[AArch64] [Windows] Use COFF stubs for calls to extern_weak functions

As the extern_weak target might be missing, resolving to the absolute
address zero, we can't use the normal direct PC-relative branch
instructions (as that would result in relocations out of range).

Improve the classifyGlobalFunctionReference method to set
MO_DLLIMPORT/MO_COFFSTUB, and simplify the existing code in
AArch64TargetLowering::LowerCall to use the return value from
classifyGlobalFunctionReference for these cases.

Add code in both AArch64FastISel and GlobalISel/IRTranslator to
bail out for function calls to extern weak functions on windows,
to let SelectionDAG handle them.

This matches what was done for X86 in 6bf108d77a3c.

Differential Revision: https://reviews.llvm.org/D71721

3 files changed, 42 insertions, 17 deletions

diff --git a/llvm/test/CodeGen/AArch64/GlobalISel/call-translator-tail-call-weak.ll b/llvm/test/CodeGen/AArch64/GlobalISel/call-translator-tail-call-weak.ll
new file mode 100644
index 00000000000..bea5961efff
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/GlobalISel/call-translator-tail-call-weak.ll
@@ -0,0 +1,15 @@
+; NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+; RUN: llc %s -stop-after=irtranslator -verify-machineinstrs -mtriple aarch64-apple-darwin -global-isel -o - 2>&1 | FileCheck %s --check-prefixes=DARWIN,COMMON
+
+; Shouldn't tail call when the OS doesn't support it.
+declare extern_weak void @extern_weak_fn()
+define void @test_extern_weak() {
+  ; DARWIN-LABEL: name: test_extern_weak
+  ; DARWIN: bb.1 (%ir-block.0):
+  ; DARWIN:   ADJCALLSTACKDOWN 0, 0, implicit-def $sp, implicit $sp
+  ; DARWIN:   BL @extern_weak_fn, csr_aarch64_aapcs, implicit-def $lr, implicit $sp
+  ; DARWIN:   ADJCALLSTACKUP 0, 0, implicit-def $sp, implicit $sp
+  ; DARWIN:   RET_ReallyLR
+  tail call void @extern_weak_fn()
+  ret void
+}
diff --git a/llvm/test/CodeGen/AArch64/GlobalISel/call-translator-tail-call.ll b/llvm/test/CodeGen/AArch64/GlobalISel/call-translator-tail-call.ll
index b49307ca216..174c6ecb899 100644
--- a/llvm/test/CodeGen/AArch64/GlobalISel/call-translator-tail-call.ll
+++ b/llvm/test/CodeGen/AArch64/GlobalISel/call-translator-tail-call.ll
@@ -193,23 +193,6 @@ define void @test_inreg(i8* inreg %ptr) {
   ret void
 }
 
-; Shouldn't tail call when the OS doesn't support it. Windows supports this,
-; so we should be able to tail call there.
-declare extern_weak void @extern_weak_fn()
-define void @test_extern_weak() {
-  ; DARWIN-LABEL: name: test_extern_weak
-  ; DARWIN: bb.1 (%ir-block.0):
-  ; DARWIN:   ADJCALLSTACKDOWN 0, 0, implicit-def $sp, implicit $sp
-  ; DARWIN:   BL @extern_weak_fn, csr_aarch64_aapcs, implicit-def $lr, implicit $sp
-  ; DARWIN:   ADJCALLSTACKUP 0, 0, implicit-def $sp, implicit $sp
-  ; DARWIN:   RET_ReallyLR
-  ; WINDOWS-LABEL: name: test_extern_weak
-  ; WINDOWS: bb.1 (%ir-block.0):
-  ; WINDOWS:   TCRETURNdi @extern_weak_fn, 0, csr_aarch64_aapcs, implicit $sp
-  tail call void @extern_weak_fn()
-  ret void
-}
-
 declare fastcc void @fast_fn()
 define void @test_mismatched_caller() {
   ; COMMON-LABEL: name: test_mismatched_caller
diff --git a/llvm/test/CodeGen/AArch64/windows-extern-weak.ll b/llvm/test/CodeGen/AArch64/windows-extern-weak.ll
new file mode 100644
index 00000000000..e1e6de49604
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/windows-extern-weak.ll
@@ -0,0 +1,27 @@
+; RUN: llc -mtriple aarch64-windows -filetype asm -o - < %s | FileCheck %s
+; RUN: llc -mtriple aarch64-windows -filetype asm -o - -fast-isel %s | FileCheck %s
+; RUN: llc -mtriple aarch64-windows -filetype asm -o - -global-isel -global-isel-abort=0 %s | FileCheck %s
+
+define void @func() {
+; CHECK-LABEL: func:
+; CHECK:      str x30, [sp, #-16]!
+; CHECK-NEXT: adrp x8, .refptr.weakfunc
+; CHECK-NEXT: ldr x8, [x8, .refptr.weakfunc]
+; CHECK-NEXT: cbz     x8, .LBB0_2
+; CHECK-NEXT: ; %bb.1:
+; CHECK-NEXT: blr     x8
+; CHECK-NEXT: .LBB0_2:
+; CHECK-NEXT: ldr x30, [sp], #16
+; CHECK-NEXT: ret
+
+  br i1 icmp ne (void ()* @weakfunc, void ()* null), label %1, label %2
+
+1:
+  call void @weakfunc()
+  br label %2
+
+2:
+  ret void
+}
+
+declare extern_weak void @weakfunc()