Introduce a @llvm.experimental.guard intrinsic

Summary:
As discussed on llvm-dev[1].

This change adds the basic boilerplate code around having this intrinsic
in LLVM:

 - Changes in Intrinsics.td, and the IR Verifier
 - A lowering pass to lower @llvm.experimental.guard to normal
   control flow
 - Inliner support

[1]: http://lists.llvm.org/pipermail/llvm-dev/2016-February/095523.html

Reviewers: reames, atrick, chandlerc, rnk, JosephTremoulet, echristo

Subscribers: mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D18527

llvm-svn: 264976

docs/LangRef.rst

@@ -12181,6 +12181,50 @@ ensure that this symbol is defined).  The call arguments to
 arguments of the specified types, and not as varargs.
 
 
+'``llvm.experimental.guard``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+::
+
+      declare void @llvm.experimental.guard(i1, ...) [ "deopt"(...) ]
+
+Overview:
+"""""""""
+
+This intrinsic, together with :ref:`deoptimization operand bundles
+<deopt_opbundles>`, allows frontends to express guards or checks on
+optimistic assumptions made during compilation.  The semantics of
+``@llvm.experimental.guard`` is defined in terms of
+``@llvm.experimental.deoptimize`` -- its body is defined to be
+equivalent to:
+
+.. code-block:: llvm
+
+	define void @llvm.experimental.guard(i1 %pred, <args...>) {
+	  %realPred = and i1 %pred, undef
+	  br i1 %realPred, label %continue, label %leave
+
+	leave:
+	  call void @llvm.experimental.deoptimize(<args...>) [ "deopt"() ]
+	  ret void
+
+	continue:
+	  ret void
+	}
+
+In words, ``@llvm.experimental.guard`` executes the attached
+``"deopt"`` continuation if (but **not** only if) its first argument
+is ``false``.  Since the optimizer is allowed to replace the ``undef``
+with an arbitrary value, it can optimize guard to fail "spuriously",
+i.e. without the original condition being false (hence the "not only
+if"); and this allows for "check widening" type optimizations.
+
+``@llvm.experimental.guard`` cannot be invoked.
+
+
 Stack Map Intrinsics
 --------------------
 

include/llvm/IR/Intrinsics.td

@@ -597,6 +597,10 @@ def int_debugtrap : Intrinsic<[]>,
 def int_experimental_deoptimize : Intrinsic<[llvm_any_ty], [llvm_vararg_ty],
                                             [Throws]>;
 
+// Support for speculative runtime guards
+def int_experimental_guard : Intrinsic<[], [llvm_i1_ty, llvm_vararg_ty],
+                                       [Throws]>;
+
 // NOP: calls/invokes to this intrinsic are removed by codegen
 def int_donothing : Intrinsic<[], [], [IntrNoMem]>;
 

include/llvm/InitializePasses.h

@@ -188,6 +188,7 @@ void initializeLoopIdiomRecognizePass(PassRegistry&);
 void initializeLowerAtomicPass(PassRegistry&);
 void initializeLowerBitSetsPass(PassRegistry&);
 void initializeLowerExpectIntrinsicPass(PassRegistry&);
+void initializeLowerGuardIntrinsicPass(PassRegistry&);
 void initializeLowerIntrinsicsPass(PassRegistry&);
 void initializeLowerInvokePass(PassRegistry&);
 void initializeLowerSwitchPass(PassRegistry&);

include/llvm/Transforms/Scalar.h

@@ -372,6 +372,12 @@ FunctionPass *createSinkingPass();
 //
 Pass *createLowerAtomicPass();
 
+//===----------------------------------------------------------------------===//
+//
+// LowerGuardIntrinsic - Lower guard intrinsics to normal control flow.
+//
+Pass *createLowerGuardIntrinsicPass();
+
 //===----------------------------------------------------------------------===//
 //
 // ValuePropagation - Propagate CFG-derived value information

lib/IR/Verifier.cpp

@@ -4106,6 +4106,14 @@ void Verifier::visitIntrinsicCallSite(Intrinsic::ID ID, CallSite CS) {
     break;
   }
 
