[InstrProf] Add the skipprofile attribute

As discussed in [0], this diff adds the `skipprofile` attribute to
prevent the function from being profiled while allowing profiled
functions to be inlined into it. The `noprofile` attribute remains
unchanged.

The `noprofile` attribute is used for functions where it is
dangerous to add instrumentation to while the `skipprofile` attribute is
used to reduce code size or performance overhead.

[0] https://discourse.llvm.org/t/why-does-the-noprofile-attribute-restrict-inlining/64108

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D130807

clang/lib/CodeGen/CodeGenFunction.h

@@ -1522,7 +1522,8 @@ public:
   /// If \p StepV is null, the default increment is 1.
   void incrementProfileCounter(const Stmt *S, llvm::Value *StepV = nullptr) {
     if (CGM.getCodeGenOpts().hasProfileClangInstr() &&
-        !CurFn->hasFnAttribute(llvm::Attribute::NoProfile))
+        !CurFn->hasFnAttribute(llvm::Attribute::NoProfile) &&
+        !CurFn->hasFnAttribute(llvm::Attribute::SkipProfile))
       PGO.emitCounterIncrement(Builder, S, StepV);
     PGO.setCurrentStmt(S);
   }

clang/lib/CodeGen/CodeGenPGO.cpp

@@ -822,6 +822,8 @@ void CodeGenPGO::assignRegionCounters(GlobalDecl GD, llvm::Function *Fn) {
   CGM.ClearUnusedCoverageMapping(D);
   if (Fn->hasFnAttribute(llvm::Attribute::NoProfile))
     return;
+  if (Fn->hasFnAttribute(llvm::Attribute::SkipProfile))
+    return;
 
   setFuncName(Fn);
 

clang/test/CodeGen/profile-function-groups.c

@@ -1,9 +1,9 @@
-// RUN: %clang -fprofile-generate -fprofile-function-groups=3 -fprofile-selected-function-group=0 -emit-llvm -S %s -o - | FileCheck %s --check-prefixes=CHECK,SELECT0
-// RUN: %clang -fprofile-generate -fprofile-function-groups=3 -fprofile-selected-function-group=1 -emit-llvm -S %s -o - | FileCheck %s --check-prefixes=CHECK,SELECT1
-// RUN: %clang -fprofile-generate -fprofile-function-groups=3 -fprofile-selected-function-group=2 -emit-llvm -S %s -o - | FileCheck %s --check-prefixes=CHECK,SELECT2
+// RUN: %clang -fprofile-generate -fprofile-function-groups=3 -fprofile-selected-function-group=0 -emit-llvm -S %s -o - | FileCheck %s --implicit-check-not="; {{.* (noprofile|skipprofile)}}" --check-prefixes=CHECK,SELECT0
+// RUN: %clang -fprofile-generate -fprofile-function-groups=3 -fprofile-selected-function-group=1 -emit-llvm -S %s -o - | FileCheck %s --implicit-check-not="; {{.* (noprofile|skipprofile)}}" --check-prefixes=CHECK,SELECT1
+// RUN: %clang -fprofile-generate -fprofile-function-groups=3 -fprofile-selected-function-group=2 -emit-llvm -S %s -o - | FileCheck %s --implicit-check-not="; {{.* (noprofile|skipprofile)}}" --check-prefixes=CHECK,SELECT2
 
 // Group 0
-// SELECT0-NOT: noprofile
+
 // SELECT1: noprofile
 // SELECT2: noprofile
 // CHECK: define {{.*}} @hoo()
@@ -11,7 +11,7 @@ void hoo() {}
 
 // Group 1
 // SELECT0: noprofile
-// SELECT1-NOT: noprofile
+
 // SELECT2: noprofile
 // CHECK: define {{.*}} @goo()
 void goo() {}
@@ -19,6 +19,6 @@ void goo() {}
 // Group 2
 // SELECT0: noprofile
 // SELECT1: noprofile
-// SELECT2-NOT: noprofile
+
 // CHECK: define {{.*}} @boo()
 void boo() {}

llvm/docs/LangRef.rst

@@ -1803,8 +1803,14 @@ example:
     startup time if the function is not called during program startup.
 ``noprofile``
     This function attribute prevents instrumentation based profiling, used for
-    coverage or profile based optimization, from being added to a function,
-    even when inlined.
+    coverage or profile based optimization, from being added to a function. It
+    also blocks inlining if the caller and callee have different values of this
+    attribute.
+``skipprofile``
+    This function attribute prevents instrumentation based profiling, used for
+    coverage or profile based optimization, from being added to a function. This
+    attribute does not restrict inlining, so instrumented instruction could end
+    up in this function.
 ``noredzone``
     This attribute indicates that the code generator should not use a
     red zone, even if the target-specific ABI normally permits it.

llvm/include/llvm/Bitcode/LLVMBitCodes.h

@@ -689,6 +689,7 @@ enum AttributeKindCodes {
   ATTR_KIND_ALLOC_KIND = 82,
   ATTR_KIND_PRESPLIT_COROUTINE = 83,
   ATTR_KIND_FNRETTHUNK_EXTERN = 84,
+  ATTR_KIND_SKIP_PROFILE = 85,
 };
 
 enum ComdatSelectionKindCodes {

llvm/include/llvm/IR/Attributes.td

@@ -186,6 +186,10 @@ def NoCfCheck : EnumAttr<"nocf_check", [FnAttr]>;
 /// Function should not be instrumented.
 def NoProfile : EnumAttr<"noprofile", [FnAttr]>;
 
