From 818d50a93dbeb4a62140f72a0f89d0295b461177 Mon Sep 17 00:00:00 2001
From: Wei Mi <wmi@google.com>
Date: Wed, 6 Sep 2017 16:05:17 +0000
Subject: [PATCH] [TailCall] Allow llvm.memcpy/memset/memmove to be tail calls
 when parent function return the intrinsics's first argument.

llvm.memcpy/memset/memmove return void but they will return the first
argument after they are expanded as libcalls. Now if the parent function
has any return value, llvm.memcpy cannot be turned into tail call after
expansion.

The patch is to handle that case in SelectionDAGBuilder so when caller
function return the same value as the first argument of llvm.memcpy,
tail call is allowed.

Differential Revision: https://reviews.llvm.org/D37406

llvm-svn: 312641
---
 llvm/lib/CodeGen/Analysis.cpp                 | 11 +++++++++
 .../CodeGen/X86/tailcall-mem-intrinsics.ll    | 24 +++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/llvm/lib/CodeGen/Analysis.cpp b/llvm/lib/CodeGen/Analysis.cpp
index c2aecc651b79..f561577e1e2e 100644
--- a/llvm/lib/CodeGen/Analysis.cpp
+++ b/llvm/lib/CodeGen/Analysis.cpp
@@ -565,6 +565,17 @@ bool llvm::returnTypeIsEligibleForTailCall(const Function *F,
     return false;
 
   const Value *RetVal = Ret->getOperand(0), *CallVal = I;
+  // Intrinsic like llvm.memcpy has no return value, but will return the
+  // first argument if it is expanded as libcall.
+  const CallInst *Call = cast<CallInst>(I);
+  if (Function *F = Call->getCalledFunction()) {
+    Intrinsic::ID IID = F->getIntrinsicID();
+    if ((IID == Intrinsic::memcpy || IID == Intrinsic::memmove ||
+         IID == Intrinsic::memset) &&
+        RetVal == Call->getArgOperand(0))
+      return true;
+  }
+
   SmallVector<unsigned, 4> RetPath, CallPath;
   SmallVector<CompositeType *, 4> RetSubTypes, CallSubTypes;
 
diff --git a/llvm/test/CodeGen/X86/tailcall-mem-intrinsics.ll b/llvm/test/CodeGen/X86/tailcall-mem-intrinsics.ll
index 8e1e4f464baa..7491ea659ba2 100644
--- a/llvm/test/CodeGen/X86/tailcall-mem-intrinsics.ll
+++ b/llvm/test/CodeGen/X86/tailcall-mem-intrinsics.ll
@@ -24,6 +24,30 @@ entry:
   ret void
 }
 
+; CHECK-LABEL: tail_memcpy_ret
+; CHECK: jmp memcpy
+define i8* @tail_memcpy_ret(i8* nocapture %p, i8* nocapture readonly %q, i32 %n) #0 {
+entry:
+  tail call void @llvm.memcpy.p0i8.p0i8.i32(i8* %p, i8* %q, i32 %n, i32 1, i1 false)
+  ret i8* %p
+}
+
+; CHECK-LABEL: tail_memmove_ret
+; CHECK: jmp memmove
+define i8* @tail_memmove_ret(i8* nocapture %p, i8* nocapture readonly %q, i32 %n) #0 {
+entry:
+  tail call void @llvm.memmove.p0i8.p0i8.i32(i8* %p, i8* %q, i32 %n, i32 1, i1 false)
+  ret i8* %p
+}
+
+; CHECK-LABEL: tail_memset_ret
+; CHECK: jmp memset
+define i8* @tail_memset_ret(i8* nocapture %p, i8 %c, i32 %n) #0 {
+entry:
+  tail call void @llvm.memset.p0i8.i32(i8* %p, i8 %c, i32 %n, i32 1, i1 false)
+  ret i8* %p
+}
+
 declare void @llvm.memcpy.p0i8.p0i8.i32(i8* nocapture, i8* nocapture readonly, i32, i32, i1) #0
 declare void @llvm.memmove.p0i8.p0i8.i32(i8* nocapture, i8* nocapture readonly, i32, i32, i1) #0
 declare void @llvm.memset.p0i8.i32(i8* nocapture, i8, i32, i32, i1) #0