MantisBT - CMake
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0015968CMakeCMakepublic2016-02-10 12:132016-06-10 14:21
Brad King 
Apple MacOS X10.4.10
CMake 3.4.3 
CMake 3.6CMake 3.6 
0015968: Ninja generator is non-deterministic
CMake's Ninja generator is non-deterministic on our project (LLVM). This is trivially reproducible by configuring llvm into a clean directory multiple times and comparing the results.

This is a very real problem for our project because the nondeterminism in the generated ninja build seems to be the cause of non-determinism in our build outputs. One of the important methods of compiler testing is what we call a 3-stage build. The process is basically:

(1) build clang
(2) use clang from step 1 to build clang again
(3) use clang from step 2 to build clang again
(4) ensure clang from step 2 and clang from step 3 are binary identical

Today this works using the CMake Unix Makefile generator, but not using the Ninja generator. This issue reproduces on CMake 3.4.x and CMake 3.5.0 rc1. I assume it probably goes back quite a long ways too.
mkdir scratch
cd scratch
cmake -G Ninja <path to llvm>
cp ..
rm -rf
cmake -G Ninja <path to llvm>
diff ../
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Issue History
2016-02-10 12:13ChrisNew Issue
2016-02-10 14:12Brad KingNote Added: 0040467
2016-02-10 17:13ChrisNote Added: 0040468
2016-02-10 17:26ChrisNote Added: 0040469
2016-02-11 08:56Brad KingNote Edited: 0040467bug_revision_view_page.php?bugnote_id=40467#r2023
2016-02-11 08:59Brad KingNote Added: 0040471
2016-02-11 16:21ChrisNote Added: 0040476
2016-02-12 08:58Brad KingNote Added: 0040477
2016-02-12 19:27ChrisNote Added: 0040480
2016-02-16 09:45Brad KingNote Added: 0040493
2016-02-16 09:45Brad KingAssigned To => Brad King
2016-02-16 09:45Brad KingStatusnew => resolved
2016-02-16 09:45Brad KingResolutionopen => fixed
2016-02-16 09:45Brad KingFixed in Version => CMake 3.6
2016-02-16 09:45Brad KingTarget Version => CMake 3.6
2016-06-10 14:21Kitware RobotNote Added: 0041252
2016-06-10 14:21Kitware RobotStatusresolved => closed

Brad King   
2016-02-10 14:12   
(edited on: 2016-02-11 08:56)
Thanks. Please try this fix:

 Ninja: Fix non-determinism in generated build statement order;a=commitdiff;h=59ade844 [^]

2016-02-10 17:13   
Even with the applied patch I'm still seeing differences in the files. A lot less differences than previously, but target orders are still different.

I don't expect that those should impact the build output. I have a test build running.

I applied your patch on top of CMake 3.5.0 rc2.
2016-02-10 17:26   
My build completed. While the ninja files are not identical, the generated binaries are.

Thanks for the rapid fix. Any chance this can get nominated for 3.5.0?
Brad King   
2016-02-11 08:59   
Re 0015968:0040469: The development window for 3.5 is closed: [^]

We typically don't add non-regression fixes after rc1 because every change risks a new regression and therefore undermines rc testing already done by others.

Meanwhile we should work further to eliminate the non-determinism you're seeing. I just generated a LLVM build tree with "-G Ninja" twice and got identical files. What differences do you see?
2016-02-11 16:21   
I'm still seeing a lot of dependencies being re-ordered.

With your patch I'm no longer seeing randomization in the order that targets appear in the file.

One example diff I'm seeing is the first run has:

build cmake_order_depends_target_llvm-tblgen: phony || lib/libLLVMSupport.a lib/libLLVMTableGen.a utils/TableGen/obj.llvm-tblgen

The second run has:

build cmake_order_depends_target_llvm-tblgen: phony || utils/TableGen/obj.llvm-tblgen lib/libLLVMSupport.a lib/libLLVMTableGen.a

This is less important than the other diffs because it doesn't impact the final built product.
Brad King   
2016-02-12 08:58   
Re 0015968:0040476: Thanks. Here is a fix for at least some such cases:

 Ninja: Fix non-determinism in generated target dependency order;a=commitdiff;h=46fa9583 [^]
2016-02-12 19:27   
With that patch the ninja files are identical. I tested by comparing 3 subsequent builds.

Thanks for the rapid fix!
Brad King   
2016-02-16 09:45   
Great, thanks for testing!
Kitware Robot   
2016-06-10 14:21   
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