melver updated this revision to Diff 351229.
melver added a comment.
As promised, some cleanups, docs, and updated test for the current version (no
other major changes yet).
While the identical-writes test is quite contrived, the currently failing
switch test is a more realistic example. The example uses AArch64, where the
optimizer does not emit a branch but instead uses cinc, which would break the
requirement of emitting a real branch.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D103958/new/
https://reviews.llvm.org/D103958
Files:
clang/include/clang/Basic/Attr.td
clang/include/clang/Basic/AttrDocs.td
clang/lib/CodeGen/CGStmt.cpp
clang/lib/CodeGen/CodeGenFunction.cpp
clang/lib/CodeGen/CodeGenFunction.h
clang/lib/Sema/SemaStmtAttr.cpp
clang/test/CodeGenCXX/attr-mustcontrol.cpp
llvm/include/llvm/IR/FixedMetadataKinds.def
llvm/include/llvm/IR/IRBuilder.h
llvm/include/llvm/IR/Intrinsics.td
llvm/include/llvm/IR/MDBuilder.h
llvm/lib/CodeGen/BranchFolding.cpp
llvm/lib/IR/IRBuilder.cpp
llvm/lib/IR/MDBuilder.cpp
Index: llvm/lib/IR/MDBuilder.cpp
===================================================================
--- llvm/lib/IR/MDBuilder.cpp
+++ llvm/lib/IR/MDBuilder.cpp
@@ -56,6 +56,8 @@
return MDNode::get(Context, None);
}
+MDNode *MDBuilder::createMustControl() { return MDNode::get(Context, None); }
+
MDNode *MDBuilder::createFunctionEntryCount(
uint64_t Count, bool Synthetic,
const DenseSet<GlobalValue::GUID> *Imports) {
Index: llvm/lib/IR/IRBuilder.cpp
===================================================================
--- llvm/lib/IR/IRBuilder.cpp
+++ llvm/lib/IR/IRBuilder.cpp
@@ -960,7 +960,7 @@
if (MDFrom) {
MDNode *Prof = MDFrom->getMetadata(LLVMContext::MD_prof);
MDNode *Unpred = MDFrom->getMetadata(LLVMContext::MD_unpredictable);
- Sel = addBranchMetadata(Sel, Prof, Unpred);
+ Sel = addBranchMetadata(Sel, Prof, Unpred, nullptr /*TODO*/);
}
if (isa<FPMathOperator>(Sel))
setFPAttrs(Sel, nullptr /* MDNode* */, FMF);
Index: llvm/lib/CodeGen/BranchFolding.cpp
===================================================================
--- llvm/lib/CodeGen/BranchFolding.cpp
+++ llvm/lib/CodeGen/BranchFolding.cpp
@@ -1217,7 +1217,20 @@
// Blocks should be considered empty if they contain only debug info;
// else the debug info would affect codegen.
static bool IsEmptyBlock(MachineBasicBlock *MBB) {
- return MBB->getFirstNonDebugInstr(true) == MBB->end();
+ if (MBB->getFirstNonDebugInstr(true) != MBB->end())
+ return false;
+
+ // Even though this block is empty, check if we should preserve it.
+ // XXX: Implementation subject to change.
+ if (const auto *BB = MBB->getBasicBlock()) {
+ for (const BasicBlock *PredBB : predecessors(BB)) {
+ const auto *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator());
+ if (PredBr && PredBr->getMetadata(LLVMContext::MD_mustcontrol))
+ return false;
+ }
+ }
+
+ return true;
}
// Blocks with only debug info and branches should be considered the same
Index: llvm/include/llvm/IR/MDBuilder.h
===================================================================
--- llvm/include/llvm/IR/MDBuilder.h
+++ llvm/include/llvm/IR/MDBuilder.h
@@ -66,6 +66,9 @@
/// Return metadata specifying that a branch or switch is unpredictable.
MDNode *createUnpredictable();
+ /// Return metadata specifying that a branch or switch must not be removed.
