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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.
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* lukas/erts/fix_threads_error_printout:
erts: Print the error reason when threads fail to start
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* kvakvs/zero-size-read_file/ERL-327/PR-1524/OTP-14637:
erts: On zero-size files attempt to read until EOF
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fix off by one error in docs
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* bjorn/speed-up-disassembler:
Add testing of erts_debug:df() to the emulator smoke tests
Speed up erts_debug:df()
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The byte_offset of sub-binaries wasn't taken into account for
ProcBins, subtly ruining the results. The test suite didn't catch
it since it didn't check for sub-binaries in particular, and only
checked for equality between variations -- not whether the output
was equal to the input.
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It is too easy to break the disassembler. Make sure that we notice.
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by ignoring reply earlier.
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* sverker/on_load-on_load-bug/OTP-14612:
erts: Fix 'on_load' tracing bug for modules with -on_load
code_SUITE:on_load_trace_on_load
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* bjorn/erts/pack-combined:
Pack combined instructions
beam_makeops: Refactor code generation
Correct disassembly of select instructions
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that is shared between _kp and _nkp versions.
Makes it easier to access in debugger.
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# Conflicts:
# erts/emulator/beam/erl_process.c
# erts/emulator/beam/sys.h
# erts/emulator/sys/common/erl_check_io.c
# erts/emulator/sys/common/erl_check_io.h
# erts/emulator/sys/unix/sys.c
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by having different structs with same name.
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and use correct cache alignment.
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* lukas/erts/remove-dirty-scheduler-defines/OTP-14613:
erts: Remove possibility to disable dirty schedulers
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The refactoring will simplify packing of combined instructions.
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Make sure to use the 'unpacked[-1]' when accessing the unpacked
arguments.
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* bjorn/erts/relative-jumps:
Pack failure labels in i_select_val2 and i_select_tuple_arity2
Optimize i_select_tuple_arity2 and is_select_lins
Rewrite select_val_bins so that its labels can be packed
Pack sequences of trailing 'f' operands
Implement packing of 'f' and 'j'
Make sure that mask literals are 64 bits
Use relative failure labels
Add information about offset to common group start position
Remove JUMP_OFFSET
Refactor instructions to support relative jumps
Introduce a new trace_jump/1 instruction for tracing
Avoid using $Src more than once
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Don't save a pointer to the default failure label. That could
relieve register pressure. Instead, if we'll need to signal an
error in i_select_tuple_arity2, delay to the execution phase.
That should be a clear win because i_select_tuple_arity2 very
rarely fails because of the term being selected is not a tuple.
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Pack sequences of trailing 'f' operands for instructions
such at jump_on_val or i_select_val_lins.
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Use the "ull" suffix for the mask literals instead of "ul"
to ensure that the literals are 64 bits also on Windows.
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Relative failure in itself is not an optimization, but we plan to
pack failure labels in the future to save memory.
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It has served its purpose.
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* maint:
stdlib: Fix jumping to beginning or end of line
Fix del_chars not considering wide chars and update buffer length before calling write_buf
Make cp_pos_to_col function aware of the ANSI escape codes
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Consider ANSI escape codes on cp_pos_to_col
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sverker/on_load-on_load-bug/OTP-14612
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Symptom: VM crash when erlang:trace_pattern(on_load, ..) is set and
module with -on_load is loaded.
Problem: Tracing and -on_load clash in their use of Export.beam[1]
Solution: Do not do call set_default_trace_pattern in finish_loading_1
for modules with -on_load. finish_after_on_load_2 will do that anyway.
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calling write_buf
After deleting the chars the function `del_chars` was considering the code points to move the cursor back and not the graphemes
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Introduce new macros that can be used for relative jumps and
use them consistently.
Test that everything works by using a non-zero constant JUMP_OFFSET.
The loader subtracts JUMP_OFFSET from loaded labels, and all
instructions that use 'f' operands add it back.
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* bjorn/erts/improve-beam-ops:
Add built-in macros $ARG_POSITION() and $IS_PACKED()
Use 't' instead of 'I' bit syntax operands
Optimize operand type for match context in i_bs_get_integer
Change operand type from 's' to 'S' for a few instructions
Use the correct name of the parameter
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Optimise equality comparisons
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* potatosalad/erts/binary_find_bif_improved/PR-1480/OTP-14610:
stdlib: Improved BIF for binary matches and split.
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maint-20
* sverker/20/binary_to_atom-utf8-crash/ERL-474/OTP-14590:
erts: Fix crash in binary_to_atom/term for invalid utf8
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