+  case Intrinsic::experimental_guard: {
+    Assert(CS.isCall(), "experimental_guard cannot be invoked", CS);
+    Assert(CS.countOperandBundlesOfType(LLVMContext::OB_deopt) == 1,
+           "experimental_guard must have exactly one "
+           "\"deopt\" operand bundle");
+    break;
+  }
+
   case Intrinsic::experimental_deoptimize: {
     Assert(CS.isCall(), "experimental_deoptimize cannot be invoked", CS);
     Assert(CS.countOperandBundlesOfType(LLVMContext::OB_deopt) == 1,

lib/Transforms/Scalar/CMakeLists.txt

@@ -32,6 +32,7 @@ add_llvm_library(LLVMScalarOpts
   LoopVersioningLICM.cpp
   LowerAtomic.cpp
   LowerExpectIntrinsic.cpp
+  LowerGuardIntrinsic.cpp
   MemCpyOptimizer.cpp
   MergedLoadStoreMotion.cpp
   NaryReassociate.cpp

lib/Transforms/Scalar/LowerGuardIntrinsic.cpp

@@ -0,0 +1,108 @@
+//===- LowerGuardIntrinsic.cpp - Lower the guard intrinsic ---------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass lowers the llvm.experimental.guard intrinsic to a conditional call
+// to @llvm.experimental.deoptimize.  Once this happens, the guard can no longer
+// be widened.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/InstIterator.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Intrinsics.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Pass.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+
+using namespace llvm;
+
+namespace {
+struct LowerGuardIntrinsic : public FunctionPass {
+  static char ID;
+  LowerGuardIntrinsic() : FunctionPass(ID) {
+    initializeLowerGuardIntrinsicPass(*PassRegistry::getPassRegistry());
+  }
+
+  bool runOnFunction(Function &F) override;
+};
+}
+
+static void MakeGuardControlFlowExplicit(Function *DeoptIntrinsic,
+                                         CallInst *CI) {
+  OperandBundleDef DeoptOB(*CI->getOperandBundle(LLVMContext::OB_deopt));
+  SmallVector<Value *, 4> Args(std::next(CI->arg_begin()), CI->arg_end());
+
+  auto *CheckBB = CI->getParent();
+  auto *DeoptBlockTerm =
+      SplitBlockAndInsertIfThen(CI->getArgOperand(0), CI, true);
+
+  auto *CheckBI = cast<BranchInst>(CheckBB->getTerminator());
+
+  // SplitBlockAndInsertIfThen inserts control flow that branches to
+  // DeoptBlockTerm if the condition is true.  We want the opposite.
+  CheckBI->swapSuccessors();
+
+  CheckBI->getSuccessor(0)->setName("guarded");
+  CheckBI->getSuccessor(1)->setName("deopt");
+
+  IRBuilder<> B(DeoptBlockTerm);
+  auto *DeoptCall = B.CreateCall(DeoptIntrinsic, Args, {DeoptOB}, "");
+
+  if (DeoptIntrinsic->getReturnType()->isVoidTy()) {
+    B.CreateRetVoid();
+  } else {
+    DeoptCall->setName("deoptcall");
+    B.CreateRet(DeoptCall);
+  }
+
+  DeoptBlockTerm->eraseFromParent();
+}
+
+bool LowerGuardIntrinsic::runOnFunction(Function &F) {
+  // Check if we can cheaply rule out the possibility of not having any work to
+  // do.
+  auto *GuardDecl = F.getParent()->getFunction(
+      Intrinsic::getName(Intrinsic::experimental_guard));
+  if (!GuardDecl || GuardDecl->use_empty())
+    return false;
+
+  SmallVector<CallInst *, 8> ToLower;
+  for (auto &I : instructions(F))
+    if (auto *CI = dyn_cast<CallInst>(&I))
+      if (auto *F = CI->getCalledFunction())
+        if (F->getIntrinsicID() == Intrinsic::experimental_guard)
+          ToLower.push_back(CI);
+
+  if (ToLower.empty())
+    return false;
+
+  auto *DeoptIntrinsic = Intrinsic::getDeclaration(
+      F.getParent(), Intrinsic::experimental_deoptimize, {F.getReturnType()});
+
+  for (auto *CI : ToLower) {
+    MakeGuardControlFlowExplicit(DeoptIntrinsic, CI);
+    CI->eraseFromParent();
+  }
+
+  return true;
+}
+
+char LowerGuardIntrinsic::ID = 0;
+INITIALIZE_PASS(LowerGuardIntrinsic, "lower-guard-intrinsic",
+                "Lower the guard intrinsic to normal control flow", false,
+                false)
+
+Pass *llvm::createLowerGuardIntrinsicPass() {
+  return new LowerGuardIntrinsic();
+}

