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author | Björn Gustavsson <[email protected]> | 2017-09-25 07:18:56 +0200 |
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committer | Björn Gustavsson <[email protected]> | 2017-09-28 06:58:16 +0200 |
commit | 281ae7c2cf7e2dfd48cf50b2f68fd76f7c6ab0e2 (patch) | |
tree | f9779bb042a63a905d716ef333f4004db729b629 /erts/emulator/beam/atom.c | |
parent | 28c7d822eeb56c9f3955cb936e3f90e8971b9a71 (diff) | |
download | otp-281ae7c2cf7e2dfd48cf50b2f68fd76f7c6ab0e2.tar.gz otp-281ae7c2cf7e2dfd48cf50b2f68fd76f7c6ab0e2.tar.bz2 otp-281ae7c2cf7e2dfd48cf50b2f68fd76f7c6ab0e2.zip |
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.
Diffstat (limited to 'erts/emulator/beam/atom.c')
0 files changed, 0 insertions, 0 deletions