Main part of this feature is a new UDA `@dynamicCompile`.

You can mark with this attribute any function (including class methods and lambdas) and compilation and optimization of this function will be deferred to runtime. Dynamic compiler will use instruction set available on host, so application can utilize the best available instructions without sacrificing users with old hardware. You only need to mark top-level function with this UDA, compiler will also defer compilation for any function called from root with body available. You need to explicitly compile `@dynamicCompile` functions before using any of them.

CompilerSettings settings;
settings.optLevel = 3; // -O3
compileDynamicCode(settings); // compile all dynamic functions

Second feature is `@dynamicCompileConst` variables.
You can mark any global non-thread local variable with this attribute. They are treated as usual from static code but if they are treated as compile-time constants by optimizer when accessed from `@dynamicCompile` function.

`@dynamicCompileConst` __gshared simdSize;

`@dynamicCompile` void foo()
  if (4 == simdSize)
    // SSE-otimized code
  else if (8 == simdSize)
    // AVX-optimized code


simdSize = 4;

Optimizer will treat `simdSize` as constant and remove checks and not taken branches.

You shouldn't change these variables from dynamic code (this is UB and will probaly crash you)

And you need to recompile dynamic code with `compileDynamicCode(...)` if you want changes to these variables take effect in dynamic code.

These are highly experimental features so expect major bugs.

Known issues:
* There is debug hook to get final asm generated for `@dynamicCompile` function but it is disabled now (but you can still get original and optimized LLVM IR for them) * Exceptions doesn't work on windows if there are dynamic functions in stack (they work on linux and osx).
* There are codegen issues on win32, use win64

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