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gccx86x86-64branch-prediction

Intel x86 0x2E/0x3E Prefix Branch Prediction actually used?

In the latest Intel software dev manual it describes two opcode prefixes:

Group 2 > Branch Hints

    0x2E: Branch Not Taken
    0x3E: Branch Taken

These allow for explicit branch prediction of Jump instructions (opcodes likeJxx)

I remember reading a couple of years ago that on x86 explicit branch prediction was essentially a no-op in the context of gccs branch prediciton intrinsics.

I am now unclear if these x86 branch hints are a new feature or whether they are essentially no-ops in practice.

Can anyone clear this up?

(That is: Does gccs branch prediction functions generate these x86 branch hints? - and do current Intel CPUs not ignore them? - and when did this happen?)

Update:

I created a quick test program:

int main(int argc, char** argv)
{
    if (__builtin_expect(argc,0))
        return 1;

    if (__builtin_expect(argc == 2, 1))
        return 2;

    return 3;
}

Disassembles to the following:

00000000004004cc <main>:
  4004cc:   55                      push   %rbp
  4004cd:   48 89 e5                mov    %rsp,%rbp
  4004d0:   89 7d fc                mov    %edi,-0x4(%rbp)
  4004d3:   48 89 75 f0             mov    %rsi,-0x10(%rbp)
  4004d7:   8b 45 fc                mov    -0x4(%rbp),%eax
  4004da:   48 98                   cltq   
  4004dc:   48 85 c0                test   %rax,%rax
  4004df:   74 07                   je     4004e8 <main+0x1c>
  4004e1:   b8 01 00 00 00          mov    $0x1,%eax
  4004e6:   eb 1b                   jmp    400503 <main+0x37>
  4004e8:   83 7d fc 02             cmpl   $0x2,-0x4(%rbp)
  4004ec:   0f 94 c0                sete   %al
  4004ef:   0f b6 c0                movzbl %al,%eax
  4004f2:   48 85 c0                test   %rax,%rax
  4004f5:   74 07                   je     4004fe <main+0x32>
  4004f7:   b8 02 00 00 00          mov    $0x2,%eax
  4004fc:   eb 05                   jmp    400503 <main+0x37>
  4004fe:   b8 03 00 00 00          mov    $0x3,%eax
  400503:   5d                      pop    %rbp
  400504:   c3                      retq   
  400505:   66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
  40050c:   00 00 00 
  40050f:   90                      nop

I don't see 2E or 3E ? Maybe gcc has elided them for some reason?

Solution

  • These instruction prefixes have no effect on modern processors (anything newer than Pentium 4). They just cost one byte of code space, and thus, not generating them is the right thing.

    For details, see Agner Fog's optimization manuals, in particular 3. Microarchitecture: http://www.agner.org/optimize/

    The "Intel® 64 and IA-32 Architectures Optimization Reference Manual" no longer mentions them in the section about optimizing branches (section 3.4.1): http://www.intel.de/content/dam/doc/manual/64-ia-32-architectures-optimization-manual.pdf

    These prefixes are a (harmless) relict of the Netburst architecture. In all-out optimization, you can use them to align code, but that's all they're good for nowadays.

    Edit 2024-09-26: As user20574 pointed out in the comments, newer Intel processors again respect these prefixes when they have no data from branch prediction.