+/// This function should not be instrumented but it is ok to inline profiled
+// functions into it.
+def SkipProfile : EnumAttr<"skipprofile", [FnAttr]>;
+
 /// Function doesn't unwind stack.
 def NoUnwind : EnumAttr<"nounwind", [FnAttr]>;
 

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -1919,6 +1919,8 @@ static Attribute::AttrKind getAttrFromCode(uint64_t Code) {
     return Attribute::NoCfCheck;
   case bitc::ATTR_KIND_NO_PROFILE:
     return Attribute::NoProfile;
+  case bitc::ATTR_KIND_SKIP_PROFILE:
+    return Attribute::SkipProfile;
   case bitc::ATTR_KIND_NO_UNWIND:
     return Attribute::NoUnwind;
   case bitc::ATTR_KIND_NO_SANITIZE_BOUNDS:

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -698,6 +698,8 @@ static uint64_t getAttrKindEncoding(Attribute::AttrKind Kind) {
     return bitc::ATTR_KIND_NOCF_CHECK;
   case Attribute::NoProfile:
     return bitc::ATTR_KIND_NO_PROFILE;
+  case Attribute::SkipProfile:
+    return bitc::ATTR_KIND_SKIP_PROFILE;
   case Attribute::NoUnwind:
     return bitc::ATTR_KIND_NO_UNWIND;
   case Attribute::NoSanitizeBounds:

llvm/lib/Transforms/Instrumentation/GCOVProfiling.cpp

@@ -797,6 +797,8 @@ bool GCOVProfiler::emitProfileNotes(
       if (isUsingScopeBasedEH(F)) continue;
       if (F.hasFnAttribute(llvm::Attribute::NoProfile))
         continue;
+      if (F.hasFnAttribute(llvm::Attribute::SkipProfile))
+        continue;
 
       // Add the function line number to the lines of the entry block
       // to have a counter for the function definition.

llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp

@@ -1574,6 +1574,8 @@ static bool InstrumentAllFunctions(
       continue;
     if (F.hasFnAttribute(llvm::Attribute::NoProfile))
       continue;
+    if (F.hasFnAttribute(llvm::Attribute::SkipProfile))
+      continue;
     auto &TLI = LookupTLI(F);
     auto *BPI = LookupBPI(F);
     auto *BFI = LookupBFI(F);

llvm/lib/Transforms/Utils/CodeExtractor.cpp

@@ -963,6 +963,7 @@ Function *CodeExtractor::constructFunction(const ValueSet &inputs,
       case Attribute::NoCfCheck:
       case Attribute::MustProgress:
       case Attribute::NoProfile:
+      case Attribute::SkipProfile:
         break;
       // These attributes cannot be applied to functions.
       case Attribute::Alignment:

llvm/test/Bitcode/attributes.ll

@@ -535,6 +535,9 @@ define void @f86() nosanitize_bounds
 ; CHECK: define void @f87() [[FNRETTHUNKEXTERN:#[0-9]+]]
 define void @f87() fn_ret_thunk_extern { ret void }
 
+; CHECK: define void @f88() [[SKIPPROFILE:#[0-9]+]]
+define void @f88() skipprofile { ret void }
+
 ; CHECK: attributes #0 = { noreturn }
 ; CHECK: attributes #1 = { nounwind }
 ; CHECK: attributes #2 = { readnone }
@@ -589,4 +592,5 @@ define void @f87() fn_ret_thunk_extern { ret void }
 ; CHECK: attributes #51 = { uwtable(sync) }
 ; CHECK: attributes #52 = { nosanitize_bounds }
 ; CHECK: attributes [[FNRETTHUNKEXTERN]] = { fn_ret_thunk_extern }
+; CHECK: attributes [[SKIPPROFILE]] = { skipprofile }
 ; CHECK: attributes #[[NOBUILTIN]] = { nobuiltin }

llvm/test/Transforms/GCOVProfiling/noprofile.ll

@@ -10,6 +10,14 @@ define dso_local i32 @no_instr(i32 %a) noprofile !dbg !9 {
   ret i32 42, !dbg !27
 }
 
+; Test that the skipprofile attribute disables profiling.
+define dso_local i32 @skip_instr(i32 %a) skipprofile {
+; CHECK-LABEL: @skip_instr(
+; CHECK-NEXT:    ret i32 52
+;
+  ret i32 52
+}
+
 define dso_local i32 @instr(i32 %a) !dbg !28 {
 ; CHECK-LABEL: @instr(
 ; CHECK-NEXT:    [[GCOV_CTR:%.*]] = load i64, ptr @__llvm_gcov_ctr, align 4, !dbg [[DBG8:![0-9]+]]

llvm/test/Transforms/PGOProfile/noprofile.ll

@@ -21,4 +21,10 @@ entry:
   ret i32 %sub
 }
 
+define i32 @test3() skipprofile {
+entry:
+; CHECK-NOT: call void @llvm.instrprof.increment
+  ret i32 101
+}
+
 attributes #0 = { noprofile }