Guaranteed tail call

[MrSmith33]

Since clang 13 there is musttail attribute for that
https://clang.llvm.org/docs/AttributeReference.html#musttail

int g(int);
int f(int x) {
    __attribute__((musttail)) return g(x);
}

In D I tried to get this via inline LLVM IR

extern(C) int foo(int a, void* f) {
    return inlineIR!(`
        %t1 = bitcast i8* %1 to i32 (i32, i8*)*
        %t2 = musttail call i32 %t1(i32 %0, i8* %1)
        ret i32 %t2`, int, int, void*)(a, f);
}

But the resulting IR doesn't have the musttail

define i32 @foo(i32 %a_arg, i8* %f_arg) {
  %t1.i = bitcast i8* %f_arg to i32 (i32, i8*)*
  %t2.i = tail call i32 %t1.i(i32 %a_arg, i8* %f_arg)
  ret i32 %t2.i
}

Is it difficult to add this to LDC?

[JohanEngelen]

Have you tried import ldc.attributes; @llvmAttr("musttail") ... ?

Ah, I now see that you want it on the call site. I don't think we have any method for that at the moment.

[kinke]

Hmm I don't think there are any statement-level pragmas yet, although supported according to the lang spec. A PragmaDeclaration has a decl list of Dsymbols it applies to - but statements are no Dsymbols...

[kinke]

Ah wait, that form is a PragmaStatement apparently. Which has a _body as Statement it applies to.

[MrSmith33]

So pragma without ; is essentially an attribute.

There is ToIRVisitor.visit(PragmaStatement) that is not implemented. So setting a flag on CallExp can be done from there. Not sure how where call expression code is generated. Is this the place?

[JohanEngelen]

In my opinion, the handling of the pragma statement should be done during semantic analysis, not during IR generation. That is also where errors should be caught (e.g. pragma(musttail) on a function or regular function call), such that it fits in with all normal semantic analysis (__traits(compiles, ...)).

(note we already have this problem with ldc's UDAs...)

[kinke]

Is this the place?

The CallExp codegen calls that function:

ldc/gen/toir.cpp

Lines 779 to 780 in 0d4d711

	DValue *result =
	DtoCallFunction(e->loc, e->type, fnval, e->arguments, sretPointer);

You'd need to add a line similar to

ldc/ir/irclass.cpp

Line 639 in 0d4d711

call->setTailCallKind(callee->isVarArg() ? llvm::CallInst::TCK_MustTail

after

ldc/gen/tocall.cpp

Line 1067 in 0d4d711

call->setAttributes(attrlist);

when passing the tail-call bla through, or ideally in that call() helper directly, where you'd need to get hold of the llvm::CallInst* from the returned DValue.

[MrSmith33]

Added PR #4366

[MrSmith33]

Problem is that callOrInvoke returns LLCallBasePtr, which is base to InvokeInst and CallInst. setTailCallKind is only present on CallInst.

[MrSmith33]

I could propagate bool isMustTail into callOrInvoke, but what to do with invoke call? Ideally compiler should produce error in that case.