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Re: [Qemu-devel] [PATCH] configure: workaround for Clang 3.5.0
From: |
Laszlo Ersek |
Subject: |
Re: [Qemu-devel] [PATCH] configure: workaround for Clang 3.5.0 |
Date: |
Thu, 29 Oct 2015 22:20:50 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 |
Aargh, clicked "Reply" instead of "Reply All". Resending.
On 10/29/15 21:29, John Snow wrote:
>
>
> On 10/29/2015 04:22 PM, John Snow wrote:
>> Clang++ 3.5 on Fedora 22 appears to have difficulty tolerating
>> D_FORTIFY_SOURCE for certain glibc headers, such as stdio.
>>
>> This interferes, currently, with any arm target build.
>>
>> Work around this by disabling FORTIFY_SOURCE for clang builds
>> if a problem is observed.
>>
>> Newer versions of clang such as 3.5.2 (As seen in debian-testing)
>> or 3.7.0 (As seen in Fedora 23 Beta) are unaffected and will not
>> trigger this workaround.
>>
>> Signed-off-by: John Snow <address@hidden>
>> ---
>
> As a meta-cover-letter, this fix is a little weird in that it will
> disable FORTIFY_SOURCE (silently!) for non-debug builds. Not great.
>
> (Maybe I could have it fail and print a warning encouraging users to
> either use --enable-debug or --disable-fortify-source?)
>
> Still, It'd be nice to have Clang builds working out of the box for ARM
> builds. It just so happens that ARM is the only target that happens to
> trip this specific unfortunate chain of events.
>
> Is it sane to check for clang-and-arm-targets only? Maybe it's a moot
> point -- newer (and older, I believe) versions of Clang won't trigger
> this at all.
>
> Anyway, see these links (Dredged up by Laszlo Ersek, thanks!)
Thanks for the credit.
I didn't notice this patch on the list (and I've been CC'd just now), so
I'm unsure if I'm supposed to (try to) review it. If so, then:
> https://llvm.org/bugs/show_bug.cgi?id=7219
> https://llvm.org/bugs/show_bug.cgi?id=16821
> https://llvm.org/bugs/show_bug.cgi?id=23277#c2
>
> Here's a good takeaway quote:
>
> 'Also, _FORTIFY_SOURCE + glibc + clang is not supported and does not
> work (for instance, it relies on __builtin_va_pack_len and friends,
> which we have no intention of supporting), so glibc compatibility is
> unlikely to be a strong motivator for a change here.'
this quote would be compelling enough for me to disable _FORTIFY_SOURCE
when clang is seen, no questions asked. The above is a public no-support
statement from an apparently core clang developer, so "it happens to
build without errors with version X.Y.Z." just don't cut it. A positive
claim (bugzilla comment, release note, etc) would be necessary.
... As far as I'm concerned, of course. :)
Cheers
Laszlo
>
> So long story short, we have a weird hacky workaround where we disable
> FORTIFY_SOURCE for compilers that don't appear to be able to support it.
>
> --js
>
>> configure | 25 ++++++++++++++++++++++++-
>> 1 file changed, 24 insertions(+), 1 deletion(-)
>>
>> diff --git a/configure b/configure
>> index 7a1d08d..7abfcc3 100755
>> --- a/configure
>> +++ b/configure
>> @@ -107,6 +107,11 @@ compile_object() {
>> do_cc $QEMU_CFLAGS $local_cflags -c -o $TMPO $TMPC
>> }
>>
>> +compile_cxx_object() {
>> + local_cflags="$1"
>> + do_cxx $QEMU_CXXFLAGS $local_cflags -c -o $TMPO $TMPC
>> +}
>> +
>> compile_prog() {
>> local_cflags="$1"
>> local_ldflags="$2"
>> @@ -4436,13 +4441,31 @@ if ! compile_object "-Werror"; then
>> fi
>>
>> ##########################################
>> +# Test that we can use FORTIFY_SOURCE,
>> +# which might break Clang.
>> +
>> +if test "$debug" = "no"; then
>> + cat > $TMPC << EOF
>> +#include <cstdio>
>> +int main(int argc, char*argv[]) {
>> + fprintf(stdout, "Hello World\n");
>> + return 0;
>> +}
>> +EOF
>> +
>> + if ! compile_cxx_object "-O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2"; then
>> + fortify_source="no";
>> + fi
>> +fi
>> +
>> +##########################################
>> # End of CC checks
>> # After here, no more $cc or $ld runs
>>
>> if test "$gcov" = "yes" ; then
>> CFLAGS="-fprofile-arcs -ftest-coverage -g $CFLAGS"
>> LDFLAGS="-fprofile-arcs -ftest-coverage $LDFLAGS"
>> -elif test "$debug" = "no" ; then
>> +elif test "$debug" = "no" && test "$fortify_source" != "no" ; then
>> CFLAGS="-O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 $CFLAGS"
>> fi
>>
>>