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, 4 insertions, 7 deletions

diff --git a/erts/emulator/beam/arith_instrs.tab b/erts/emulator/beam/arith_instrs.tab
index 7c9cd47e28..3db19df3c4 100644
--- a/erts/emulator/beam/arith_instrs.tab
+++ b/erts/emulator/beam/arith_instrs.tab
@@ -51,8 +51,7 @@ plus.fetch(Op1, Op2) {
 plus.execute(Fail, Live, Dst) {
     if (ERTS_LIKELY(is_both_small(PlusOp1, PlusOp2))) {
         Sint i = signed_val(PlusOp1) + signed_val(PlusOp2);
-        ASSERT(MY_IS_SSMALL(i) == IS_SSMALL(i));
-        if (ERTS_LIKELY(MY_IS_SSMALL(i))) {
+        if (ERTS_LIKELY(IS_SSMALL(i))) {
             $Dst = make_small(i);
             $NEXT0();
         }
@@ -74,8 +73,7 @@ minus.fetch(Op1, Op2) {
 minus.execute(Fail, Live, Dst) {
     if (ERTS_LIKELY(is_both_small(MinusOp1, MinusOp2))) {
         Sint i = signed_val(MinusOp1) - signed_val(MinusOp2);
-        ASSERT(MY_IS_SSMALL(i) == IS_SSMALL(i));
-        if (ERTS_LIKELY(MY_IS_SSMALL(i))) {
+        if (ERTS_LIKELY(IS_SSMALL(i))) {
             $Dst = make_small(i);
             $NEXT0();
         }
@@ -100,8 +98,7 @@ increment.execute(IncrementVal, Live, Dst) {
     increment_val = $IncrementVal;
     if (ERTS_LIKELY(is_small(increment_reg_val))) {
         Sint i = signed_val(increment_reg_val) + increment_val;
-        ASSERT(MY_IS_SSMALL(i) == IS_SSMALL(i));
-        if (ERTS_LIKELY(MY_IS_SSMALL(i))) {
+        if (ERTS_LIKELY(IS_SSMALL(i))) {
             $Dst = make_small(i);
             $NEXT0();
         }
@@ -142,7 +139,7 @@ i_int_div(Fail, Live, Op1, Op2, Dst) {
         $BIF_ERROR_ARITY_2($Fail, BIF_intdiv_2, op1, op2);
     } else if (ERTS_LIKELY(is_both_small(op1, op2))) {
         Sint ires = signed_val(op1) / signed_val(op2);
-        if (ERTS_LIKELY(MY_IS_SSMALL(ires))) {
+        if (ERTS_LIKELY(IS_SSMALL(ires))) {
             $Dst = make_small(ires);
             $NEXT0();
         }