+ MDNode *createMustControl();
+
/// Return metadata containing the entry \p Count for a function, a boolean
/// \Synthetic indicating whether the counts were synthetized, and the
/// GUIDs stored in \p Imports that need to be imported for sample PGO, to
Index: llvm/include/llvm/IR/Intrinsics.td
===================================================================
--- llvm/include/llvm/IR/Intrinsics.td
+++ llvm/include/llvm/IR/Intrinsics.td
@@ -1326,7 +1326,8 @@
// has having opaque side effects. This may be inserted into loops to ensure
// that they are not removed even if they turn out to be empty, for languages
// which specify that infinite loops must be preserved.
-def int_sideeffect : DefaultAttrsIntrinsic<[], [], [IntrInaccessibleMemOnly, IntrWillReturn]>;
+def int_sideeffect : DefaultAttrsIntrinsic<[], [llvm_vararg_ty],
+ [IntrInaccessibleMemOnly, IntrWillReturn]>;
// The pseudoprobe intrinsic works as a place holder to the block it probes.
// Like the sideeffect intrinsic defined above, this intrinsic is treated by the
Index: llvm/include/llvm/IR/IRBuilder.h
===================================================================
--- llvm/include/llvm/IR/IRBuilder.h
+++ llvm/include/llvm/IR/IRBuilder.h
@@ -934,15 +934,18 @@
//===--------------------------------------------------------------------===//
private:
- /// Helper to add branch weight and unpredictable metadata onto an
- /// instruction.
+ /// Helper to add branch weight, unpredictable, and mustcontrol metadata onto
+ /// an instruction.
/// \returns The annotated instruction.
template <typename InstTy>
- InstTy *addBranchMetadata(InstTy *I, MDNode *Weights, MDNode *Unpredictable) {
+ InstTy *addBranchMetadata(InstTy *I, MDNode *Weights, MDNode *Unpredictable,
+ MDNode *MustControl) {
if (Weights)
I->setMetadata(LLVMContext::MD_prof, Weights);
if (Unpredictable)
I->setMetadata(LLVMContext::MD_unpredictable, Unpredictable);
+ if (MustControl)
+ I->setMetadata(LLVMContext::MD_mustcontrol, MustControl);
return I;
}
@@ -980,9 +983,10 @@
/// instruction.
BranchInst *CreateCondBr(Value *Cond, BasicBlock *True, BasicBlock *False,
MDNode *BranchWeights = nullptr,
- MDNode *Unpredictable = nullptr) {
+ MDNode *Unpredictable = nullptr,
+ MDNode *MustControl = nullptr) {
return Insert(addBranchMetadata(BranchInst::Create(True, False, Cond),
- BranchWeights, Unpredictable));
+ BranchWeights, Unpredictable, MustControl));
}
/// Create a conditional 'br Cond, TrueDest, FalseDest'
@@ -991,9 +995,10 @@
Instruction *MDSrc) {
BranchInst *Br = BranchInst::Create(True, False, Cond);
if (MDSrc) {
- unsigned WL[4] = {LLVMContext::MD_prof, LLVMContext::MD_unpredictable,
- LLVMContext::MD_make_implicit, LLVMContext::MD_dbg};
- Br->copyMetadata(*MDSrc, makeArrayRef(&WL[0], 4));
+ unsigned WL[5] = {LLVMContext::MD_prof, LLVMContext::MD_unpredictable,
+ LLVMContext::MD_make_implicit, LLVMContext::MD_dbg,
+ LLVMContext::MD_mustcontrol};
+ Br->copyMetadata(*MDSrc, makeArrayRef(&WL[0], 5));
}
return Insert(Br);
}
@@ -1003,9 +1008,10 @@
/// allocation).
