[Monotone-devel] Re: illegal instruction again [was: Re: monotone 0.11 test failures]

[Peter Simons]

Jonathan Matthew writes:

 > AC_CHECK_HEADERS(emmintrin.h, [CXXFLAGS_MSSE2="-msse2"])
 > test "z$CXXFLAGS" = "z" && CXXFLAGS="-O2 -fno-strict-aliasing 
 > $CXXFLAGS_MSSE2"

A (somewhat) minor problem with this code is that it tests
for the existence of $CXXFLAGS in a sub-shell. The variable
will not be visible for the test unless it has been
exported. You cannot say

    CXXFLAGS=-O0 ./configure

but have to use:

    export CXXFLAGS=-O0
    ./configure

This is uncommon for configure scripts, so I'd recommend
using the shell-internal mechanism for the test instead:

  CXXFLAGS=${CXXFLAGS:--O2 -fno-strict-aliasing $CXXFLAGS_MSSE2}

Oh, and about the '-msse2' flag: I wouldn't enable MSSE
support, just because some header file exists! Personally, I
would never enable it automatically but provide an
--enable-msse2 option to configure. If one wants to check, I
think

    grep msse /proc/cpuinfo

is a better test -- at least on Linux. But all that fails
miserable when you are cross-compiling.

Peter