[MemoryBuiltin] Add an API for checking if an unused allocation can be removed [NFC]

Not all allocation functions are removable if unused.  An example of a non-removable allocation would be a direct call to the replaceable global allocation function in C++.  An example of a removable one - at least according to historical practice - would be malloc.

4 files changed, 16 insertions, 2 deletions

diff --git a/llvm/lib/Analysis/MemoryBuiltins.cpp b/llvm/lib/Analysis/MemoryBuiltins.cpp
index 213fdc5a..48c01fb 100644
--- a/llvm/lib/Analysis/MemoryBuiltins.cpp
+++ b/llvm/lib/Analysis/MemoryBuiltins.cpp
@@ -298,6 +298,17 @@ bool llvm::isStrdupLikeFn(const Value *V, const TargetLibraryInfo *TLI) {
   return getAllocationData(V, StrDupLike, TLI).hasValue();
 }
 
+bool llvm::isAllocRemovable(const CallBase *CB, const TargetLibraryInfo *TLI) {
+  assert(isAllocationFn(CB, TLI));
+
+  // Note: Removability is highly dependent on the source language.  For
+  // example, recent C++ requires direct calls to the global allocation
+  // [basic.stc.dynamic.allocation] to be observable unless part of a new
+  // expression [expr.new paragraph 13].
+
+  // Historically we've treated the C family allocation routines as removable
+  return isAllocLikeFn(CB, TLI);
+}
 
 Value *llvm::getAllocAlignment(const CallBase *V,
                                const TargetLibraryInfo *TLI) {
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 7904242..22c736f 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -2761,7 +2761,8 @@ Instruction *InstCombinerImpl::visitCallBase(CallBase &Call) {
             Call, Builder.CreateBitOrPointerCast(ReturnedArg, CallTy));
     }
 
-  if (isAllocLikeFn(&Call, &TLI))
+  if (isAllocationFn(&Call, &TLI) &&
+      isAllocRemovable(&cast<CallBase>(Call), &TLI))
     return visitAllocSite(Call);
 
   // Handle intrinsics which can be used in both call and invoke context.
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 31329ae..6d5548d 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2696,6 +2696,8 @@ static bool isAllocSiteRemovable(Instruction *AI,
 }
 
 Instruction *InstCombinerImpl::visitAllocSite(Instruction &MI) {
+  assert(isa<AllocaInst>(MI) || isAllocRemovable(&cast<CallBase>(MI), &TLI));
+
   // If we have a malloc call which is only used in any amount of comparisons to
   // null and free calls, delete the calls and replace the comparisons with true
   // or false as appropriate.
diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index efc5396..3ea82e1 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -492,7 +492,7 @@ bool llvm::wouldInstructionBeTriviallyDead(Instruction *I,
     }
   }
 
-  if (isAllocLikeFn(I, TLI))
+  if (isAllocationFn(I, TLI) && isAllocRemovable(cast<CallBase>(I), TLI))
     return true;
 
   if (CallInst *CI = isFreeCall(I, TLI))