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authorBjörn Gustavsson <[email protected]>2017-09-25 07:18:56 +0200
committerBjörn Gustavsson <[email protected]>2017-09-28 06:58:16 +0200
commit281ae7c2cf7e2dfd48cf50b2f68fd76f7c6ab0e2 (patch)
treef9779bb042a63a905d716ef333f4004db729b629 /erts/emulator/beam/erl_gc.c
parent28c7d822eeb56c9f3955cb936e3f90e8971b9a71 (diff)
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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/erl_gc.c')
-rw-r--r--erts/emulator/beam/erl_gc.c4
1 files changed, 2 insertions, 2 deletions
diff --git a/erts/emulator/beam/erl_gc.c b/erts/emulator/beam/erl_gc.c
index 8344c164fa..97a1ca915f 100644
--- a/erts/emulator/beam/erl_gc.c
+++ b/erts/emulator/beam/erl_gc.c
@@ -337,7 +337,7 @@ erts_heap_sizes(Process* p)
for (i = num_heap_sizes-1; i >= 0; i--) {
n += 2;
- if (!MY_IS_SSMALL(heap_sizes[i])) {
+ if (!IS_SSMALL(heap_sizes[i])) {
big += BIG_UINT_HEAP_SIZE;
}
}
@@ -352,7 +352,7 @@ erts_heap_sizes(Process* p)
Eterm num;
Sint sz = heap_sizes[i];
- if (MY_IS_SSMALL(sz)) {
+ if (IS_SSMALL(sz)) {
num = make_small(sz);
} else {
num = uint_to_big(sz, bigp);