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

From: Alex Bennée
Subject: Re: [PATCH v2 3/6] configure: add option to enable LTO
Date: Wed, 28 Oct 2020 09:35:36 +0000
User-agent: mu4e 1.5.6; emacs 28.0.50

Daniele Buono <dbuono@linux.vnet.ibm.com> writes:

> 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.

sparc64-linux-user? I think that might be in a bit of a bit rotted state
- we had to disable running check-tcg on it in CI because of instability
so I wouldn't be surprised if messing around with LTO has dug up even
more gremlins.

> 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.

Breakage in both system and linux-user emulation probably points at
something in the instruction decode being broken. Shame we don't have a
working risu setup for sparc64 to give the instruction handling a proper
work out.

> I briefly tested with gcc-9 and that seemed to work ok, buy your mileage
> may vary
> 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

Alex Bennée

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