zero((const char *)s, size))
+    return -1;
+  // Slow path: loop through again to find the location.
   for (uptr i = 0; i < size; ++i)
     if (s[i])
       return i;
   return -1;
 }
 
 void __msan_check_mem_is_initialized(const void *x, uptr size) {
   if (!__msan::flags()->report_umrs) return;
   sptr offset = __msan_test_shadow(x, size);
   if (offset < 0)
     return;
 
   GET_CALLER_PC_BP_SP;
   (void)sp;
   ReportUMRInsideAddressRange(__func__, x, size, offset);
   __msan::PrintWarningWithOrigin(pc, bp,
                                  __msan_get_origin(((const char *)x) + offset));
   if (__msan::flags()->halt_on_error) {
     Printf("Exiting\n");
     Die();
   }
 }
 
 int __msan_set_poison_in_malloc(int do_poison) {
   int old = flags()->poison_in_malloc;
   flags()->poison_in_malloc = do_poison;
   return old;
 }
 
 int __msan_has_dynamic_component() { return false; }
 
 NOINLINE
 void __msan_clear_on_return() {
   __msan_param_tls[0] = 0;
 }