Re: bug or documentation missing boost posix_time time_duration not_a_date_time

From: Erik Thiele (erik.thiele_at_[hidden])
Date: 2019-05-24 13:41:21

I assume that time_duration is implemented as an integer. Some values
of this integer are then reserved for special values like
not_a_date_time.

this can be seen here:

https://github.com/boostorg/date_time/blob/develop/include/boost/date_time/int_adapter.hpp

this should have the consequence that when studying the assembler code
for comparing two time_durations, all that happens is the assembler
code of "less than compare" for integer type. if additional comparisons
and branchings take place, then I assume the code is not meeting the
design criteria of being optimized.

Well... this is what I did, copy paste this code to justcompare.cpp:

#include <boost/date_time/posix_time/ptime.hpp>

using namespace boost::posix_time;

bool compare_time_duration(time_duration a, time_duration b)
{
  return a < b;
}

bool compare_int(int a, int b)
{
  return a < b;
}

EOF

g++ -Wall -O2 -c justcompare.cpp
objdump -S --demangle justcompare.o

justcompare.o:     file format elf64-x86-64

Disassembly of section .text:

0000000000000000
<compare_time_duration(boost::posix_time::time_duration, boost::posix_time::time_duration)>:
   0:   48 8b 17                mov    (%rdi),%rdx
   3:   48 b9 ff ff ff ff ff    movabs $0x7fffffffffffffff,%rcx
   a:   ff ff 7f 
   d:   48 8d 04 0a             lea    (%rdx,%rcx,1),%rax
  11:   48 83 f8 fd             cmp    $0xfffffffffffffffd,%rax
  15:   77 29                   ja     40
  17:   48 bf fe ff ff ff ff    movabs $0x7ffffffffffffffe,%rdi
  1e:   ff ff 7f 
  21:   31 c0                   xor    %eax,%eax
  23:   48 39 fa                cmp    %rdi,%rdx
  26:   74 60                   je     88
  28:   48 8b 36                mov    (%rsi),%rsi
  2b:   48 01 f1                add    %rsi,%rcx
  2e:   48 83 f9 fd             cmp    $0xfffffffffffffffd,%rcx
  32:   77 20                   ja     54
  34:   48 39 fe                cmp    %rdi,%rsi
  37:   74 0a                   je     43
  39:   48 39 f2                cmp    %rsi,%rdx
  3c:   0f 9c c0                setl   %al
  3f:   c3                      retq   
  40:   48 8b 36                mov    (%rsi),%rsi
  43:   48 b9 fe ff ff ff ff    movabs $0x7ffffffffffffffe,%rcx
  4a:   ff ff 7f 
  4d:   31 c0                   xor    %eax,%eax
  4f:   48 39 ce                cmp    %rcx,%rsi
  52:   74 3c                   je     90
  54:   48 bf 00 00 00 00 00    movabs $0x8000000000000000,%rdi
  5b:   00 00 80 
  5e:   48 39 fa                cmp    %rdi,%rdx
  61:   74 35                   je     98
  63:   48 b9 ff ff ff ff ff    movabs $0x7fffffffffffffff,%rcx
  6a:   ff ff 7f 
  6d:   48 39 ce                cmp    %rcx,%rsi
  70:   0f 94 c0                sete   %al
  73:   48 39 ca                cmp    %rcx,%rdx
  76:   41 0f 95 c0             setne  %r8b
  7a:   44 20 c0                and    %r8b,%al
  7d:   74 21                   je     a0
  7f:   f3 c3                   repz retq 
  81:   0f 1f 80 00 00 00 00    nopl   0x0(%rax)
  88:   f3 c3                   repz retq 
  8a:   66 0f 1f 44 00 00       nopw   0x0(%rax,%rax,1)
  90:   f3 c3                   repz retq 
  92:   66 0f 1f 44 00 00       nopw   0x0(%rax,%rax,1)
  98:   48 39 d6                cmp    %rdx,%rsi
  9b:   0f 95 c0                setne  %al
  9e:   c3                      retq   
  9f:   90                      nop
  a0:   48 39 ca                cmp    %rcx,%rdx
  a3:   74 da                   je     7f
  a5:   48 39 fe                cmp    %rdi,%rsi
  a8:   75 8f                   jne    39
  aa:   f3 c3                   repz retq 
  ac:   0f 1f 40 00             nopl   0x0(%rax)

00000000000000b0 <compare_int(int, int)>:
  b0:   39 f7                   cmp    %esi,%edi
  b2:   0f 9c c0                setl   %al
  b5:   c3                      retq   

(i removed the verbose jump target strings from the output to make the
email readable)

see the difference between compare_time_duration and compare_int...

I have a bad feeling in my stomach... sorry, something is going very
wrong here :-)

cya
erik