[llvm-dev] Is clang+llvm deterministisc?

章明 via llvm-dev llvm-dev at lists.llvm.org
Mon Jul 17 00:36:39 PDT 2017

I searched source code of LLVM/Clang 4.0.0 for 'random_seed' with grep. It seems the -frandom-seed option is not supported.

The -rng-seed option appears to be defined in ./lib/Support/RandomNumberGenerator.cpp, which is source code for class RandomNumberGenerator. The constructor of class RandomNumberGenerator is private and is only called by Module::createRNG (defined in lib/IR/Module.cpp). But Module::createRNG does not seem to be called anywhere, except by a unit test.

I also tried adding a line to print a message in Module::createRNG. The modified code compiles without any error. However, when I run clang and llc to compile a simple C program, the message is not printed out. This confirms that Module::createRNG is not called by clang or llc.

-----Original Messages-----
From:"Alexandre Isoard" <alexandre.isoard at gmail.com>
Sent Time:2017-07-17 03:49:48 (Monday)
To: "章明" <editing at zju.edu.cn>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Is clang+llvm deterministisc?

Hi Ming Zhang,

If you don't want to rely on Clang reproducibility, you could save the IR into a .bc file. Clang can directly take a .bc file as input.

You then:
- instrument a copy of that .bc file and run your counting
- add control flow checking on an other copy of the original .bc file and you have your final binary

For the reproducibility, I think we try to preserve that, but sometime we lose it, you may have to specify -frandom-seed.

On Sun, Jul 16, 2017 at 4:22 AM, 章明 via llvm-dev <llvm-dev at lists.llvm.org> wrote:

Hi, there,

I am working on a project on software control flow checking, which instruments a program to check if the control flow at runtime matches the control flow graph computed at compile-time.

My instrumentation process has to make use of control flow information, including as control flow graph and dominator/post-dominator trees, so it is better part of the compiler. On the other hand, I don't want any transformation pass to mess up the additional instrumentation code, so my instrumentation process has to be run after other transformation passes are complete. Therefore, I'd like to implement my instrumentation process as the last pass before the machine intermediate representation (MIR) is translated to native assembly code.

My instrumentation process also needs to take basic block execution frequencies into consideration. So I have to compile the same program twice. First, the program is compiled, adding code to collect execution frequencies. Then, when the execution frequencies have been collected, the same program is compiled again to add control flow checking instructions, which takes execution frequencies into consideration. Obviously, the program profiled to collect execution frequencies and the program instrumented with control flow checking instructions have to be consistent. At least, they have to have the same basic blocks and identical control flow graphs. So my question is this: If I compile the same program twice using Clang, with the same command line, is it guaranteed that, at the point right before the MIRs are converted to native assembly code, the MIRs are identical?

Thank you!

Ming Zhang

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Alexandre Isoard
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