AArch64: correct preserve_most and preserve_all on Windows (#166436)

This fixes register information handling for the `preserve_most` and
`preserve_all` calling conventions on Windows ARM64.

The root issue was cascading `if` statements whose behavior depended on
their order. This patch makes the minimal, tactical change needed for
Swift’s two calling conventions, unblocking current work. A broader
refactor to remove the ordering dependency is still desired and will
follow in a subsequent PR.

Author: compnerd

llvm/lib/Target/AArch64/AArch64CallingConvention.td

@@ -601,6 +601,12 @@ def CSR_Win_AArch64_AAPCS_SwiftError
 def CSR_Win_AArch64_AAPCS_SwiftTail
     : CalleeSavedRegs<(sub CSR_Win_AArch64_AAPCS, X20, X22)>;
 
+def CSR_Win_AArch64_RT_MostRegs
+    : CalleeSavedRegs<(add CSR_Win_AArch64_AAPCS, (sequence "X%u", 9, 15))>;
+
+def CSR_Win_AArch64_RT_AllRegs
+    : CalleeSavedRegs<(add CSR_Win_AArch64_RT_MostRegs, (sequence "Q%u", 8, 31))>;
+
 // The Control Flow Guard check call uses a custom calling convention that also
 // preserves X0-X8 and Q0-Q7.
 def CSR_Win_AArch64_CFGuard_Check : CalleeSavedRegs<(add CSR_Win_AArch64_AAPCS,

llvm/lib/Target/AArch64/AArch64RegisterInfo.cpp

@@ -90,6 +90,16 @@ AArch64RegisterInfo::getCalleeSavedRegs(const MachineFunction *MF) const {
   if (MF->getSubtarget<AArch64Subtarget>().isTargetDarwin())
     return getDarwinCalleeSavedRegs(MF);
 
+  if (MF->getFunction().getCallingConv() == CallingConv::PreserveMost)
+    return MF->getSubtarget<AArch64Subtarget>().isTargetWindows()
+               ? CSR_Win_AArch64_RT_MostRegs_SaveList
+               : CSR_AArch64_RT_MostRegs_SaveList;
+
+  if (MF->getFunction().getCallingConv() == CallingConv::PreserveAll)
+    return MF->getSubtarget<AArch64Subtarget>().isTargetWindows()
+               ? CSR_Win_AArch64_RT_AllRegs_SaveList
+               : CSR_AArch64_RT_AllRegs_SaveList;
+
   if (MF->getFunction().getCallingConv() == CallingConv::CFGuard_Check)
     return CSR_Win_AArch64_CFGuard_Check_SaveList;
   if (MF->getSubtarget<AArch64Subtarget>().isTargetWindows()) {
@@ -138,10 +148,6 @@ AArch64RegisterInfo::getCalleeSavedRegs(const MachineFunction *MF) const {
     return CSR_AArch64_AAPCS_SwiftError_SaveList;
   if (MF->getFunction().getCallingConv() == CallingConv::SwiftTail)
     return CSR_AArch64_AAPCS_SwiftTail_SaveList;
-  if (MF->getFunction().getCallingConv() == CallingConv::PreserveMost)
-    return CSR_AArch64_RT_MostRegs_SaveList;
-  if (MF->getFunction().getCallingConv() == CallingConv::PreserveAll)
-    return CSR_AArch64_RT_AllRegs_SaveList;
   if (MF->getFunction().getCallingConv() == CallingConv::Win64)
     // This is for OSes other than Windows; Windows is a separate case further
     // above.

llvm/test/CodeGen/AArch64/preserve_mostcc.ll

@@ -1,4 +1,5 @@
-; RUN: llc < %s -mtriple=arm64-apple-ios-8.0.0 | FileCheck %s
+; RUN: llc < %s -mtriple=arm64-apple-ios-8.0.0 | FileCheck -check-prefix CHECK -check-prefix CHECK-DARWIN %s
+; RUN: llc < %s -mtriple=aarch64-unknown-windiws-msvc | FileCheck -check-prefix CHECK -check-prefix CHECK-WIN %s
 
 declare void @standard_cc_func()
 declare preserve_mostcc void @preserve_mostcc_func()
@@ -8,18 +9,26 @@ declare preserve_mostcc void @preserve_mostcc_func()
 define preserve_mostcc void @preserve_mostcc1() nounwind {
 entry:
 ;CHECK-LABEL: preserve_mostcc1
-;CHECK-NOT:   stp
-;CHECK-NOT:   str
-;CHECK:       str     x15
-;CHECK-NEXT:  stp     x14, x13,
-;CHECK-NEXT:  stp     x12, x11,
-;CHECK-NEXT:  stp     x10, x9,
-;CHECK:       bl      _standard_cc_func
+;CHECK-DARWIN-NOT:   stp
+;CHECK-DARWIN-NOT:   str
+;CHECK-DARWIN:       str     x15
+;CHECK-DARWIN-NEXT:  stp     x14, x13,
+;CHECK-DARWIN-NEXT:  stp     x12, x11,
+;CHECK-DARWIN-NEXT:  stp     x10, x9,
+;CHECK-WIN:       stp     x15, x14
+;CHECK-WIN-NEXT:  stp     x13, x12,
+;CHECK-WIN-NEXT:  stp     x11, x10,
+;CHECK-WIN-NEXT:  stp     x9, x30
+;CHECK:       bl      {{_?}}standard_cc_func
   call void @standard_cc_func()
-;CHECK:       ldp     x10, x9,
-;CHECK-NEXT:  ldp     x12, x11,
-;CHECK-NEXT:  ldp     x14, x13,
-;CHECK-NEXT:  ldr     x15
+;CHECK-DARWIN:       ldp     x10, x9,
+;CHECK-DARWIN-NEXT:  ldp     x12, x11,
+;CHECK-DARWIN-NEXT:  ldp     x14, x13,
+;CHECK-DARWIN-NEXT:  ldr     x15
+;CHECK-WIN:       ldp     x9, x30
+;CHECK-WIN-NEXT:  ldp     x11, x10,
+;CHECK-WIN-NEXT:  ldp     x13, x12,
+;CHECK-WIN-NEXT:  ldp     x15, x14,
   ret void
 }
 
@@ -31,9 +40,10 @@ define preserve_mostcc void @preserve_mostcc2() nounwind {
 entry:
 ;CHECK-LABEL: preserve_mostcc2
 ;CHECK-NOT: x14
-;CHECK:     stp     x29, x30,
+;CHECK-DARWIN:     stp     x29, x30,
+;CHECK-WIN:     str     x30
 ;CHECK-NOT: x14
-;CHECK:     bl      _preserve_mostcc_func
+;CHECK:     bl      {{_?}}preserve_mostcc_func
   call preserve_mostcc void @preserve_mostcc_func()
   ret void
 }