From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Kris Jurka <books(at)ejurka(dot)com> |
Cc: | pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Need -fwrapv or -fno-strict-overflow for gcc-4.3 |
Date: | 2008-03-10 15:39:46 |
Message-ID: | 24549.1205163586@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Kris Jurka <books(at)ejurka(dot)com> writes:
> Gcc 4.3 has started to perform optimizations based on the denial of the
> existence of signed overflow.
> ...
> I don't understand the difference between -fwrapv and
> -fno-strict-aliasing, but it seems we need at least one of them.
I don't see -fno-strict-overflow listed at all in the manual for gcc 4.1.
So I think we should go for -fwrapv, which is defined thus:
`-fwrapv'
This option instructs the compiler to assume that signed arithmetic
overflow of addition, subtraction and multiplication wraps around
using twos-complement representation. This flag enables some
optimizations and disables others. This option is enabled by
default for the Java front-end, as required by the Java language
specification.
and so doesn't sound nearly as bad as Jakub painted it ;-). If we use
the other, we are assuming that there are no problems in 4.1, which
feels to me like a dangerous assumption. 4.1 *did* break mysql,
remember; and we have no regression tests checking most of these
security-related overflow tests, so we have no direct proof that we
are not broken.
regards, tom lane
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