SwitchInst *CreateSwitch(Value *V, BasicBlock *Dest, unsigned NumCases = 10,
MDNode *BranchWeights = nullptr,
- MDNode *Unpredictable = nullptr) {
+ MDNode *Unpredictable = nullptr,
+ MDNode *MustControl = nullptr) {
return Insert(addBranchMetadata(SwitchInst::Create(V, Dest, NumCases),
- BranchWeights, Unpredictable));
+ BranchWeights, Unpredictable, MustControl));
}
/// Create an indirect branch instruction with the specified address
Index: llvm/include/llvm/IR/FixedMetadataKinds.def
===================================================================
--- llvm/include/llvm/IR/FixedMetadataKinds.def
+++ llvm/include/llvm/IR/FixedMetadataKinds.def
@@ -42,3 +42,4 @@
LLVM_FIXED_MD_KIND(MD_vcall_visibility, "vcall_visibility", 28)
LLVM_FIXED_MD_KIND(MD_noundef, "noundef", 29)
LLVM_FIXED_MD_KIND(MD_annotation, "annotation", 30)
+LLVM_FIXED_MD_KIND(MD_mustcontrol, "mustcontrol", 31)
Index: clang/test/CodeGenCXX/attr-mustcontrol.cpp
===================================================================
--- /dev/null
+++ clang/test/CodeGenCXX/attr-mustcontrol.cpp
@@ -0,0 +1,54 @@
+// RUN: %clang_cc1 -O2 -S -emit-llvm %s -triple x86_64-unknown-linux-gnu -o - | FileCheck %s
+// RUN: %clang_cc1 -O2 -S -emit-llvm %s -triple x86_64-unknown-linux-gnu -o - | opt -verify
+//
+// TODO: move to .ll test
+// RUN: %clang_cc1 -O2 -S %s -triple aarch64-unknown-linux-gnu -o - | FileCheck %s -check-prefix=ARM
+
+#if __has_attribute(mustcontrol)
+#define mustcontrol [[clang::mustcontrol]]
+#else
+#error "mustcontrol not supported!"
+#endif
+
+volatile int x, y;
+int z;
+
+// CHECK-LABEL: define{{.*}}IfThenIdenticalWrites
+void IfThenIdenticalWrites() {
+ z = 42;
+
+ // CHECK: br{{.*}}mustcontrol
+ // ARM: cbz
+ mustcontrol if (x) {
+ y = z;
+ } else {
+ y = z;
+ }
+}
+
+// CHECK-LABEL: define{{.*}}SupportWhile
+void SupportWhile() {
+ mustcontrol while (x) { y = 1; }
+}
+
+// CHECK-LABEL: define{{.*}}SupportFor
+void SupportFor() {
+ mustcontrol for (; x; y++) { }
+}
+
+// FIXME: This test currently fails because the optimizer turns the switch into
+// a cinc!!!
+//
+// CHECK-LABEL: define{{.*}}SupportSwitch
+// ARM-LABEL: _Z13SupportSwitchv:
+void SupportSwitch() {
+ // ARM-NOT: cinc
+ mustcontrol switch (x) {
+ case 0:
+ y = 1;
+ break;
+ default:
+ y = 2;
+ break;
+ }
+}
Index: clang/lib/Sema/SemaStmtAttr.cpp
===================================================================
--- clang/lib/Sema/SemaStmtAttr.cpp
+++ clang/lib/Sema/SemaStmtAttr.cpp
@@ -233,6 +233,11 @@
return ::new (S.Context) UnlikelyAttr(S.Context, A);
}
+static Attr *handleMustControl(Sema &S, Stmt *St, const ParsedAttr &A,
+ SourceRange Range) {
+ return ::new (S.Context) MustControlAttr(S.Context, A);
+}
+
#define WANT_STMT_MERGE_LOGIC
#include "clang/Sema/AttrParsedAttrImpl.inc"
#undef WANT_STMT_MERGE_LOGIC
@@ -424,6 +429,8 @@
return handleLikely(S, St, A, Range);
case ParsedAttr::AT_Unlikely:
return handleUnlikely(S, St, A, Range);
+ case ParsedAttr::AT_MustControl:
+ return handleMustControl(S, St, A, Range);
default:
// N.B., ClangAttrEmitter.cpp emits a diagnostic helper that ensures a
// declaration attribute is not written on a statement, but this code is
Index: clang/lib/CodeGen/CodeGenFunction.h
===================================================================
--- clang/lib/CodeGen/CodeGenFunction.h
+++ clang/lib/CodeGen/CodeGenFunction.h
@@ -524,6 +524,10 @@
// applies to. nullptr if there is no 'musttail' on the current statement.
const CallExpr *MustTailCall = nullptr;
+ // The Stmt within the current statement that the mustcontrol attribute
+ // applies to. nullptr if there is no 'mustcontrol' on the current statement.
