AArch64: allocate small fixed args properly in varargs functions.

[t.p.northover]

Summary

Quite an embarrassing one here, vargargs calls have been broken in some edge-cases for quite a while...

On Darwin, function arguments occupy their real size when passed on the stack (e.g. an i16 only consumes 2 bytes). This means that, even for fixed args in varargs calls we need to keep track of the original type being passed before any DAG/GISel promotions. Existing logic only applied this fix to the non-varargs case leading to mismatch between caller & callee in those situations.

On Linux & Windows these arguments always occupy a 64-bit slot anyway so there's no special handling needed.

[Gerolf]

LGTM for both (GISel & SDAG).

[t.p.northover]

Thanks, pushed to main (3ed58d4df65e).

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

for (unsigned i = 0; i != NumArgs; ++i) {
     if (IsVarArg) {
       // On Windows, the fixed arguments in a vararg call are passed in GPRs
       // too, so use the vararg CC to force them to integer registers.
       if (IsCalleeWin64) {
         UseVarArgCC = true;
       } else {
         UseVarArgCC = !Outs[i].IsFixed;
       }
-    } else {
+    }
+
+    if (!UseVarArgCC) {
       // Get type of the original argument.
       EVT ActualVT =
           TLI.getValueType(DAG.getDataLayout(), CLI.Args[Outs[i].OrigArgIndex].Ty,
                        /*AllowUnknown*/ true);
       MVT ActualMVT = ActualVT.isSimple() ? ActualVT.getSimpleVT() : ArgVT;
       // If ActualMVT is i1/i8/i16, we should set LocVT to i8/i8/i16.
       if (ActualMVT == MVT::i1 || ActualMVT == MVT::i8)
         ArgVT = MVT::i8;

llvm/lib/Target/AArch64/GISel/AArch64CallLowering.cpp

struct AArch64OutgoingValueAssigner
 
   bool assignArg(unsigned ValNo, EVT OrigVT, MVT ValVT, MVT LocVT,
                  CCValAssign::LocInfo LocInfo,
                  const CallLowering::ArgInfo &Info, ISD::ArgFlagsTy Flags,
                  CCState &State) override {
     bool IsCalleeWin = Subtarget.isCallingConvWin64(State.getCallingConv());
     bool UseVarArgsCCForFixed = IsCalleeWin && State.isVarArg();
 
-    if (!State.isVarArg() && !UseVarArgsCCForFixed && !IsReturn)
-      applyStackPassedSmallTypeDAGHack(OrigVT, ValVT, LocVT);
-
     bool Res;
-    if (Info.IsFixed && !UseVarArgsCCForFixed)
+    if (Info.IsFixed && !UseVarArgsCCForFixed) {
+      if (!IsReturn)
+        applyStackPassedSmallTypeDAGHack(OrigVT, ValVT, LocVT);
       Res = AssignFn(ValNo, ValVT, LocVT, LocInfo, Flags, State);
-    else
+    } else
       Res = AssignFnVarArg(ValNo, ValVT, LocVT, LocInfo, Flags, State);
 
     StackOffset = State.getNextStackOffset();
     return Res;
   }
 };
 
 struct IncomingArgHandler : public CallLowering::IncomingValueHandler {

llvm/test/CodeGen/AArch64/varargs-fixed-i16.ll

+; RUN: llc -mtriple=arm64-apple-macosx %s -o - | FileCheck %s --check-prefix=CHECK-DARWIN
+; RUN: llc -mtriple=arm64-apple-macosx %s -o - -global-isel | FileCheck %s --check-prefix=CHECK-DARWIN
+; RUN: llc -mtriple=aarch64-linux-gnu %s -o - | FileCheck %s --check-prefix=CHECK-LINWIN
+; RUN: llc -mtriple=aarch64-linux-gnu %s -o - -global-isel | FileCheck %s --check-prefix=CHECK-LINWIN
+; RUN: llc -mtriple=aarch64-windows-msvc %s -o - | FileCheck %s --check-prefix=CHECK-LINWIN
+; RUN: llc -mtriple=aarch64-windows-msvc %s -o - -global-isel | FileCheck %s --check-prefix=CHECK-LINWIN
+
+declare i16 @foo([8 x i64], i16 signext, i16 signext %a, ...)
+
+define void @bar() {
+; CHECK-DARWIN-LABEL: bar:
+; CHECK-LINWIN-LABEL: bar:
+
+; CHECK-DARWIN: mov [[TMP:w[0-9]+]], #2752512
+; CHECK-DARWIN: str [[TMP]], [sp]
+
+; CHECK-LINWIN: mov [[TMP:w[0-9]+]], #42
+; CHECK-LINWIN: str{{h?}} wzr, [sp]
+; CHECK-LINWIN: str{{h?}} [[TMP]], [sp, #8]
+
+  call i16([8 x i64], i16, i16, ...) @foo([8 x i64] poison, i16 signext 0, i16 signext 42)
+  ret void
+}