[llvm-dev] [RFC] Adding CPS call support

[Gor Nishanov via llvm-dev]

Here is an example of emulating a CPS call using LLVM Coroutines:

https://gist.github.com/GorNishanov/d7caa7697681b96180e4648ed9408bc9

It corresponds to:

void f(int val) {
  partA(val);
  int result = cps_call_bar(41);
  partB(result)
}

int cps_call_bar(int val) {
  partC(val)
  return 42;
}

int main() {
   f(40);
}

Run: opt -enable-coroutines -O2 cps.ll -S

cps_call will look like:

define void @cps_call_bar(i8* %caller, i32* nocapture %retval.addr, i32 %n)
local_unnamed_addr {
  tail call void @partC(i32 %n)
  store i32 42, i32* %retval.addr, align 4
  %1 = bitcast i8* %caller to { i8*, i8* }*
  %2 = getelementptr inbounds { i8*, i8* }, { i8*, i8* }* %1, i32 0, i32 0
  %3 = load i8*, i8** %2
  %4 = bitcast i8* %3 to void (i8*)*
  tail call fastcc void %4(i8* %caller)
  ret void
}

main will get optimized to:

define i32 @main() local_unnamed_addr {
entry:
  %alloc.i = tail call i8* @malloc(i32 24)
  tail call void @partA(i32 40)
  tail call void @partC(i32 41)
  tail call void @partB(i32 42)
  tail call void @free(i8* nonnull %alloc.i)
  ret i32 0
}

and resume part of f will be:

define internal fastcc void @f.resume(%f.Frame* %FramePtr) {
entry.resume:
  %vFrame = bitcast %f.Frame* %FramePtr to i8*
  tail call void @partB(i32 42)
  tail call void @free(i8* %vFrame)
  ret void
}

Note that this is using LLVM coroutines as is. To support CPS we would need
to add a couple of intrinsics along the lines I mentioned in my earlier
post.
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