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

From: Richard Kenner
Subject: Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."
Date: Tue, 02 Jan 2007 10:43:05 EST

> Well, while the effect of -fstrict-aliasing is hard to describe
> (TBAA _is_ a complex part of the standard), -fno-strict-aliasing
> rules are simple.  All loads and stores alias each other if they
> cannot be proven not to alias by points-to analysis.

Yes, the rules are "simple", but are written in terms of OPTIMIZATIONS, not
of language concepts: you can't talk about "points-to-analysis" in a language

In other words, -fwrapv says that we are modifying the language semantics to
define signed overflows as wrapping and this will have effects on the
optimizer (so the language effect is primary), while -fno-strict-aliasing
says what the optimizer will do and hence how we're modifying the language
(meaning the optimizer effect is primary).

There's isn't that big a difference here, admittedly and I agree that it's
unfortunate that we do this for strict aliasing, but I doing it for -fwrapv
would be even worse for the above reason.

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