assembly - Which is faster, imm64 or m64 for x86-64?

Question

After testing about 10 billion times, if imm64 is 0.1 nanoseconds faster than m64 for AMD64, The m64 seems to be faster, but I don't really understand. Isn't the address of val_ptr in the following code an immediate value itself?

# Text section
.section __TEXT,__text,regular,pure_instructions
# 64-bit code
.code64
# Intel syntax
.intel_syntax noprefix
# Target macOS High Sierra
.macosx_version_min 10,13,0

# Make those two test functions global for the C measurer
.globl _test1
.globl _test2

# Test 1, imm64
_test1:
  # Move the immediate value 0xDEADBEEFFEEDFACE to RAX (return value)
  movabs rax, 0xDEADBEEFFEEDFACE
  ret
# Test 2, m64
_test2:
  # Move from the RAM (val_ptr) to RAX (return value)
  mov rax, qword ptr [rip + val_ptr]
  ret
# Data section
.section __DATA,__data
val_ptr:
  .quad 0xDEADBEEFFEEDFACE

The measurement code is:

#include <stdio.h>            // For printf
#include <stdlib.h>           // For EXIT_SUCCESS
#include <math.h>             // For fabs
#include <stdint.h>           // For uint64_t
#include <stddef.h>           // For size_t
#include <string.h>           // For memset
#include <mach/mach_time.h>   // For time stuff

#define FUNCTION_COUNT  2     // Number of functions to test
#define TEST_COUNT      0x10000000  // Number of times to test each function

// Type aliases
typedef uint64_t rettype_t;
typedef rettype_t(*function_t)();

// External test functions (defined in Assembly)
rettype_t test1();
rettype_t test2();

// Program entry point
int main() {

  // Time measurement stuff
  mach_timebase_info_data_t info;
  mach_timebase_info(&info);

  // Sums to divide by the test count to get average
  double sums[FUNCTION_COUNT];

  // Initialize sums to 0
  memset(&sums, 0, FUNCTION_COUNT * sizeof (double));

  // Functions to test
  function_t functions[FUNCTION_COUNT] = {test1, test2};

  // Useless results (should be 0xDEADBEEFFEEDFACE), but good to have
  rettype_t results[FUNCTION_COUNT];

  // Function loop, may get unrolled based on optimization level
  for (size_t test_fn = 0; test_fn < FUNCTION_COUNT; test_fn++) {
    // Test this MANY times
    for (size_t test_num = 0; test_num < TEST_COUNT; test_num++) {
      // Get the nanoseconds before the action
      double nanoseconds = mach_absolute_time();
      // Do the action
      results[test_fn] = functions[test_fn]();
      // Measure the time it took
      nanoseconds = mach_absolute_time() - nanoseconds;

      // Convert it to nanoseconds
      nanoseconds *= info.numer;
      nanoseconds /= info.denom;

      // Add the nanosecond count to the sum
      sums[test_fn] += nanoseconds;
    }
  }
  // Compute the average
  for (size_t i = 0; i < FUNCTION_COUNT; i++) {
    sums[i] /= TEST_COUNT;
  }

  if (FUNCTION_COUNT == 2) {
    // Print some fancy information
    printf("Test 1 took %f nanoseconds average.
", sums[0]);
    printf("Test 2 took %f nanoseconds average.
", sums[1]);
    printf("Test %d was faster, with %f nanoseconds difference
", sums[0] < sums[1] ? 1 : 2, fabs(sums[0] - sums[1]));
  } else {
    // Else, just print something
    for (size_t fn_i = 0; fn_i < FUNCTION_COUNT; fn_i++) {
      printf("Test %zu took %f clock ticks average.
", fn_i + 1, sums[fn_i]);
    }
  }

  // Everything went fine!
  return EXIT_SUCCESS;
}

So, which really is fastest, m64 or imm64?

By the way, I'm using Intel Core i7 Ivy Bridge and DDR3 RAM. I'm running macOS High Sierra.

EDIT: I inserted the ret instructions, and now imm64 turned out to be faster.

Answer 1

You don't show the actual loop you tested with, or say anything about how you measured time. Apparently you measured wall-clock time, not core clock cycles (with performance counters). So your sources of measurement noise include turbo / power-saving as well as sharing a physical core with another logical thread (on an i7).

---

On Intel IvyBridge:

movabs rax, 0xDEADBEEFFEEDFACE is an ALU instruction

• Take 10 bytes of code-size (which might or might not matter depending on surrounding code).
• Decodes to 1 uop for any ALU port (p0, p1, or p5). (max throughput = 3 per clock)
• Takes 2 entries in the uop cache (because of the 64-bit immediate), and takes 2 cycles to read from the uop cache. (So running from the loop buffer is a significant advantage for front-end throughput, if that's the bottleneck in code containing this).

mov rax, [RIP + val_ptr] is a load

• takes 7 bytes (REX + opcode + modrm + rel32)
• decodes to 1 uop for either load port (p2 or p3). (max throughput = 2 per clock)
• fits in 1 entry in the uop cache (no immediate and 32 or 32small address offset).
• runs a lot slower if the load is split across a page boundary, even on Skylake.
• can miss in cache the first time.

Source: Agner Fog's microarch pdf and instruction tables. See Table 9.1 for uop-cache stuff. See also other performance links in the x86 tag wiki.

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Compilers usually choose to generate 64-bit constants with a mov r64, imm64. (Related: What are the best instruction sequences to generate vector constants on the fly?, but in practice those never come up for scalar integer because there's no short single-instruction way to get a 64-bit -1.)

That's generally the right choice, although in a long-running loop where you expect the constant to stay hot in cache it could be a win to load it from .rodata. Especially if that lets you do something like and rax, [constant] instead of movabs r8, imm64 / and rax, r8.

If your 64-bit constant is an address, use a RIP-relative lea instead, if possible. lea rax, [rel my_symbol] in NASM syntax, lea my_symbol(%rip), %rax in AT&T.

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The surrounding code matters a lot when considering tiny sequences of asm, especially when they compete for different throughput resources.