└── README.md


/README.md:
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 1 | ct_choose: Constant-time choose between two variables
 2 | =======================================================
 3 | This project contains Clang/LLVM patches/passes for a new built-in function:
 4 | ```c
 5 | Type val = __builtin_ct_choose(bool condition, Type valTrue, Type valFalse);
 6 | ```
 7 | 
 8 | This function returns `valTrue` if `condition` is true, else `valFalse`, and does so in constant time.
 9 | The X86_64 backend will compile this function into a `CMOV` after other optimizations have run: this should ensure that 
10 | the transformation is *not* altered. 
11 | 
12 | For other backends, there is a default 
13 | implementation as `return (trueVal & ~condition) | (falseVal & condition)` with `condition={0,1}`.
14 | Unlike the x86_64, this transformation happens after other IR passes have run, but before the backend passes have: this means IR 
15 | passes will not alter the transformation, but there is a risk the backend optimizations will.
16 | 
17 | For more information, refer to the paper "[What you get is what you C: Controlling side effects in mainstream C compilers](https://drive.google.com/file/d/1jsOolD1C_Fu9oNVvhkB1_RQ9GlrFSGcN/view)".
18 |  
19 | The setup below was tested on Ubuntu trusty 14.04.5 LTS x86_64. I suggest you install this as a VM before reading further.
20 | 
21 | Pre-requesites:
22 | ---------------
23 | 	1. Install a VM running Ubuntu trusty 14.04.5 LTS x86_64. Allocate 32GB of disk.
24 | 	2. $sudo apt-get install git cmake g++ binutils-dev
25 | 
26 | Download, compile and install Clang/LLVM:
27 | -----------------------------------------
28 | 	$git clone https://github.com/lmrs2/llvm.git -b ctchoose_38 --single-branch --depth 1 
29 | 	$cd llvm/tools
30 | 	$git clone https://github.com/lmrs2/clang.git -b ctchoose_38 --single-branch --depth 1 
31 | 	$cd ..
32 | 	$mkdir build
33 | 	$cd build
34 | 	$cmake -DLLVM_BINUTILS_INCDIR=/usr/include -DBUILD_SHARED_LIBS=ON -DCMAKE_BUILD_TYPE=Debug -DLLVM_TARGETS_TO_BUILD="X86" ../
35 | 	$cmake --build .
36 | 	$sudo make install
37 | 
38 | Now Clang has the new built-in function `__builtin_ct_choose()` which you can call. If you're on x86_64, it should be compiled into a `CMOV`.
39 | 


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