Re: [PATCH v2 3/6] configure: add option to enable LTO

[Daniel P . Berrangé]

On Tue, Oct 27, 2020 at 10:57:14AM -0400, Daniele Buono wrote:
> In terms of ar and linker, if you don't have the right mix it will just
> stop at link time with an error.
> 
> In terms of using gcc the errors may be a bit more subtle, similar to
> what Daniel mentioned. Succesfully compiling but then showing issues at
> runtime or in the test suite.
> 
> I'm using ubuntu 18.04 and the stock compiler (based on gcc 7.5) issues
> a bunch of warnings but compile succesfully with LTO.
> However, the tcg binary for sparc64 is broken. System-wide emulation
> stops in OpenFirmware with an exception. User emulation triggers a
> segmentation fault in some of the test cases. If I compile QEMU with
> --enable-debug the tests magically work.
> 
> I briefly tested with gcc-9 and that seemed to work ok, buy your mileage
> may vary

This why we shouldn't artificially block use of LTO with GCC in
the configure script. It blocks completely legitimate usage of
LTO with GCC versions where it works.

The user can detect if their version of GCC is broken by running the
test suite during their build process, which is best practice already,
and actually testing the result.

> 
> On 10/26/2020 11:50 AM, Daniel P. Berrangé wrote:
> > On Mon, Oct 26, 2020 at 10:51:43AM +0100, Paolo Bonzini wrote:
> > > On 23/10/20 22:06, Daniele Buono wrote:
> > > > This patch allows to compile QEMU with link-time optimization (LTO).
> > > > Compilation with LTO is handled directly by meson. This patch adds 
> > > > checks
> > > > in configure to make sure the toolchain supports LTO.
> > > > 
> > > > Currently, allow LTO only with clang, since I have found a couple of 
> > > > issues
> > > > with gcc-based LTO.
> > > > 
> > > > In case fuzzing is enabled, automatically switch to llvm's linker (lld).
> > > > The standard bfd linker has a bug where function wrapping (used by the 
> > > > fuzz*
> > > > targets) is used in conjunction with LTO.
> > > > 
> > > > Tested with all major versions of clang from 6 to 12
> > > > 
> > > > Signed-off-by: Daniele Buono <dbuono@linux.vnet.ibm.com>
> > > 
> > > What are the problems like if you have GCC or you ar/linker are not up
> > > to the job?  I wouldn't mind omitting the tests since this has to be
> > > enabled explicitly by the user.
> > 
> > We temporarily disabled LTO in Fedora rawhide due to GCC bugs causing
> > wierd test suite asserts. Those were pre-release versions of GCC/binutils
> > though. I've just tested again and LTO works correctly, so I've enabled
> > LTO once again.
> > 
> > Regards,
> > Daniel
> > 
> 

Regards,
Daniel
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