+ const Stmt *MustControlStmt = nullptr;
+
/// Returns true if a function must make progress, which means the
/// mustprogress attribute can be added.
bool checkIfFunctionMustProgress() {
@@ -4488,7 +4492,8 @@
/// evaluate to true based on PGO data.
void EmitBranchOnBoolExpr(const Expr *Cond, llvm::BasicBlock *TrueBlock,
llvm::BasicBlock *FalseBlock, uint64_t TrueCount,
- Stmt::Likelihood LH = Stmt::LH_None);
+ Stmt::Likelihood LH = Stmt::LH_None,
+ bool MustControl = false);
/// Given an assignment `*LHS = RHS`, emit a test that checks if \p RHS is
/// nonnull, if \p LHS is marked _Nonnull.
Index: clang/lib/CodeGen/CodeGenFunction.cpp
===================================================================
--- clang/lib/CodeGen/CodeGenFunction.cpp
+++ clang/lib/CodeGen/CodeGenFunction.cpp
@@ -1606,11 +1606,9 @@
/// statement) to the specified blocks. Based on the condition, this might try
/// to simplify the codegen of the conditional based on the branch.
/// \param LH The value of the likelihood attribute on the True branch.
-void CodeGenFunction::EmitBranchOnBoolExpr(const Expr *Cond,
- llvm::BasicBlock *TrueBlock,
- llvm::BasicBlock *FalseBlock,
- uint64_t TrueCount,
- Stmt::Likelihood LH) {
+void CodeGenFunction::EmitBranchOnBoolExpr(
+ const Expr *Cond, llvm::BasicBlock *TrueBlock, llvm::BasicBlock *FalseBlock,
+ uint64_t TrueCount, Stmt::Likelihood LH, bool MustControl) {
Cond = Cond->IgnoreParens();
if (const BinaryOperator *CondBOp = dyn_cast<BinaryOperator>(Cond)) {
@@ -1795,8 +1793,10 @@
CondV = EvaluateExprAsBool(Cond);
}
+ llvm::MDBuilder MDHelper(getLLVMContext());
llvm::MDNode *Weights = nullptr;
llvm::MDNode *Unpredictable = nullptr;
+ llvm::MDNode *MustControlMD = nullptr;
// If the branch has a condition wrapped by __builtin_unpredictable,
// create metadata that specifies that the branch is unpredictable.
@@ -1804,10 +1804,8 @@
auto *Call = dyn_cast<CallExpr>(Cond->IgnoreImpCasts());
if (Call && CGM.getCodeGenOpts().OptimizationLevel != 0) {
auto *FD = dyn_cast_or_null<FunctionDecl>(Call->getCalleeDecl());
- if (FD && FD->getBuiltinID() == Builtin::BI__builtin_unpredictable) {
- llvm::MDBuilder MDHelper(getLLVMContext());
+ if (FD && FD->getBuiltinID() == Builtin::BI__builtin_unpredictable)
Unpredictable = MDHelper.createUnpredictable();
- }
}
// If there is a Likelihood knowledge for the cond, lower it.
