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digitalmars.D.bugs - [Issue 936] New: Optimization by compiler hints

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=936

           Summary: Optimization by compiler hints
           Product: D
           Version: unspecified
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P5
         Component: DMD
        AssignedTo: bugzilla digitalmars.com
        ReportedBy: arkangath gmail.com


The Microsoft C++ compiler used compiler hints such as assume(var<[constant]);
to help the compiler optimize code. DMD could also benefit from similar hints
to improve optimizations.

Actually, rather than removing "assert"s from debug code, why not use it's
conditions to optimize code in release version?


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Feb 06 2007
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=936






If such things are added I would hope they are added as pragmas rather than
language keywords, becuase these sorts of hints have a way of becoming useless
and even counterproductive as compilers get smarter.  C.f. the C 'register'
keyword.


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Feb 07 2007
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=936


arkangath gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  BugsThisDependsOn|                            |677
             Status|NEW                         |ASSIGNED




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Feb 09 2007
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=936


arkangath gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  BugsThisDependsOn|677                         |
             Status|ASSIGNED                    |NEW




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Feb 09 2007
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=936


smjg iname.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |smjg iname.com





The reporter's point is that one of these ways in which compilers can become
smarter is by using assert expressions as optimisation hints.  Why add a pragma
as an extra way of doing the same thing?


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Feb 09 2007
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=936






I've never heard of "assume" and don't know what it does, so I thought he was
suggesting to add a new "assume" keyword and other "compiler hints such as"
that to D.

Taking hints from existing expressions like assert() would be fine by me.
Taking hints from contracts would also be cool.


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Feb 09 2007
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=936








 The reporter's point is that one of these ways in which compilers can become
 smarter is by using assert expressions as optimisation hints.  Why add a pragma
 as an extra way of doing the same thing?


An assert indicates "this is true".
A pragma could add a way to say "this is *probably* true".
The compiler could then perhaps emit code optimized to run the most likely path
as fast as possible, at the expense of less likely paths.

For instance, you could tell it that it's unlikely a function returned an error
code (let's assume this was a C function that doesn't know about exceptions) so
perhaps the code for when the error occurs should be the one performing a jump
instruction[1].


[1]: I read somewhere that conditional jumps are less expensive when the
condition does not hold. I have no idea if this is still the case for modern
processors, this is just an example.


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Feb 10 2007
prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=936






When debugging code,conditions in asserts are expected to be *always* true, and
not probably. If we intent to have probable conditions, the release code would
have some sort of handling of such situations. Pragmas for probabilities are
not so useful. Consider that in a switch a case is more probable than other,
the programmer would just change the order of cases, and not adding a pragma to
do the same thing which would just bloat the code.

As an example, in a getCurrentDay() function, we expect the return value never
to be bigger than 31, and out{} contract would be perfect to (possibly)
optimize the code. It is a certainty, not a probability.


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Feb 10 2007