[PATCH] D78659: Add nomerge function attribute to supress tail merge optimization in simplifyCFG

[Zequan Wu via Phabricator via cfe-commits]

zequanwu created this revision.
zequanwu added reviewers: asbirlea, rnk.
zequanwu added projects: LLVM, clang.
Herald added subscribers: cfe-commits, hiraditya.

We want to add a way to avoid tail calls to noreturn function from being 
merged. 
This patch add nomerge to disable the optimization in IR level.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D78659

Files:
  clang/include/clang/Basic/Attr.td
  clang/include/clang/Basic/AttrDocs.td
  clang/lib/CodeGen/CGCall.cpp
  llvm/include/llvm/IR/Attributes.td
  llvm/include/llvm/IR/InstrTypes.h
  llvm/lib/IR/Attributes.cpp
  llvm/lib/IR/Verifier.cpp
  llvm/lib/Transforms/Utils/SimplifyCFG.cpp


Index: llvm/lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1287,6 +1287,10 @@
     if (!TTI.isProfitableToHoist(I1) || !TTI.isProfitableToHoist(I2))
       return Changed;
 
+    // If any of the two call sites has nomerge attribute, stop hoisting.
+    if (C1->cannotMerge() || C2->cannotMerge())
+      return Changed;
+
     if (isa<DbgInfoIntrinsic>(I1) || isa<DbgInfoIntrinsic>(I2)) {
       assert (isa<DbgInfoIntrinsic>(I1) && isa<DbgInfoIntrinsic>(I2));
       // The debug location is an integral part of a debug info intrinsic
Index: llvm/lib/IR/Verifier.cpp
===================================================================
--- llvm/lib/IR/Verifier.cpp
+++ llvm/lib/IR/Verifier.cpp
@@ -1509,6 +1509,7 @@
 /// Return true if this attribute kind only applies to functions.
 static bool isFuncOnlyAttr(Attribute::AttrKind Kind) {
   switch (Kind) {
+  case Attribute::NoMerge:
   case Attribute::NoReturn:
   case Attribute::NoSync:
   case Attribute::WillReturn:
Index: llvm/lib/IR/Attributes.cpp
===================================================================
--- llvm/lib/IR/Attributes.cpp
+++ llvm/lib/IR/Attributes.cpp
@@ -368,6 +368,8 @@
     return "noinline";
   if (hasAttribute(Attribute::NonLazyBind))
     return "nonlazybind";
+  if (hasAttribute(Attribute::NoMerge))
+    return "nomerge";
   if (hasAttribute(Attribute::NonNull))
     return "nonnull";
   if (hasAttribute(Attribute::NoRedZone))
Index: llvm/include/llvm/IR/InstrTypes.h
===================================================================
--- llvm/include/llvm/IR/InstrTypes.h
+++ llvm/include/llvm/IR/InstrTypes.h
@@ -1717,6 +1717,9 @@
     addAttribute(AttributeList::FunctionIndex, Attribute::NoDuplicate);
   }
 
+  /// Determine if the invoke cannot be tail merged.
+  bool cannotMerge() const { return hasFnAttr(Attribute::NoMerge); }
+
   /// Determine if the invoke is convergent
   bool isConvergent() const { return hasFnAttr(Attribute::Convergent); }
   void setConvergent() {
Index: llvm/include/llvm/IR/Attributes.td
===================================================================
--- llvm/include/llvm/IR/Attributes.td
+++ llvm/include/llvm/IR/Attributes.td
@@ -97,6 +97,9 @@
 /// Function is called early and/or often, so lazy binding isn't worthwhile.
 def NonLazyBind : EnumAttr<"nonlazybind">;
 
+/// Disable tail merge for this function
+def NoMerge : EnumAttr<"nomerge">;
+
 /// Pointer is known to be not null.
 def NonNull : EnumAttr<"nonnull">;
 
Index: clang/lib/CodeGen/CGCall.cpp
===================================================================
--- clang/lib/CodeGen/CGCall.cpp
+++ clang/lib/CodeGen/CGCall.cpp
@@ -1905,6 +1905,8 @@
       FuncAttrs.addAttribute(llvm::Attribute::NoDuplicate);
     if (TargetDecl->hasAttr<ConvergentAttr>())
       FuncAttrs.addAttribute(llvm::Attribute::Convergent);
+    if (TargetDecl->hasAttr<NoMergeAttr>())
+      FuncAttrs.addAttribute(llvm::Attribute::NoMerge);
 
     if (const FunctionDecl *Fn = dyn_cast<FunctionDecl>(TargetDecl)) {
       AddAttributesFromFunctionProtoType(
Index: clang/include/clang/Basic/AttrDocs.td
===================================================================
--- clang/include/clang/Basic/AttrDocs.td
+++ clang/include/clang/Basic/AttrDocs.td
@@ -350,6 +350,14 @@
   }];
 }
 
+def NoMergeDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+A function declared as ``nomerge`` shall not be tail merged at call sites during
+optimization phase.
+  }];
+}
+
 def AssertCapabilityDocs : Documentation {
   let Category = DocCatFunction;
   let Heading = "assert_capability, assert_shared_capability";
Index: clang/include/clang/Basic/Attr.td
===================================================================
--- clang/include/clang/Basic/Attr.td
+++ clang/include/clang/Basic/Attr.td
@@ -1793,6 +1793,13 @@
   let Documentation = [Undocumented];
 }
 
+def NoMerge : InheritableAttr {
+  let Spellings = [Clang<"nomerge">];
+  let Subjects = SubjectList<[Function]>;
+  let Documentation = [NoMergeDocs];
+  let SimpleHandler = 1;
+}
+
 def NoInstrumentFunction : InheritableAttr {
   let Spellings = [GCC<"no_instrument_function">];
   let Subjects = SubjectList<[Function]>;

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