[FastISel] Respect musttail over "disable-tail-calls"

[aeubanks]

Summary

musttail should be honored even in the presence of attributes like "disable-tail-calls". SelectionDAG properly handles this.

Update LangRef to explicitly mention that this is the semantics of musttail.

[RKSimon]

Is this explicitly documented anywhere? I can't find anything in langref

[aeubanks]

https://llvm.org/docs/LangRef.html#call-instruction
The musttail marker means that the call must be tail call optimized in order for the program to be correct. I read this as regardless of any attributes
And SelectionDAG properly handles this

[RKSimon]

In D132193#3735757, @aeubanks wrote:

https://llvm.org/docs/LangRef.html#call-instruction
The musttail marker means that the call must be tail call optimized in order for the program to be correct. I read this as regardless of any attributes
And SelectionDAG properly handles this

Should the description be updated to make that explicit? Worth adding the doc tweak to this patch?

[aeubanks]

add langref clarification

[rnk]

lgtm

llvm/docs/LangRef.rst

 """"""""""
 
 This instruction requires several arguments:
 
 #. The optional ``tail`` and ``musttail`` markers indicate that the optimizers
    should perform tail call optimization. The ``tail`` marker is a hint that
    `can be ignored <CodeGenerator.html#sibcallopt>`_. The ``musttail`` marker
    means that the call must be tail call optimized in order for the program to
-   be correct. The ``musttail`` marker provides these guarantees:
+   be correct. This is true even in the presence of attributes like
+   "disable-tail-calls". The ``musttail`` marker provides these guarantees:
 
    #. The call will not cause unbounded stack growth if it is part of a
       recursive cycle in the call graph.
    #. Arguments with the :ref:`inalloca <attr_inalloca>` or
       :ref:`preallocated <attr_preallocated>` attribute are forwarded in place.
    #. If the musttail call appears in a function with the ``"thunk"`` attribute
       and the caller and callee both have varargs, than any unprototyped
       arguments in register or memory are forwarded to the callee. Similarly,

llvm/lib/CodeGen/SelectionDAG/FastISel.cpp

for (auto i = CI->arg_begin(), e = CI->arg_end(); i != e; ++i) {
     Args.push_back(Entry);
   }
 
   // Check if target-independent constraints permit a tail call here.
   // Target-dependent constraints are checked within fastLowerCall.
   bool IsTailCall = CI->isTailCall();
   if (IsTailCall && !isInTailCallPosition(*CI, TM))
     IsTailCall = false;
-  if (IsTailCall && MF->getFunction()
-                            .getFnAttribute("disable-tail-calls")
-                            .getValueAsBool())
+  if (IsTailCall && !CI->isMustTailCall() &&
+      MF->getFunction().getFnAttribute("disable-tail-calls").getValueAsBool())
     IsTailCall = false;
 
   CallLoweringInfo CLI;
   CLI.setCallee(RetTy, FuncTy, CI->getCalledOperand(), std::move(Args), *CI)
       .setTailCall(IsTailCall);
 
   diagnoseDontCall(*CI);
 

llvm/test/CodeGen/X86/fast-isel-disable-tail-calls.ll

+; RUN: llc -O0 -fast-isel -mtriple=x86_64-unknown-unknown < %s | FileCheck %s
+
+; CHECK-NOT: retq
+; CHECK: jmpq
+
+define void @f(ptr %this) "disable-tail-calls"="true" {
+  musttail call void %this(ptr %this)
+  ret void
+}