cmake: Add LTO functionality

[hebasto]

The LTO_FLAGS CMake variable might be used to override CMake's default LTO flags.

The following build system configuration:

cmake -B build -DLTO=ON -DLTO_FLAGS="-flto"

is comparable with the master branch amended with bitcoin#28876.

[hebasto]

Rebased.

[fanquake]

Repeating my comments from bitcoin#28876. I'm not a fan of this interface. If we are going to have a dedicated LTO option, it shouldn't also require manually passing flags into non-standard variables.

It's also not clear to me why:

cmake -B build -DLTO=ON -DLTO_FLAGS="-flto"

is any better than just:

cmake -B build CXX_FLAGS="-flto"

In this branch, is LTO_FLAGS meant to be for any flags, i.e compile and link?

[hebasto]

It's also not clear to me why:

cmake -B build -DLTO=ON -DLTO_FLAGS="-flto"

is any better than just:

cmake -B build CXX_FLAGS="-flto"

In this branch, is LTO_FLAGS meant to be for any flags, i.e compile and link?

1. In this branch, the LTO_FLAGS is meant to be for flags that are supposed to be passed to a compiler driver for any language (C or C++) in any mode (compiling or linking).

2. The default CMake LTO flags are better for everyday developer builds than just -flto for both GCC and Clang. They need to be adjusted only for release builds to be on-par with the master branch.

3. The LTO CMake variable was chosen to accomplish feature parity requirement for a new build system. Agree with you that LTO_FLAGS looks ugly.

4. Both ways:

CXXFLAGS="-flto" cmake -B build

and

cmake -B build -DCMAKE_CXX_FLAGS="-flto"

have the common drawback, which is no support for C code. That means that all object files in the build/CMakeFiles/secp256k1.dir/src/secp256k1/src are compiled without LTO flags.

---

I'm not a fan of this interface.

Happy to hear other interface ideas / prototypes :)

[fanquake]

Right. So we've still got to use LDFLAGS to pass through normal LTO link options, with your current approach? So why not just keep using the other standard *FLAGS vars? Otherwise now we've also got to mix and match.

What are the default CMake LTO flags? Why are they better? Are they stable (not changing from release to release)?

which is no support for C code.

Why wouldn't it support C code; that's what CFLAGS is for? I only omitted it here to simplify examples.

[hebasto]

What are the default CMake LTO flags? Why are they better?

For GCC:

-flto=auto -fno-fat-lto-objects

with the main benefit of parallelizing of LTRANS jobs, which in turn means:

• quicker build
• no "lto-wrapper: warning: using serial compilation of 128 LTRANS jobs" warnings

For Clang:

-flto-thin

with benefits as follows:

• quicker build
• compatibility with OSS-Fuzz

Are they stable (not changing from release to release)?

No, they are not.

---

If I understand your point correctly, this PR could be closed and all needed LTO flags are to be provided via standard variables, right?

[fanquake]

I think the PR should stay open for further discussion/other ideas, but yes, my main point is that we shouldn't be creating a special interface, for a subset of optimisation flags, without a good reason too. Especially if it might still requires you to use standard env vars anyways.

[hebasto]

Rebased.

[hebasto]

Rebased.

[hebasto]

Marking this PR as a draft in the light of the bitcoin#29185.

[hebasto]

Closing. See bitcoin#29185.