RFR: 8343698: Linux x86_64 lto build gives a lot of warnings and fails lto-wrapper: fatal error: make returned 2 exit status [v3]

[Julian Waters]

On Mon, 18 Nov 2024 08:50:59 GMT, Matthias Baesken <mbaesken at openjdk.org> wrote:

>> The old LTO used to set -O3 directly on both LDFLAGS and CFLAGS, bypassing OPTIMIZATION variable entirely. Just wanted to put this out there, since the topic of traditional gcc LTO came up. I changed that to set OPTIMIZATION to HIGHEST_JVM some time ago, and I'm not sure whether my change played any part in breaking LTO
>> 
>> LTO is currently only supported for VC and gcc. I can look into how to implement it for clang, though not now. First I'm committed to fixing all the issues with LTO, and making sure Kim's worry of poisoning virtually disappearing when LTO is enabled doesn't become a reality
>
> Agree, we should support LTO for gcc AND clang on Linux/macOS.
> Regarding the longer link/build times , I can add some info how much longer it takes in our environment (SUSE Linux/gcc).

I can add some info here: On Windows, with LTO enabled it takes gcc about 15-20 minutes to link the JVM, without it takes anywhere between 8-10 minutes to do so. ld is very slow on Windows due to a bug, so the numbers are skewed towards the high end. I don't have any numbers for VC, but I can assume that even LTO with VC and link.exe is very fast, because VC in general is an extremely fast compiler

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PR Review Comment: https://git.openjdk.org/jdk/pull/22069#discussion_r1846588693