[CGP] Extend `dupRetToEnableTailCallOpts` to known intrinsics

Hint further tail call optimization opportunities when the examined
returned value is the return value of a known intrinsic or library
function, and it appears as first function argument.

Fixes: https://github.com/llvm/llvm-project/issues/75455.

1 files changed, 64 insertions, 6 deletions

diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index 09c4922..32a25b4 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -2522,8 +2522,40 @@ bool CodeGenPrepare::optimizeCallInst(CallInst *CI, ModifyDT &ModifiedDT) {
   return false;
 }
 
+static bool isIntrinsicOrLFToBeTailCalled(const TargetLibraryInfo *TLInfo,
+                                          const CallInst *CI) {
+  assert(CI && CI->use_empty());
+
+  if (const auto *II = dyn_cast<IntrinsicInst>(CI))
+    switch (II->getIntrinsicID()) {
+    case Intrinsic::memset:
+    case Intrinsic::memcpy:
+    case Intrinsic::memmove:
+      return true;
+    default:
+      return false;
+    }
+
+  LibFunc LF;
+  Function *Callee = CI->getCalledFunction();
+  if (Callee && TLInfo && TLInfo->getLibFunc(*Callee, LF))
+    switch (LF) {
+    case LibFunc_strcpy:
+    case LibFunc_strncpy:
+    case LibFunc_strcat:
+    case LibFunc_strncat:
+      return true;
+    default:
+      return false;
+    }
+
+  return false;
+}
+
 /// Look for opportunities to duplicate return instructions to the predecessor
-/// to enable tail call optimizations. The case it is currently looking for is:
+/// to enable tail call optimizations. The case it is currently looking for is
+/// the following one. Known intrinsics or library function that may be tail
+/// called are taken into account as well.
 /// @code
 /// bb0:
 ///   %tmp0 = tail call i32 @f0()
@@ -2580,8 +2612,6 @@ bool CodeGenPrepare::dupRetToEnableTailCallOpts(BasicBlock *BB,
     }
 
     PN = dyn_cast<PHINode>(V);
-    if (!PN)
-      return false;
   }
 
   if (PN && PN->getParent() != BB)
@@ -2620,8 +2650,30 @@ bool CodeGenPrepare::dupRetToEnableTailCallOpts(BasicBlock *BB,
       // Make sure the phi value is indeed produced by the tail call.
       if (CI && CI->hasOneUse() && CI->getParent() == PredBB &&
           TLI->mayBeEmittedAsTailCall(CI) &&
-          attributesPermitTailCall(F, CI, RetI, *TLI))
+          attributesPermitTailCall(F, CI, RetI, *TLI)) {
         TailCallBBs.push_back(PredBB);
+      } else {
+        // Consider the cases in which the phi value is indirectly produced by
+        // the tail call, for example when encountering memset(), memmove(),
+        // strcpy(), whose return value may have been optimized out. In such
+        // cases, the value needs to be the first function argument.
+        //
+        // bb0:
+        //   tail call void @llvm.memset.p0.i64(ptr %0, i8 0, i64 %1)
+        //   br label %return
+        // return:
+        //   %phi = phi ptr [ %0, %bb0 ], [ %2, %entry ]
+        if (PredBB && PredBB->getSingleSuccessor() == BB)
+          CI = dyn_cast_or_null<CallInst>(
+              PredBB->getTerminator()->getPrevNonDebugInstruction(true));
+
+        if (CI && CI->use_empty() &&
+            isIntrinsicOrLFToBeTailCalled(TLInfo, CI) &&
+            IncomingVal == CI->getArgOperand(0) &&
+            TLI->mayBeEmittedAsTailCall(CI) &&
+            attributesPermitTailCall(F, CI, RetI, *TLI))
+          TailCallBBs.push_back(PredBB);
+      }
     }
   } else {
     SmallPtrSet<BasicBlock *, 4> VisitedBBs;
@@ -2631,8 +2683,14 @@ bool CodeGenPrepare::dupRetToEnableTailCallOpts(BasicBlock *BB,
       if (Instruction *I = Pred->rbegin()->getPrevNonDebugInstruction(true)) {
         CallInst *CI = dyn_cast<CallInst>(I);
         if (CI && CI->use_empty() && TLI->mayBeEmittedAsTailCall(CI) &&
-            attributesPermitTailCall(F, CI, RetI, *TLI))
-          TailCallBBs.push_back(Pred);
+            attributesPermitTailCall(F, CI, RetI, *TLI)) {
+          // Either we return void or the return value must be the first
+          // argument of a known intrinsic or library function.
+          if (!V || (isIntrinsicOrLFToBeTailCalled(TLInfo, CI) &&
+                     V == CI->getArgOperand(0))) {
+            TailCallBBs.push_back(Pred);
+          }
+        }
       }
     }
   }