Thread overview
New LDC feature: dynamic compilation
Nov 14
kinke
November 13
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.

Example:
`@dynamicCompileConst` __gshared simdSize;

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

...

simdSize = 4;
compileDynamicCode();

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
November 13
On Monday, 13 November 2017 at 19:04:16 UTC, Ivan Butygin wrote:

> You need to explicitly compile `@dynamicCompile` functions before using any of them.

Interesting feature.  So is the executable linked to an installed instance of LDC/LLVM to make this happen, or is there some limited compiler embedded in the executable or Druntime?  More details about the implementation please.

Mike

November 13
On Monday, 13 November 2017 at 19:58:29 UTC, Michael V. Franklin wrote:
> Interesting feature.  So is the executable linked to an installed instance of LDC/LLVM to make this happen, or is there some limited compiler embedded in the executable or Druntime?  More details about the implementation please.
>
> Mike

Jit runtime (ldc-jit.dll/so) uses llvm optimizer and backend.

Forgot to mention, user need to pass `-enable-dynamic-compile` compiler switch to enable this feature (and jit runtime will only be linked in this case).
November 14
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