[llvm-dev] How to configure cmake to not rebuild .inc (tablegen) files?

[Jie Zhou via llvm-dev]

Dear all,

I’m developing Clang&LLVM on MacOS Catalina 10.15.

Now every time I "make clang", the building systems rebuilds

all the .inc files generated by cmake, which essentially

rebuilds all the tablegen files in llvm, although I did not

change any of the tablegen files. The building is unnecessary

and slow, especially when it builds the X86GenSubtargetInfo.inc.

Is there a way to prevent cmake generating Makefiles that

rebuilds all the tablegen files?

Here is my command line to run cmake for generating Makefiles:

cmake -G “Unix Makefiles” -DLLVM_TARGETS_TO_BUILD=“X86" -DCMAKE_BUILD_TYPE=Debug $(LLVM_SRC)

The cmake version is 3.13.4. And I did not experience the same behavior on a Linux.

Thanks,

- Jie

[Joan Lluch via llvm-dev]

Hi Jie,

I’m using a Mac with Xcode and found that specifying DLLVM_OPTIMIZED_TABLEGEN=On helped. Incremental builds are faster but tablegen files are run every time anyway, so it’s not actually a fix. I posted this issue before but it seems solving this is not that easy, at least for Xcode. 

In the case of Xcode I found big speed improvements after saving the project xcuserdata folder in a safe place, deleting the entire “build” folder, re-creating a fresh “build” with cmake, and moving the xcuserdata folder I saved before in place. Unfortunately, this stops working as soon as I checkout branches, or pull commits from origin, so I need to repeat the same procedure to get fast compiles again.

John, 

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[Mehdi AMINI via llvm-dev]

On Sun, Oct 20, 2019 at 4:21 PM Jie Zhou via llvm-dev <llvm...@lists.llvm.org> wrote:

Dear all,

I’m developing Clang&LLVM on MacOS Catalina 10.15.

Now every time I "make clang", the building systems rebuilds

all the .inc files generated by cmake,

Are you saying that if you re-run `make clang` immediately after the first invocation complete you see these files rebuilt? You're not changing anything in the repo or running `git pull` (or equivalent)? This would really not be expected.

-- 

Mehdi

 

which essentially

rebuilds all the tablegen files in llvm, although I did not

change any of the tablegen files. The building is unnecessary

and slow, especially when it builds the X86GenSubtargetInfo.inc.

Is there a way to prevent cmake generating Makefiles that

rebuilds all the tablegen files?

Here is my command line to run cmake for generating Makefiles:

cmake -G “Unix Makefiles” -DLLVM_TARGETS_TO_BUILD=“X86" -DCMAKE_BUILD_TYPE=Debug $(LLVM_SRC)

The cmake version is 3.13.4. And I did not experience the same behavior on a Linux.

Thanks,

- Jie

[Jie Zhou via llvm-dev]

On Oct 21, 2019, at 14:16, Mehdi AMINI <joke...@gmail.com> wrote:

On Sun, Oct 20, 2019 at 4:21 PM Jie Zhou via llvm-dev <llvm...@lists.llvm.org> wrote:

Dear all,

I’m developing Clang&LLVM on MacOS Catalina 10.15.

Now every time I "make clang", the building systems rebuilds

all the .inc files generated by cmake,

Are you saying that if you re-run `make clang` immediately after the first invocation complete you see these files rebuilt? You're not changing anything in the repo or running `git pull` (or equivalent)? This would really not be expected.

Yes the tablegen files get recompiled immediately after I run a `make clang`. I checked on a Ubuntu and did not see the same behavior.

But luckily Joan's suggestion works: the -DLLVM_OPTIMIZED_TABLEGEN for cmake will do the trick.

- Jie