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Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."

From: Robert Dewar
Subject: Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."
Date: Sat, 30 Dec 2006 20:07:09 -0500
User-agent: Thunderbird (Windows/20061207)

(first, sorry for breaking the thread, pilot error on my part)
Here is the message again properly threaded. The other broken
thread by dewar's evil twin can be discarded.

Andrew Pinski wrote:

It does buy you something for code like:

if (a - 10 < 20)

 Well that particular example is far fetched in code that people
 expect to run efficiently, but with a bit of effort you could
 come up with a more realistic example.

 Compilers are always full of such optimizations which are simple to do
 and get put in without any sound basis for determining that they actually

 Now if they don't change semantics, then unhelpful optimizations (by this
 I mean optimizations which don't in practice help real applications), are
 only slightly damaging (they damage maintainability and reliability by
 creating additional unnecessary special cases).

 But if they are NOT semantically neutral in terms of expected usage, then
 the burden should be much higher.

 In my view, this comparison optimization should not have been put in
 without justification given that it clearly does affect the semantics
 of real code. Indeed if you really see code like

   if (a - 10 < 20)

 in place of

   if (a < 30)

 a very likely explanation is that you are deliberately doing something
 strange with wrap around, and should leave it alone.

 You say "I don't want to see this optimization removed", why not?
 The answer should hopefully be something like

   because test # 5 in benchmark suite xxx takes a 3% hit

 or even better

   awk processing expressions like bla bla takes an n% hit

 However, I suspect the real answer is

 "it's a neat optimization, I did it, and it can only help. I don't know
 if it really helps, but I can concoct an example where it would help,
 so why not leave it in, every little bit helps when it comes to high

 And that's to me not good enough for optimziations that break existing

 Note that

   if (a - 10 < 20)

 in the presence of wrap around semantics means something like

    if (a < 30 && a > minint + 9) ...

 and perhaps that was exactly what was meant!

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