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[Scm-discuss] building SCM with recent GCC versions

From: Steve VanDevender
Subject: [Scm-discuss] building SCM with recent GCC versions
Date: Sun, 7 Jul 2019 23:34:39 -0700

After upgrading my systems to the just-released Debian Buster with GCC
version 8.3, I found that rebuilding SCM with "gcc -O" produced a number
of test errors relating to floating-point printing and bignums.

After some trial and error and an inspired guess, I found that
"gcc -O -fwrapv" builds a version of SCM that compiles and passes all
tests.  This also produces working builds when higher optimization
levels like -O2 or -O3 are used.

"-fwrapv" tells GCC to assume that arithmetic "wraps around" mod 2^N
where N is the number of bits in an integer type.  This is apparently no
longer the default assumption in GCC.

I don't know yet which GCC versions might have this behavior other than
8.3.  "-fwrapv" has been an option for quite a while (I see it in a
system with GCC 4.4, at least).

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