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

From: Daniele Buono
Subject: Re: [PATCH v2 3/6] configure: add option to enable LTO
Date: Tue, 27 Oct 2020 10:57:14 -0400
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.12.1

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

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.


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