CODEWITHSUNDEEP
c++assemblyx86clangx86-64
A. K.
A. K.

Reputation: 38214

What is the semantics of testb $1, %al

I'm trying to understand what this testb instruction (x86-64) will do.

testb $1, %al

What is the value of $1 here. is it all ones (0xFF) or a single 1 (0x1)?

The assembly is produced by clang for the following program:

#include <atomic>
std::atomic<bool> flag_atomic{false};

extern void f1();
extern void f2();

void foo() {
    bool b = flag_atomic.load(std::memory_order_relaxed);
    if (b == false) {
        f1();
    } else {
        f2();
    }   
}

The relevant assembly with (clang++ -s test.cpp -O3) is the following:

Lcfi2:
  .cfi_def_cfa_register %rbp
  movb  _flag_atomic(%rip), %al 
  testb $1, %al ; <<<<------------
  jne LBB0_2

Upvotes: 0

Views: 1347

Answers (2)

Matteo Italia
Matteo Italia

Reputation: 126867

In AT&T syntax $ is the prefix for immediate values (see also); $1 is a plain 1, so your instruction sets the flags according to the least significant bit of al.

is it all ones (0xFF) or a single 1 (0x1)?

All ones would be

testb $-1, %al

or (exact same machine code, just disassembly preference)

testb $0xff, %al

which incidentally would have the exact same semantic as

testb %al, %al

(as a mask of 0xff over an 8 bit register doesn't mask anything), and in this case is also valid for your code, as for a boolean there should be no need to mask anything out to check if it's true (and indeed gcc prefers this last version for your code).

movb  _flag_atomic(%rip), %al 
testb $1, %al
jne LBB0_2

in Intel syntax (no prefixes, no suffixes, dest, source operands order, explicit memory addressing syntax) this is

mov al, [rip+_flag_atomic]
test al, 1
jne LBB0_2

And, in pseudo-C:

%al = _flag_atomic;
if(%al & 1 != 0) goto LBB0_2;

(jne is an alias of jnz, which is probably more clear in this case).

Upvotes: 3

Sandeep
Sandeep

Reputation: 353

0x is prefixed with hexadecimal number. 0 is prefixed with Octal number and if you do not mentioned any prefix it would be decimal number system. In your case this is 1 in decimal number system.

Upvotes: 0

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