[AArch64][GlobalISel] Add support for sibcalling callees with varargs

[paquette]

Summary

This adds support for tail calling callees with varargs, equivalent to how it is done in AArch64ISelLowering.

This only works for sibling calls, and does not add the necessary support for musttail with varargs. (See r345641 for equivalent ISelLowering support.) This should be implemented when we stop falling back on musttail.

Update call-translator-tail-call.ll to show that we can now tail call varargs.

llvm/lib/Target/AArch64/AArch64CallLowering.cpp

bool AArch64CallLowering::areCalleeOutgoingArgsTailCallable(
   // Verify that the parameters in callee-saved registers match.
   // TODO: Port this over to CallLowering as general code once swiftself is
   // supported.
   auto TRI = MF.getSubtarget<AArch64Subtarget>().getRegisterInfo();
   const uint32_t *CallerPreservedMask = TRI->getCallPreservedMask(MF, CallerCC);
 
   for (auto &ArgLoc : OutLocs) {
     // If it's not a register, it's fine.
-    if (!ArgLoc.isRegLoc())
+    if (!ArgLoc.isRegLoc()) {
+      if (Info.IsVarArg) {
+        // Be conservative and disallow variadic memory operands to match SDAG's
+        // behaviour.
+        // FIXME: If the caller's calling convention is C, then we can
+        // potentially use its argument area. However, for cases like fastcc,
+        // we can't do anything.
+        LLVM_DEBUG(
+            dbgs()
+            << "... Cannot tail call vararg function with stack arguments\n");
+        return false;
+      }
       continue;
+    }
 
     Register Reg = ArgLoc.getLocReg();
 
     // Only look at callee-saved registers.
     if (MachineOperand::clobbersPhysReg(CallerPreservedMask, Reg))
       continue;
 
     // TODO: Port the remainder of this check from TargetLowering to support
if (Info.SwiftErrorVReg) {
     return false;
   }
 
   if (!mayTailCallThisCC(CalleeCC)) {
     LLVM_DEBUG(dbgs() << "... Calling convention cannot be tail called.\n");
     return false;
   }
 
-  if (Info.IsVarArg) {
-    LLVM_DEBUG(dbgs() << "... Tail calling varargs not supported yet.\n");
-    return false;
-  }
-
   // Byval parameters hand the function a pointer directly into the stack area
   // we want to reuse during a tail call. Working around this *is* possible (see
   // X86).
   //
   // FIXME: In AArch64ISelLowering, this isn't worked around. Can/should we try
   // it?
   //
   // On Windows, "inreg" attributes signify non-aggregate indirect returns.
bool AArch64CallLowering::isEligibleForTailCallOptimization(
   // true. However, we don't have full tail call support yet. So, continue
   // checking. We want to emit a sibling call.
 
   // I want anyone implementing a new calling convention to think long and hard
   // about this assert.
   assert((!Info.IsVarArg || CalleeCC == CallingConv::C) &&
          "Unexpected variadic calling convention");
 
+  // Before we can musttail varargs, we need to forward parameters like in
+  // r345641. Make sure that we don't enable musttail with varargs without
+  // addressing that!
+  assert(!(Info.IsVarArg && Info.IsMustTailCall) &&
+         "musttail support for varargs not implemented yet!");
+
   // Verify that the incoming and outgoing arguments from the callee are
   // safe to tail call.
   if (!doCallerAndCalleePassArgsTheSameWay(Info, MF, InArgs)) {
     LLVM_DEBUG(
         dbgs()
         << "... Caller and callee have incompatible calling conventions.\n");
     return false;
   }

llvm/test/CodeGen/AArch64/GlobalISel/call-translator-tail-call.ll

 define i32 @test_nonvoid_ret() {
   ; COMMON-LABEL: name: test_nonvoid_ret
   ; COMMON: bb.1 (%ir-block.0):
   ; COMMON:   TCRETURNdi @nonvoid_ret, 0, csr_aarch64_aapcs, implicit $sp
   %call = tail call i32 @nonvoid_ret()
   ret i32 %call
 }
 
-; Right now, this should not be tail called.
-; TODO: Support this.
 declare void @varargs(i32, double, i64, ...)
 define void @test_varargs() {
-  ; COMMON-LABEL: name: test_varargs
-  ; COMMON: bb.1 (%ir-block.0):
-  ; COMMON:   [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 42
-  ; COMMON:   [[C1:%[0-9]+]]:_(s64) = G_FCONSTANT double 1.000000e+00
-  ; COMMON:   [[C2:%[0-9]+]]:_(s64) = G_CONSTANT i64 12
-  ; COMMON:   ADJCALLSTACKDOWN 0, 0, implicit-def $sp, implicit $sp
-  ; COMMON:   $w0 = COPY [[C]](s32)
-  ; COMMON:   $d0 = COPY [[C1]](s64)
-  ; COMMON:   $x1 = COPY [[C2]](s64)
-  ; COMMON:   BL @varargs, csr_aarch64_aapcs, implicit-def $lr, implicit $sp, implicit $w0, implicit $d0, implicit $x1
-  ; COMMON:   ADJCALLSTACKUP 0, 0, implicit-def $sp, implicit $sp
-  ; COMMON:   RET_ReallyLR
+  ; On Darwin, everything is passed on the stack. Since the caller has no stack,
+  ; we don't tail call.
+  ; DARWIN-LABEL: name: test_varargs
+  ; DARWIN: bb.1 (%ir-block.0):
+  ; DARWIN:   [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 42
+  ; DARWIN:   [[C1:%[0-9]+]]:_(s64) = G_FCONSTANT double 1.000000e+00
+  ; DARWIN:   [[C2:%[0-9]+]]:_(s64) = G_CONSTANT i64 12
+  ; DARWIN:   ADJCALLSTACKDOWN 0, 0, implicit-def $sp, implicit $sp
+  ; DARWIN:   $w0 = COPY [[C]](s32)
+  ; DARWIN:   $d0 = COPY [[C1]](s64)
+  ; DARWIN:   $x1 = COPY [[C2]](s64)
+  ; DARWIN:   BL @varargs, csr_aarch64_aapcs, implicit-def $lr, implicit $sp, implicit $w0, implicit $d0, implicit $x1
+  ; DARWIN:   ADJCALLSTACKUP 0, 0, implicit-def $sp, implicit $sp
+  ; DARWIN:   RET_ReallyLR
+
+  ; Windows uses registers, so we don't need to worry about using the stack.
+  ; WINDOWS-LABEL: name: test_varargs
+  ; WINDOWS: bb.1 (%ir-block.0):
+  ; WINDOWS:   [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 42
+  ; WINDOWS:   [[C1:%[0-9]+]]:_(s64) = G_FCONSTANT double 1.000000e+00
+  ; WINDOWS:   [[C2:%[0-9]+]]:_(s64) = G_CONSTANT i64 12
+  ; WINDOWS:   $w0 = COPY [[C]](s32)
+  ; WINDOWS:   $d0 = COPY [[C1]](s64)
+  ; WINDOWS:   $x1 = COPY [[C2]](s64)
+  ; WINDOWS:   TCRETURNdi @varargs, 0, csr_aarch64_aapcs, implicit $sp, implicit $w0, implicit $d0, implicit $x1
   tail call void(i32, double, i64, ...) @varargs(i32 42, double 1.0, i64 12)
   ret void
 }
 
 ; Unsupported calling convention for tail calls. Make sure we never tail call
 ; it.
 declare ghccc void @bad_call_conv_fn()
 define void @test_bad_call_conv() {