You can write assert(x*x >= 0.f) as a compile-time promise instead of a runtime check as follows in GNU C: #include <cmath> float test1 (float x) { float tmp = x*x; if (!(tmp >= 0.0f)) __builtin_unreachable(); return std::sqrt(tmp); } (related: What optimizations does __builtin_unreachable facilitate? You could also wrap if(!x)__builtin_unreachable() in a macro and call … Read more
Are two store buffer entries needed for split line/page stores on recent Intel?
No, a vpcmpeqb into a mask register does not trigger slow mode if you use a zmm register as one of the comparands, at least on SKX. This is also true of any of any other instruction (as far as I tested) which only reads the key 512-bit registers (the key registers being zmm0 – … Read more
Accessing local variable or parameter is a single step operation: take a variable located at offset N on the stack. If you function has 2 arguments (simplified): N = 0 – this N = 1 – first argument N = 2 – second argument N = 3 – first local variable N = 4 – … Read more
In modern CPUs, rep stosb‘s and rep movsb‘s microcoded implementation actually uses stores that are wider than 1B, so it can go much faster than one byte per clock. (Note this only applies to stos and movs, not repe cmpsb or repne scasb. They’re still slow, unfortunately, like at best 2 cycles per byte compared … Read more
It depends… Documentation says, “If you use array_push() to add one element to the array it’s better to use $array[] = because in that way there is no overhead of calling a function.” Source: http://us2.php.net/array_push So it boils down to how much data you want to cram into that array at any particular moment. Additionally, … Read more
an actual answer for you: Intel 64 and IA-32 Architectures Optimization Reference Manual Section 3.5.1.7 is where you want to look. In short there are situations where an xor or a mov may be preferred. The issues center around dependency chains and preservation of condition codes. In processors based on Intel Core microarchitecture, a number … Read more
Very odd, perhaps .NET optimization is backfireing in your case: The author disassembled several versions of ternary expressions and found that they are identical to if-statements, with one small difference. The ternary statement sometimes produces code that tests the opposite condition that you would expect, as in it tests that the subexpression is false instead … Read more
The bytecodes are not significantly more or less efficient if you use final because Java bytecode compilers typically do little in the way optimization. The efficiency bonus (if any) will be in the native code produced by the JIT compiler1. In theory, using the final provides a hint to the JIT compiler that should help … Read more
Comparing the generated bytecodes is mostly meaningless, since most of the optimization happens in run time with the JIT compiler. I’m going to guess that in this case, either expression is equally fast. If there’s any difference, it’s negligible. This is not something that you need to worry about. Look for big picture optimizations.