Date: Mon, 15 Nov 2010 11:10:35 +0100 From: Erik Cederstrand <erik@cederstrand.dk> To: Dimitry Andric <dim@FreeBSD.org> Cc: freebsd-toolchain@freebsd.org Subject: Re: Clang and -frandom-seed Message-ID: <FD61324A-1A22-4731-A563-B0BE8AB93069@cederstrand.dk> In-Reply-To: <4CE06C4F.7000002@FreeBSD.org> References: <E135940E-F79C-4DFA-811D-18CCBF3C609E@cederstrand.dk> <4CE06C4F.7000002@FreeBSD.org>
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Den 15/11/2010 kl. 00.10 skrev Dimitry Andric:
> On 2010-11-14 23:29, Erik Cederstrand wrote:
>> I noticed that two consecutive builds of (GCC-built) Clang don't =
produce identical binaries. This is true for clang, clang++ and tblgen. =
I asked on the llvm-dev list yesterday, and it turns out it's because =
GCC uses a random seed on some symbols. Apparently, this can be =
controlled with the -frandom-seed flag. I haven't tested if this is also =
the case for Clang-built Clang.
>>=20
> [...]
> So this is all on purpose, and I think it would be a bad idea to =
disable
> it, unless we fully understand the consequences.
>=20
> On the other hand, the requirement "The string should be different for
> every file you compile", could possibly be fulfilled. Maybe by using
> the filename, relative to $SRCDIR, that is being compiled as "seed"?
>=20
> This would be unique for each compiled file, but still give the same
> result for each build, and also be independent of the particular =
machine
> you are building on.
I was thinking of something along the same lines. I think we agree that =
it only needs to be random across files, not across builds. Someone on =
llvm-dev also suggested using the path (either full or relative to src/) =
as a seed.
Where in the build scripts would I need to add this flag? Something =
like:
CXXFLAGS +=3D -frandom-seed=3D${.TARGET}
in src.conf?
Thanks,
Erik=
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