[RISCV] Permit tail call to an externally-defined function with weak linkage

As described in D45395 `This has been modeled after ARM's tail call opt.`
ARM's abi seems to limit weak symbol.

I did not find the limitation for RISCV. (Please correct me if I am wrong)

gcc seems to use the tail-call opt: https://godbolt.org/z/bjWE68n5o

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D143137

Author: ChunyuLiao

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -12745,7 +12745,6 @@ bool RISCVTargetLowering::isEligibleForTailCallOptimization(
     CCState &CCInfo, CallLoweringInfo &CLI, MachineFunction &MF,
     const SmallVector<CCValAssign, 16> &ArgLocs) const {
 
-  auto &Callee = CLI.Callee;
   auto CalleeCC = CLI.CallConv;
   auto &Outs = CLI.Outs;
   auto &Caller = MF.getFunction();
@@ -12782,16 +12781,6 @@ bool RISCVTargetLowering::isEligibleForTailCallOptimization(
   if (IsCallerStructRet || IsCalleeStructRet)
     return false;
 
-  // Externally-defined functions with weak linkage should not be
-  // tail-called. The behaviour of branch instructions in this situation (as
-  // used for tail calls) is implementation-defined, so we cannot rely on the
-  // linker replacing the tail call with a return.
-  if (GlobalAddressSDNode *G = dyn_cast<GlobalAddressSDNode>(Callee)) {
-    const GlobalValue *GV = G->getGlobal();
-    if (GV->hasExternalWeakLinkage())
-      return false;
-  }
-
   // The callee has to preserve all registers the caller needs to preserve.
   const RISCVRegisterInfo *TRI = Subtarget.getRegisterInfo();
   const uint32_t *CallerPreserved = TRI->getCallPreservedMask(MF, CallerCC);

llvm/test/CodeGen/RISCV/tail-calls.ll

@@ -109,15 +109,11 @@ entry:
   ret void
 }
 
-; Externally-defined functions with weak linkage should not be tail-called.
-; The behaviour of branch instructions in this situation (as used for tail
-; calls) is implementation-defined, so we cannot rely on the linker replacing
-; the tail call with a return.
+; Perform tail call optimization for external weak symbol.
 declare extern_weak void @callee_weak()
 define void @caller_weak() nounwind {
 ; CHECK-LABEL: caller_weak
-; CHECK-NOT: tail callee_weak
-; CHECK: call callee_weak
+; CHECK: tail callee_weak
 entry:
   tail call void @callee_weak()
   ret void