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
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.
It’s a bug in your code, nothing wrong with the compiler or the optimization levels. The cinch is with expression templates (like the ones used by Boost Proto, and hence by Boost Spirit). They are only valid to the end of their enclosing full expression [1] The canonical workaound is: BOOST_SPIRIT_AUTO(ana, *~qi::char_(‘*’) > +qi::char_(‘*’)); You … 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
From your question + the compiler-generated asm from your answer: fill(0) is an ERMSB rep stosb which will use 256b stores in an optimized microcoded loop. (Works best if the buffer is aligned, probably to at least 32B or maybe 64B). fill(1) is a simple 128-bit movaps vector store loop. Only one store can execute … Read more
The compiler is not allowed to perform “optimizations”, which would result in a different value computed, than the one computed according to abstract machine semantics. 5.1.2.3 Program execution [#1] The semantic descriptions in this International Standard describe the behavior of an abstract machine in which issues of optimization are irrelevant. [#3] In the abstract machine, … Read more
I guess the question that needs to be answered why well-intentioned people would write the checks in the first place. The most common case is probably if you have a class that is part of a naturally occurring recursive call. If you had: struct Node { Node* left; Node* right; }; in C, you might … Read more
That’s a good problem. In order to solve that problem you will also have to disable ASLR otherwise the address of g() will be unpredictable. Disable ASLR: sudo bash -c ‘echo 0 > /proc/sys/kernel/randomize_va_space’ Disable canaries: gcc overflow.c -o overflow -fno-stack-protector After canaries and ASLR are disabled it should be a straight forward attack like … Read more
By default compilers optimize for “average” processor. Since different processors favor different instruction sequences, compiler optimizations enabled by -O2 might benefit average processor, but decrease performance on your particular processor (and the same applies to -Os). If you try the same example on different processors, you will find that on some of them benefit from … 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