C++ backend¶

Rationale (vs LLVM IR, direct machine code, bytecode etc):

C is close enough in execution model (static allocations etc.) - can leave many hard problems to be solved by C compiler
portable! (not the case for LLVM IR)
performance and memory efficiency of native code

C vs C++:

C++ easier to emit, as well as more readable (e.g., can declare variables anywhere) - C99 not supported by Open Watcom!
C++ compiler availability pretty good, but might still exclude some obscure platforms/compilers
need a possibility to manage C vs C++ function linkage – both for imported and exported functions

==> Plain C output option would be nice, but is not high priority

Original backend¶

The main thing to watch out for is that not every Propel expression translates to a C++ expression. Consider the following example:

(def foo (+ (bar)
  (if baz
      1
      (begin
        (print "Warning: not baz!")
        2))))

This must transpile to something like:

int tmp1 = bar();
int tmp2;
if (baz) {
    tmp2 = 1;
}
else {
    print("Warning: not baz!");
    tmp2 = 2;
}
int foo = tmp1 + tmp2;

The current backend is broken – it can re-order expressions incorrectly.

format-form works by returning a pair of values: