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This change is the first in a series of changes improving LLVM's Block Frequency propogation implementation to not lose probability mass in branchy code when propogating block frequency information from a basic block to its successors. This patch is a simple infrastructure improvement that does not actually modify the block frequency algorithm. The specific changes are: 1. Changes the division algorithm used when scaling block frequencies by branch probabilities to a short division algorithm. This gives us the remainder for free as well as provides a nice speed boost. When I benched the old routine and the new routine on a Sandy Bridge iMac with disabled turbo mode performing 8192 iterations on an array of length 32768, I saw ~600% increase in speed in mean/median performance. 2. Exposes a scale method that returns a remainder. This is important so we can ensure that when we scale a block frequency by some branch probability BP = N/D, the remainder from the division by D can be retrieved and propagated to other children to ensure no probability mass is lost (more to come on this). git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194950 91177308-0d34-0410-b5e6-96231b3b80d8
Design Of lib/System ==================== The software in this directory is designed to completely shield LLVM from any and all operating system specific functionality. It is not intended to be a complete operating system wrapper (such as ACE), but only to provide the functionality necessary to support LLVM. The software located here, of necessity, has very specific and stringent design rules. Violation of these rules means that cracks in the shield could form and the primary goal of the library is defeated. By consistently using this library, LLVM becomes more easily ported to new platforms since the only thing requiring porting is this library. Complete documentation for the library can be found in the file: llvm/docs/SystemLibrary.html or at this URL: http://llvm.org/docs/SystemLibrary.html While we recommend that you read the more detailed documentation, for the impatient, here's a high level summary of the library's requirements. 1. No system header files are to be exposed through the interface. 2. Std C++ and Std C header files are okay to be exposed through the interface. 3. No exposed system-specific functions. 4. No exposed system-specific data. 5. Data in lib/System classes must use only simple C++ intrinsic types. 6. Errors are handled by returning "true" and setting an optional std::string 7. Library must not throw any exceptions, period. 8. Interface functions must not have throw() specifications. 9. No duplicate function impementations are permitted within an operating system class. To accomplish these requirements, the library has numerous design criteria that must be satisfied. Here's a high level summary of the library's design criteria: 1. No unused functionality (only what LLVM needs) 2. High-Level Interfaces 3. Use Opaque Classes 4. Common Implementations 5. Multiple Implementations 6. Minimize Memory Allocation 7. No Virtual Methods