Note that: Builtin operators && and || perform short-circuit evaluation (do not evaluate the second operand if the result is known after evaluating the first), but overloaded operators behave like regular function calls and always evaluate both operands. … Because the short-circuiting properties of operator&& and operator|| do not apply to overloads, and because types … Read more
Exactly why should it be able to optimize the code? You’re assuming that any transformation that works will be done. That’s not at all how optimizers work. They’re not Artificial Intelligences. They simply work by parametrically replacing known patterns. E.g. the “Common Subexpression Elimination” scans an expression for common subexpressions, and moves them forwards, if … Read more
Known bug. Related to http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6196102 http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6357214 and others. I think they’re considered low-priority to fix because they don’t come up in the real world.
The overarching difference here is that the first version is branchless. 16 isn’t the length of any string here (the longer one, with NUL, is only 15 bytes long); it’s an offset into the return object (whose address is passed in RDI to support RVO), used to indicate that the small-string optimization is in use … Read more
Conservative If you take a closer look at the defaults of gcc, the oldest compiler in your list, you’ll realize that they are very conservative: By default, on x86, only SSE 2 is activated; not even SSE 4. The set of flags in -Wall and -Wextra has not changed for years; there are new useful … 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
If you generate code for a little-endian platform, optimizing four-byte memcmp for inequality to a single DWORD comparison is invalid. When memcmp compares individual bytes it goes from low-addressed bytes to high-addressed bytes, regardless of the platform. In order for memcmp to return zero all four bytes must be identical. Hence, the order of comparison … Read more
Source Case 1: When you declare a const in your program, int const x = 2; Compiler can optimize away this const by not providing storage for this variable; instead it can be added to the symbol table. So a subsequent read just needs indirection into the symbol table rather than instructions to fetch value … Read more
This is due to the small string optimization. When the string data is less than or equal 16 characters, including the null terminator, it is stored in a buffer local to the std::string object itself. Otherwise, it allocates memory on the heap and stores the data over there. The first string “ABCDEFGHIJKLMNO” plus the null … Read more
As others have pointed out, the test is flawed in many ways. You did not tell us exactly how you did this test. However, I tried to implement a “naive” test (no offense) like this: class PrePostIncrement { public static void main(String args[]) { for (int j=0; j<3; j++) { for (int i=0; i<5; i++) … Read more