------- Additional Comments From schlie at comcast dot net 2004-10-21 01:04 ------- Subject: Re: usual arithmetic conversion not applying correctly
Ok, a more basic observation/recommendation: the front end should not be masking true operand types by promoting them prematurely, as it generates very little value, while needlessly complicating the back-end's ability to easily generate optimal code for non-integer width machines; as with efficient access to the true source operand precision, and true operation result precision requirements, the back end can then easily and directly leverage the target's rtl machine description to generate fully conforming precision code, with out the burden of having to back-out prematurely applied type conversions (which seem wholly counterproductive). (which of course is the point of being able to specify optimal QI, and in many cases HI mode rtl instruction mappings, which have been made much less efficient by the front-end's choice to needlessly prematurely type promote operands prior to the literal requirement to do so.) Thanks for hearing me out, and by the way this whole discussion began by observing that GCC was not utilizing efficient rtl QI mode instruction descriptions, resulting in the classification of the bug as a "missed optimization" bug, as opposed to an "incorrect code" bug, with the explanation that although the source and destination operands specified in C could be implemented as specified in the rtl instruction mapping specification and be fully conforming, it wasn't recognized because the back-end didn't find the "optimization" because the front-end needlessly prematurely type converted all operands passed to the back-end. And for what little it may be worth, for good or bad, the factual reality is that there are 100's of times more small processors programmed and deployed in products than there are 32+ bit processors, therefore it would be nice if GCC didn't needless restrict itself to being most optimally applicable to larger machines at the expense of smaller machines (as there far more commercial 8 and 16 bit processors available from companies interested in support than their are 32-64 bit processor companies, and likely to remain that way, so it's not likely good business to bite the hand that may feed you). -paul- > From: jsm at polyomino dot org dot uk <[EMAIL PROTECTED]> > Reply-To: <[EMAIL PROTECTED]> > Date: 20 Oct 2004 23:03:15 -0000 > To: <[EMAIL PROTECTED]> > Subject: [Bug tree-optimization/18065] usual arithmetic conversion not > applying correctly > > > ------- Additional Comments From jsm at polyomino dot org dot uk 2004-10-20 > 23:03 ------- > Subject: Re: C integer promotion semantics / > front end problems. > > I presume you sent your message directly to me by mistake, so am sending > the reply back to the bug database so it can benefit more than one person. > Technical messages about public GCC versions should, absent any more > specific support arrangements in a particular case, go to the mailing > lists, not individuals; questions are answered in public GCC development > for the benefit of the public. > > On Wed, 20 Oct 2004, Paul Schlie wrote: > >> Sorry to do this to you again, but where do you see in the standard the >> requirement to promote, or license to needlessly promote all operands >> to int? > > C99 subclause 6.3.1.1 paragraph 2. signed char has rank less than that of > int, so it *must* be promoted to int. What happens afterwards is nothing > to do with the front end. The front end does *too many* optimisations; it > should purely generate GIMPLE representing the standard meaning of the > code and leave it to the later stages to tell that such conversions - or > indeed conversions between int and long where they are the same size - can > be optimised away in some cases. > > -- > > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18065 > > ------- You are receiving this mail because: ------- > You reported the bug, or are watching the reporter. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18065
