Is the amalgamation compiler-and environment-specific?

[ethindp]

I'm not sure if I've asked this before (so apologies if I have). However, the amalgamation feature is really, really cool. (Okay: I love it.) I was wondering though: many amalgamated libraries (i.e., sqlite) make it possible to just drop them into your source tree and go. Is this possible with Botan, or is the generated amalgamation specific to the compiler/environment it was generated with/in?

[randombit]

The amalgamation is specific to exactly the target you are compiling for. sqlite has an easier time here since they (afaik) have very little in the way of hardware specific codepaths. In some ideal world our amalgamation would work in the same way. The main issue is that right now we make various "decisions" about the target characteristics at configure time (injecting these decisions into the source using a couple of headers that are generated at that time). Kind of similar to the autoconf model, I guess. So at the point you are running the compiler, various decisions have already been made, and there isn't logic to choose these later.

Certainly some improvements are possible here. Recently for example we moved the decision of "is the target CPU little or big endian" from configure-time to compile-time (#4735), so you can for example (and I know this is is an extremely obscure situation), compile the same amalgamation for Linux MIPS big-endian and Linux MIPS little-endian now.

And there are some that would be relatively easy to change the decision point to compile time. For example we have a macro BOTAN_BUILD_COMPILER_IS_XXX. But in practice this is only used to detect Visual C++ since Visual C++ is weird and different from GCC/Clang/XLC/etc and requires special code at spots. We could probably skip generating this macro and just look for _MSC_VER at compile time.

The main blocker with a lot of what's left is the lack of any reliable way to introspect on the details of the target at compile time. As a simple example, we would like to check if getentropy exists. There is not afaik any way in C++ to write a function

consteval bool has_getentropy() {
#if __has_include(<sys/random.h>)
   // ok at least the header exists --- but now what?
   return true; // maybe?
#else
   return false;
#endif
}