[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

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64Subtarget.cpp b/llvm/lib/Target/AArch64/AArch64Subtarget.cpp
index 8e131b0caa6..931f0c0ffb8 100644
--- a/llvm/lib/Target/AArch64/AArch64Subtarget.cpp
+++ b/llvm/lib/Target/AArch64/AArch64Subtarget.cpp
@@ -251,6 +251,10 @@ unsigned AArch64Subtarget::classifyGlobalFunctionReference(
       !TM.shouldAssumeDSOLocal(*GV->getParent(), GV))
     return AArch64II::MO_GOT;
 
+  // Use ClassifyGlobalReference for setting MO_DLLIMPORT/MO_COFFSTUB.
+  if (getTargetTriple().isOSWindows())
+    return ClassifyGlobalReference(GV, TM);
+
   return AArch64II::MO_NO_FLAG;
 }