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programs on targets with large register files. The root of the compile time overhead was in the use of llvm::SmallVector to hold PhysRegEntries, which resulted in slow-down from calling llvm::SmallVector::assign(N, 0). In contrast std::vector uses the faster __platform_bzero to zero out primitive buffers when assign is called, while SmallVector uses an iterator. The fix for this was simply to replace the SmallVector with a dynamically allocated buffer and to initialize or reinitialize the buffer based on the total registers that the target architecture requires. The changes support cases where a pass manager may be reused for different targets, and note that the PhysRegEntries is allocated using calloc mainly for good for, and also to quite tools like Valgrind (see comments for more info on this). There is an rdar to track the fact that SmallVector doesn't have platform specific speedup optimizations inside of it for things like this, and I'll create a bugzilla entry at some point soon as well. TL;DR: This fix replaces the expensive llvm::SmallVector<unsigned char>::assign(N, 0) with a call to calloc for N bytes which is much faster because SmallVector's assign uses iterators. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200917 91177308-0d34-0410-b5e6-96231b3b80d8
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