Avoid bextr in dump_constants

When dump_constants is compiled for newer AMD processors it can end up using a
bextr instruction that may not be present on other makes/models of processors.

The act of calling std::cout on a numeric type ends up in
num_put<,>::do_put() which does:

const unsigned __nbuf = (numeric_limits<unsigned long>::digits / 3)
                          + ((numeric_limits<unsigned long>::digits % 3) != 0)
                          + ((__iob.flags() & ios_base::showbase) != 0) + 1;

ios_base::showbase is 0x0200, so the bit test against showbase results
in:

bextr  $0x109,0x8(%rdx),%r12d

which says to extract 1 bit from bit offset 9.  A workaround is to
use to_string() to avoid passing numeric types to cout. This alternative
is straightforward and the implementation is simpler (to_string() does
not have formatting nor locale options), though it doesn't prevent
the problem from occurring again in the future (e.g., when the source or
compiler changes).

Bug: None
Test: `make tests`. Ran dump_constants on arm32, arm64, and x64 in
CrOS build environment.

Change-Id: Ic16574141f6c935bff958e5d1f27c81994a993ab

1 files changed, 7 insertions, 4 deletions

diff --git a/dump_constants.cc b/dump_constants.cc
index 80c8664a..f3d70749 100644
--- a/dump_constants.cc
+++ b/dump_constants.cc
@@ -15,10 +15,12 @@
 #include "libsyscalls.h"
 
 int main() {
+  // Numeric values are passed to std::cout via std::to_string() to avoid
+  // the use of 'bextr' asm instruction (when compiled with -march=bdver4).
   std::cout << "{\n";
-  std::cout << "  \"arch_nr\": " << MINIJAIL_ARCH_NR << ",\n";
+  std::cout << "  \"arch_nr\": " << std::to_string(MINIJAIL_ARCH_NR) << ",\n";
   std::cout << "  \"arch_name\": \"" << MINIJAIL_ARCH_NAME << "\",\n";
-  std::cout << "  \"bits\": " << MINIJAIL_ARCH_BITS << ",\n";
+  std::cout << "  \"bits\": " << std::to_string(MINIJAIL_ARCH_BITS) << ",\n";
   std::cout << "  \"syscalls\": {\n";
   bool first = true;
   for (const struct syscall_entry* entry = syscall_table; entry->name;
@@ -27,7 +29,7 @@ int main() {
       first = false;
     else
       std::cout << ",\n";
-    std::cout << "    \"" << entry->name << "\": " << entry->nr;
+    std::cout << "    \"" << entry->name << "\": " << std::to_string(entry->nr);
   }
   std::cout << "\n  },\n";
   std::cout << "  \"constants\": {\n";
@@ -38,7 +40,8 @@ int main() {
       first = false;
     else
       std::cout << ",\n";
-    std::cout << "    \"" << entry->name << "\": " << entry->value;
+    std::cout << "    \"" << entry->name << "\": "
+	      << std::to_string(entry->value);
   }
   std::cout << "\n  }\n";
   std::cout << "}\n";