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Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."
- From: kenner at vlsi1 dot ultra dot nyu dot edu (Richard Kenner)
- To: vincent+gcc at vinc17 dot org
- Cc: autoconf-patches at gnu dot org, bernds_cb1 at t-online dot de, bug-gnulib at gnu dot org, dewar at adacore dot com, ebb9 at byu dot net, eggert at cs dot ucla dot edu, gcc at gcc dot gnu dot org, gdr at integrable-solutions dot net, pinskia at physics dot uc dot edu, richard dot guenther at gmail dot com
- Date: Sun, 31 Dec 2006 10:19:59 EST
- Subject: Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."
- References: <200612310042.kBV0g5pt027400@localhost.localdomain> <45970D3D.1090700@adacore.com> <20061231120933.GS32297@ay.vinc17.org> <4597B5F0.5050504@adacore.com> <10612311408.AA19449@vlsi1.ultra.nyu.edu> <20061231144821.GW32297@ay.vinc17.org>
> If done in unsigned, this won't lead to any optimization, as unsigned
> arithmetic doesn't have overflows. So, if you write "a - 10" where a
> is unsigned, the compiler can't assume anything, whereas is a is
> signed, the compiler can assume that a >= INT_MIN + 10, reducing
> the range for a, and possibly allowing some optimizations.
I don't follow. The purpose of this hand-optimization (whch is also done
by GCC) is to replace two comparisons with one subtraction and one
comparison, which is faster on most machines. It's not CORRECT to do this
transformation in signed unless you know there won't be overflow.
In general, for modern compilers it's best not to do this transformation
AT ALL and let the compiler figure out the best way to do the range tst.