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
diff --git a/dump_constants.cc b/dump_constants.cc
index 80c8664..f3d7074 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 @@
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 @@
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";
