I have done some experimentation in order to see when Hotspot is able to stack allocate. It turns out that its stack allocation is quite a bit more limited than what you might expect based on the available documentation. The referenced paper by Choi “Escape Analysis for Java” suggests that an object that is only … Read more
The C11 standard says this, 6.8.5/6: An iteration statement whose controlling expression is not a constant expression,156) that performs no input/output operations, does not access volatile objects, and performs no synchronization or atomic operations in its body, controlling expression, or (in the case of a for statement) its expression-3, may be assumed by the implementation … Read more
Indeed this is a known problem in gcc. gcc is notorious for reporting incorrect uninitialized variables. The shortcomings have been duly noted and there is a initiative to overcome the shortcomings: Better Uninitialized Warnings: The GNU Compiler Collection warns about the use of uninitialized variables with the option -Wuninitialized. However, the current implementation has some … Read more
EBO is important in the context of policy based design, where you generally inherit privately from multiple policy classes. If we take the example of a thread safety policy, one could imagine the pseudo-code : class MTSafePolicy { public: void lock() { mutex_.lock(); } void unlock() { mutex_.unlock(); } private: Mutex mutex_; }; class MTUnsafePolicy … Read more
The GCC documentation describes what these options do very explicitly. -O3 tries to optimize code very heavily for performance. It includes all of the optimizations -O2 includes, plus some more. -Os, on the other hand, instructs GCC to “optimize for size.” It enables all -O2 optimizations which do not increase the size of the executable, … Read more
TL;DR: I made an experimental analysis on Numpy 1.21.1. Experimental results show that np.sum does NOT (really) make use of SIMD instructions: no SIMD instruction are used for integers, and scalar SIMD instructions are used for floating-point numbers! Moreover, Numpy converts the integers to 64-bits values for smaller integer types by default so to avoid … Read more
I found it here: Ox and O2 are almost identical. They differ only in the fact that O2 also throws GF and Gy. There is almost no reason to avoid throwing these two switches.
None; no copies are done. The address of the caller’s Data return value is actually passed as a hidden argument to the function, and the createData function simply writes into the caller’s stack frame. This is known as the named return value optimisation. Also see the c++ faq on this topic. commercial-grade C++ compilers implement … Read more
Re-posting a modified version of my answer from optimized sum of an array of doubles in C, since that question got voted down to -5. The OP of the other question phrased it more as “what else is possible”, so I took him at his word and info-dumped about vectorizing and tuning for current CPU … Read more
Yes, HotSpot JIT can eliminate redundant allocations in a local context. This optimization is provided by the Escape Analysis enabled since JDK 6u23. It is often confused with on-stack allocation, but in fact it is much more powerful, since it allows not only to allocate objects on stack, but to eliminate allocation altogether by replacing … Read more