Eliminate MY_IS_SSMALL()

For a long time, there has been the two macros IS_SSMALL() and
MY_IS_SSMALL() that do exactly the same thing.

There should only be one, and it should be called IS_SSMALL().

However, we must decide which implementation to use. When
MY_IS_SSMALL() was introduced a long time ago, it was the most
efficient. In a modern C compiler, there might not be any
difference.

To find out, I used the following small C program to examine
the code generation:

    #include <stdio.h>

    typedef unsigned int Uint32;
    typedef unsigned long Uint64;
    typedef long Sint;

    #define SWORD_CONSTANT(Const) Const##L
    #define SMALL_BITS	(64-4)
    #define MAX_SMALL	((SWORD_CONSTANT(1) << (SMALL_BITS-1))-1)
    #define MIN_SMALL	(-(SWORD_CONSTANT(1) << (SMALL_BITS-1)))

    #define MY_IS_SSMALL32(x) (((Uint32) ((((x)) >> (SMALL_BITS-1)) + 1)) < 2)
    #define MY_IS_SSMALL64(x) (((Uint64) ((((x)) >> (SMALL_BITS-1)) + 1)) < 2)
    #define MY_IS_SSMALL(x) (sizeof(x) == sizeof(Uint32) ? MY_IS_SSMALL32(x) : MY_IS_SSMALL64(x))

    #define IS_SSMALL(x)      (((x) >= MIN_SMALL) && ((x) <= MAX_SMALL))

    void original(Sint n)
    {
        if (IS_SSMALL(n)) {
            printf("yes\n");
        }
    }

    void enhanced(Sint n)
    {
        if (MY_IS_SSMALL(n)) {
            printf("yes\n");
        }
    }

gcc 7.2 produced the following code for the original() function:

.LC0:
  .string "yes"
original(long):
  movabs rax, 576460752303423488
  add rdi, rax
  movabs rax, 1152921504606846975
  cmp rdi, rax
  jbe .L4
  rep ret
.L4:
  mov edi, OFFSET FLAT:.LC0
  jmp puts

clang 5.0.0 produced the following code which is slightly better:

original(long):
  movabs rax, 576460752303423488
  add rax, rdi
  shr rax, 60
  jne .LBB0_1
  mov edi, .Lstr
  jmp puts # TAILCALL
.LBB0_1:
  ret
.Lstr:
  .asciz "yes"

However, in the context of beam_emu.c, clang could produce
similar to what gcc produced.

gcc 7.2 produced the following code when MY_IS_SSMALL() was used:

.LC0:
  .string "yes"
enhanced(long):
  sar rdi, 59
  add rdi, 1
  cmp rdi, 1
  jbe .L4
  rep ret
.L4:
  mov edi, OFFSET FLAT:.LC0
  jmp puts

clang produced similar code.

This code seems to be the cheapest. There are four instructions, and
there is no loading of huge integer constants.

1 files changed, 1 insertions, 1 deletions

diff --git a/erts/emulator/beam/external.c b/erts/emulator/beam/external.c
index 60cf09dc07..970158933f 100644
--- a/erts/emulator/beam/external.c
+++ b/erts/emulator/beam/external.c
@@ -3115,7 +3115,7 @@ dec_term(ErtsDistExternal *edep,
 #if defined(ARCH_64)
 		*objp = make_small(sn);
 #else
-		if (MY_IS_SSMALL(sn)) {
+		if (IS_SSMALL(sn)) {
 		    *objp = make_small(sn);
 		} else {
 		    *objp = small_to_big(sn, hp);