The reordering that GCC may do is unrelated to the reordering an (x86) CPU may do. Let’s start off with compiler reordering. The C language rules are such that GCC is forbidden from reordering volatile loads and store memory accesses with respect to each other, or deleting them, when a sequence point occurs between them … Read more
As advocated by @dulacc in his comment, __attribute__ ((optnone)) works on clang 9.0.0 on Mac’s High Sierra.
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
Related: Does a compiler always produce an assembly code? – more about why some compilers do compile only to asm, instead of straight to machine code in some object file format. There are several reasons that compiling to asm instead of machine code makes a compiler’s job easier and a compiler more easily portable. But … Read more
With -O2 enabled, gcc will perform tail call optimization, if possible. Now, the question is: When is it possible? It is possible to eliminate a single recursive call whenever the recursive call happens either immediately before or inside the (single) return statement, so there are no observable side effects afterwards other than possibly returning a … Read more
The 00 00 00 00 (relative) target address in e8 00 00 00 00 is intended to be filled in by the linker. It doesn’t mean that the call falls through. It just means you are disassembling an object file that has not been linked yet. Also, a call to the next instruction, if that … Read more
Reading a variable (that was not declared as volatile) is not considered to be a “side effect” as specified by the C standard. So the program is free to read a location and then discard the result, as far as the C standard is concerned. This is very common. Suppose you request 1 byte of … Read more
I will attempt one myself. I will gladly accept a better answer from Travis Brown or Miles Sabin. Nat can currently not be used to represent large numbers In the current implementation of Nat, the value corresponds to the number of nested shapeless.Succ[] types: scala> Nat(3) res10: shapeless.Succ[shapeless.Succ[shapeless.Succ[shapeless._0]]] = Succ() So to represent the number … Read more
Yes, it is possible. For example, for gcc you can use __builtin_unreachable to tell the compiler about impossible conditions, like so: if (value < 0 || value > 36) __builtin_unreachable(); We can wrap the condition above in a macro: #define assume(cond) do { if (!(cond)) __builtin_unreachable(); } while (0) And use it like so: assume(x … Read more
Am I correct in assuming that compiler ‘introspected my string’, or there is another explanation of this? Yes, this is exactly what happens. It’s a pretty simple and common optimization done by the compiler. Since your first printf() call is just: printf(“Hello world\n”); It’s equivalent to: puts(“Hello world”); Since puts() does not need to scan … Read more