Add nomerge function attribute to supress tail merge optimization in simplifyCFG

We want to add a way to avoid merging identical calls so as to keep the
separate debug-information for those calls. There is also an asan
usecase where having this attribute would be beneficial to avoid
alternative work-arounds.

Here is the link to the feature request:
https://bugs.llvm.org/show_bug.cgi?id=42783.

`nomerge` is different from `noline`. `noinline` prevents function from
inlining at callsites, but `nomerge` prevents multiple identical calls
from being merged into one.

This patch adds `nomerge` to disable the optimization in IR level. A
followup patch will be needed to let backend understands `nomerge` and
avoid tail merge at backend.

Reviewed By: asbirlea, rnk

Differential Revision: https://reviews.llvm.org/D78659

docs/LangRef.rst

@@ -1528,6 +1528,14 @@ example:
     This attribute indicates that the inliner should never inline this
     function in any situation. This attribute may not be used together
     with the ``alwaysinline`` attribute.
+``nomerge``
+    This attribute indicates that calls to this function should never be merged
+    during optimization. For example, it will prevent tail merging otherwise
+    identical code sequences that raise an exception or terminate the program.
+    Tail merging normally reduces the precision of source location information,
+    making stack traces less useful for debugging. This attribute gives the
+    user control over the tradeoff between code size and debug information
+    precision.
 ``nonlazybind``
     This attribute suppresses lazy symbol binding for the function. This
     may make calls to the function faster, at the cost of extra program

include/llvm/Bitcode/LLVMBitCodes.h

@@ -634,6 +634,7 @@ enum AttributeKindCodes {
   ATTR_KIND_NOSYNC = 63,
   ATTR_KIND_SANITIZE_MEMTAG = 64,
   ATTR_KIND_PREALLOCATED = 65,
+  ATTR_KIND_NO_MERGE = 66,
 };
 
 enum ComdatSelectionKindCodes {

include/llvm/IR/Attributes.td

@@ -103,6 +103,9 @@ def NoInline : EnumAttr<"noinline">;
 /// Function is called early and/or often, so lazy binding isn't worthwhile.
 def NonLazyBind : EnumAttr<"nonlazybind">;
 
+/// Disable merging for call sites
+def NoMerge : EnumAttr<"nomerge">;
+
 /// Pointer is known to be not null.
 def NonNull : EnumAttr<"nonnull">;
 

include/llvm/IR/InstrTypes.h

@@ -1715,6 +1715,9 @@ public:
     addAttribute(AttributeList::FunctionIndex, Attribute::NoDuplicate);
   }
 
+  /// Determine if the call 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() {

lib/AsmParser/LLLexer.cpp

@@ -658,6 +658,7 @@ lltok::Kind LLLexer::LexIdentifier() {
   KEYWORD(noinline);
   KEYWORD(norecurse);
   KEYWORD(nonlazybind);
+  KEYWORD(nomerge);
   KEYWORD(nonnull);
   KEYWORD(noredzone);
   KEYWORD(noreturn);

lib/AsmParser/LLParser.cpp

@@ -1306,6 +1306,7 @@ bool LLParser::ParseFnAttributeValuePairs(AttrBuilder &B,
       B.addAttribute(Attribute::NoImplicitFloat); break;
     case lltok::kw_noinline: B.addAttribute(Attribute::NoInline); break;
     case lltok::kw_nonlazybind: B.addAttribute(Attribute::NonLazyBind); break;
+    case lltok::kw_nomerge: B.addAttribute(Attribute::NoMerge); break;
     case lltok::kw_noredzone: B.addAttribute(Attribute::NoRedZone); break;
     case lltok::kw_noreturn: B.addAttribute(Attribute::NoReturn); break;
     case lltok::kw_nosync: B.addAttribute(Attribute::NoSync); break;
@@ -1698,6 +1699,7 @@ bool LLParser::ParseOptionalParamAttrs(AttrBuilder &B) {
     case lltok::kw_noimplicitfloat:
     case lltok::kw_noinline:
     case lltok::kw_nonlazybind:
+    case lltok::kw_nomerge:
     case lltok::kw_noredzone:
     case lltok::kw_noreturn:
     case lltok::kw_nocf_check:
@@ -1797,6 +1799,7 @@ bool LLParser::ParseOptionalReturnAttrs(AttrBuilder &B) {
     case lltok::kw_noimplicitfloat:
     case lltok::kw_noinline:
     case lltok::kw_nonlazybind:
+    case lltok::kw_nomerge:
     case lltok::kw_noredzone:
     case lltok::kw_noreturn:
     case lltok::kw_nocf_check:

lib/AsmParser/LLToken.h

@@ -204,6 +204,7 @@ enum Kind {
   kw_noinline,
   kw_norecurse,
   kw_nonlazybind,
+  kw_nomerge,
   kw_nonnull,
   kw_noredzone,
   kw_noreturn,

lib/Bitcode/Reader/BitcodeReader.cpp

@@ -1442,6 +1442,8 @@ static Attribute::AttrKind getAttrFromCode(uint64_t Code) {
     return Attribute::NoInline;
   case bitc::ATTR_KIND_NO_RECURSE:
     return Attribute::NoRecurse;
+  case bitc::ATTR_KIND_NO_MERGE:
+    return Attribute::NoMerge;
   case bitc::ATTR_KIND_NON_LAZY_BIND:
     return Attribute::NonLazyBind;
   case bitc::ATTR_KIND_NON_NULL:

lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -647,6 +647,8 @@ static uint64_t getAttrKindEncoding(Attribute::AttrKind Kind) {
     return bitc::ATTR_KIND_NO_INLINE;
   case Attribute::NoRecurse:
     return bitc::ATTR_KIND_NO_RECURSE;
+  case Attribute::NoMerge:
+    return bitc::ATTR_KIND_NO_MERGE;
   case Attribute::NonLazyBind:
     return bitc::ATTR_KIND_NON_LAZY_BIND;
   case Attribute::NonNull:

lib/IR/Attributes.cpp

@@ -375,6 +375,8 @@ std::string Attribute::getAsString(bool InAttrGrp) const {
     return "noinline";
   if (hasAttribute(Attribute::NonLazyBind))
     return "nonlazybind";
+  if (hasAttribute(Attribute::NoMerge))
+    return "nomerge";
   if (hasAttribute(Attribute::NonNull))
     return "nonnull";
   if (hasAttribute(Attribute::NoRedZone))

lib/IR/Verifier.cpp

@@ -1514,6 +1514,7 @@ void Verifier::visitModuleFlagCGProfileEntry(const MDOperand &MDO) {
 /// 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:

lib/Transforms/Utils/CodeExtractor.cpp

@@ -874,6 +874,7 @@ Function *CodeExtractor::constructFunction(const ValueSet &inputs,
       case Attribute::NoAlias:
       case Attribute::NoBuiltin:
       case Attribute::NoCapture:
+      case Attribute::NoMerge:
       case Attribute::NoReturn:
       case Attribute::NoSync:
       case Attribute::None:

lib/Transforms/Utils/SimplifyCFG.cpp

@@ -1300,6 +1300,14 @@ bool SimplifyCFGOpt::HoistThenElseCodeToIf(BranchInst *BI,
     if (!TTI.isProfitableToHoist(I1) || !TTI.isProfitableToHoist(I2))
       return Changed;
 
+    // If any of the two call sites has nomerge attribute, stop hoisting.
+    if (const auto *CB1 = dyn_cast<CallBase>(I1))
+      if (CB1->cannotMerge())
+        return Changed;
+    if (const auto *CB2 = dyn_cast<CallBase>(I2))
+      if (CB2->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
@@ -1485,8 +1493,9 @@ static bool canSinkInstructions(
     // Conservatively return false if I is an inline-asm instruction. Sinking
     // and merging inline-asm instructions can potentially create arguments
     // that cannot satisfy the inline-asm constraints.
+    // If the instruction has nomerge attribute, return false.
     if (const auto *C = dyn_cast<CallBase>(I))
-      if (C->isInlineAsm())
+      if (C->isInlineAsm() || C->cannotMerge())
         return false;
 
     // Each instruction must have zero or one use.

test/Transforms/SimplifyCFG/nomerge.ll

@@ -0,0 +1,71 @@
+; RUN: opt < %s -O1 -S | FileCheck %s
+
+; The attribute nomerge prevents the 3 bar() calls from being sunk/hoisted into 
+; one inside a function. Check that there are still 3 tail calls.
+
+; Test case for preventing sinking
+; CHECK-LABEL: define void @sink
+; CHECK: if.then:
+; CHECK-NEXT: tail call void @bar()
+; CHECK: if.then2:
+; CHECK-NEXT: tail call void @bar()
+; CHECK: if.end3:
+; CHECK-NEXT: tail call void @bar()
+define void @sink(i32 %i) {
+entry:
+  switch i32 %i, label %if.end3 [
+    i32 5, label %if.then
+    i32 7, label %if.then2
+  ]
+
+if.then:
+  tail call void @bar() #0
+  br label %if.end3
+
+if.then2:
+  tail call void @bar() #0
+  br label %if.end3
+
+if.end3:
+  tail call void @bar() #0
+  ret void
+}
+
+; Test case for preventing hoisting
+; CHECK-LABEL: define void @hoist
+; CHECK: if.then:
+; CHECK-NEXT: tail call void @bar()
+; CHECK: if.then2:
+; CHECK-NEXT: tail call void @bar()
+; CHECK: if.end:
+; CHECK-NEXT: tail call void @bar()
+define void @hoist(i32 %i) {
+entry:
+  %i.addr = alloca i32, align 4
+  store i32 %i, i32* %i.addr, align 4
+  %0 = load i32, i32* %i.addr, align 4
+  %cmp = icmp eq i32 %0, 5
+  br i1 %cmp, label %if.then, label %if.else
+
+if.then:
+  tail call void @bar() #1
+  unreachable
+
+if.else:
+  %1 = load i32, i32* %i.addr, align 4
+  %cmp1 = icmp eq i32 %i, 7
+  br i1 %cmp1, label %if.then2, label %if.end
+
+if.then2:
+  tail call void @bar() #1
+  unreachable
+
+if.end:
+  tail call void @bar() #1
+  unreachable
+}
+
+declare void @bar()
+
+attributes #0 = { nomerge }
+attributes #1 = { noreturn nomerge }