@@ -1821,7 +1819,11 @@
Weights = createProfileWeights(TrueCount, CurrentCount - TrueCount);
}
- Builder.CreateCondBr(CondV, TrueBlock, FalseBlock, Weights, Unpredictable);
+ if (MustControl)
+ MustControlMD = MDHelper.createMustControl();
+
+ Builder.CreateCondBr(CondV, TrueBlock, FalseBlock, Weights, Unpredictable,
+ MustControlMD);
}
/// ErrorUnsupported - Print out an error that codegen doesn't support the
Index: clang/lib/CodeGen/CGStmt.cpp
===================================================================
--- clang/lib/CodeGen/CGStmt.cpp
+++ clang/lib/CodeGen/CGStmt.cpp
@@ -657,6 +657,7 @@
void CodeGenFunction::EmitAttributedStmt(const AttributedStmt &S) {
bool nomerge = false;
const CallExpr *musttail = nullptr;
+ const Stmt *mustcontrol = nullptr;
for (const auto *A : S.getAttrs()) {
if (A->getKind() == attr::NoMerge) {
@@ -667,9 +668,12 @@
const ReturnStmt *R = cast<ReturnStmt>(Sub);
musttail = cast<CallExpr>(R->getRetValue()->IgnoreParens());
}
+ if (A->getKind() == attr::MustControl)
+ mustcontrol = S.getSubStmt();
}
SaveAndRestore<bool> save_nomerge(InNoMergeAttributedStmt, nomerge);
SaveAndRestore<const CallExpr *> save_musttail(MustTailCall, musttail);
+ SaveAndRestore<const Stmt *> save_mustcontrol(MustControlStmt, mustcontrol);
EmitStmt(S.getSubStmt(), S.getAttrs());
}
@@ -755,10 +759,21 @@
uint64_t Count = getProfileCount(S.getThen());
if (!Count && CGM.getCodeGenOpts().OptimizationLevel)
LH = Stmt::getLikelihood(S.getThen(), S.getElse());
- EmitBranchOnBoolExpr(S.getCond(), ThenBlock, ElseBlock, Count, LH);
+ EmitBranchOnBoolExpr(S.getCond(), ThenBlock, ElseBlock, Count, LH,
+ &S == MustControlStmt);
// Emit the 'then' code.
EmitBlock(ThenBlock);
+ if (&S == MustControlStmt) {
+ // XXX: Implementation subject to change.
+ // TODO: Make arg unique, so that optimizer doesn't get the bright idea to
+ // collapse nested mustcontrol statement blocks.
+ // TODO: Make LLVM emit this during optimization, so that mustcontrol
+ // doesn't just work for Clang.
+ Builder.CreateCall(CGM.getIntrinsic(llvm::Intrinsic::sideeffect),
+ {llvm::ConstantInt::get(IntPtrTy, 42)});
+ }
+
incrementProfileCounter(&S);
{
RunCleanupsScope ThenScope(*this);
Index: clang/include/clang/Basic/AttrDocs.td
===================================================================
--- clang/include/clang/Basic/AttrDocs.td
+++ clang/include/clang/Basic/AttrDocs.td
@@ -469,6 +469,25 @@
}];
}
+def MustControlDocs : Documentation {
+ let Category = DocCatStmt;
+ let Content = [{
+Marking a conditional control statement as ``mustcontrol`` indicates that the
+compiler must generate a conditional branch in machine code, and such
+conditional branch is placed *before* conditionally executed instructions. The
+attribute may be ignored if the condition of the control statement is a
+constant expression.
+
+This typically affects optimizations that would cause removal of a conditional
+branch. However, it also ensures that a conditional branch and subsequent
+instructions are not replaced with non-branching conditional instructions.
+
+Requesting the generation of a branch may be required in execution environments
+where execution of a specific conditional branch inhibits speculation or has
+observable side-effects of interest otherwise.
+ }];
+}
+
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
@@ -1385,6 +1385,13 @@
let Subjects = SubjectList<[ReturnStmt], ErrorDiag, "return statements">;
}
+def MustControl : StmtAttr {
+ let Spellings = [Clang<"mustcontrol">];
+ let Documentation = [MustControlDocs];
+ let Subjects = SubjectList<[IfStmt, SwitchStmt, ForStmt, WhileStmt, DoStmt],
+ ErrorDiag, "conditional control statements">;
+}
+
def FastCall : DeclOrTypeAttr {
let Spellings = [GCC<"fastcall">, Keyword<"__fastcall">,
Keyword<"_fastcall">];
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