lib/Transforms/Scalar/Scalar.cpp

@@ -62,6 +62,7 @@ void llvm::initializeScalarOpts(PassRegistry &Registry) {
   initializeLoopIdiomRecognizePass(Registry);
   initializeLowerAtomicPass(Registry);
   initializeLowerExpectIntrinsicPass(Registry);
+  initializeLowerGuardIntrinsicPass(Registry);
   initializeMemCpyOptPass(Registry);
   initializeMergedLoadStoreMotionPass(Registry);
   initializeNaryReassociatePass(Registry);

lib/Transforms/Utils/InlineFunction.cpp

@@ -428,12 +428,14 @@ static BasicBlock *HandleCallsInBlockInlinedThroughInvoke(
       continue;
 
     // We do not need to (and in fact, cannot) convert possibly throwing calls
-    // to @llvm.experimental_deoptimize into invokes.  The caller's "segment" of
-    // the deoptimization continuation attached to the newly inlined
-    // @llvm.experimental_deoptimize call should contain the exception handling
-    // logic, if any.
+    // to @llvm.experimental_deoptimize (resp. @llvm.experimental.guard) into
+    // invokes.  The caller's "segment" of the deoptimization continuation
+    // attached to the newly inlined @llvm.experimental_deoptimize
+    // (resp. @llvm.experimental.guard) call should contain the exception
+    // handling logic, if any.
     if (auto *F = CI->getCalledFunction())
-      if (F->getIntrinsicID() == Intrinsic::experimental_deoptimize)
+      if (F->getIntrinsicID() == Intrinsic::experimental_deoptimize ||
+          F->getIntrinsicID() == Intrinsic::experimental_guard)
         continue;
 
     if (auto FuncletBundle = CI->getOperandBundle(LLVMContext::OB_funclet)) {

test/Transforms/Inline/guard-intrinsic.ll

@@ -0,0 +1,39 @@
+; RUN: opt -S -always-inline < %s | FileCheck %s
+
+declare void @llvm.experimental.guard(i1, ...)
+
+define i8 @callee(i1* %c_ptr) alwaysinline {
+  %c = load volatile i1, i1* %c_ptr
+  call void(i1, ...) @llvm.experimental.guard(i1 %c, i32 1) [ "deopt"(i32 1) ]
+  ret i8 5
+}
+
+define void @caller_0(i1* %c, i8* %ptr) {
+; CHECK-LABEL: @caller_0(
+entry:
+; CHECK:  [[COND:%[^ ]+]] = load volatile i1, i1* %c
+; CHECK-NEXT:  call void (i1, ...) @llvm.experimental.guard(i1 [[COND]], i32 1) [ "deopt"(i32 2, i32 1) ]
+; CHECK-NEXT:  store i8 5, i8* %ptr
+
+  %v = call i8 @callee(i1* %c)  [ "deopt"(i32 2) ]
+  store i8 %v, i8* %ptr
+  ret void
+}
+
+define i32 @caller_1(i1* %c, i8* %ptr) personality i8 3 {
+; CHECK-LABEL: @caller_1(
+; CHECK:  [[COND:%[^ ]+]] = load volatile i1, i1* %c
+; CHECK-NEXT:  call void (i1, ...) @llvm.experimental.guard(i1 [[COND]], i32 1) [ "deopt"(i32 3, i32 1) ]
+; CHECK-NEXT:  br label %normal
+entry:
+  %v = invoke i8 @callee(i1* %c)  [ "deopt"(i32 3) ] to label %normal
+       unwind label %unwind
+
+unwind:
+  %lp = landingpad i32 cleanup
+  ret i32 43
+
+normal:
+  store i8 %v, i8* %ptr
+  ret i32 42
+}

test/Transforms/LowerGuardIntrinsic/basic.ll

@@ -0,0 +1,62 @@
+; RUN: opt -S -lower-guard-intrinsic < %s | FileCheck %s
+
+declare void @llvm.experimental.guard(i1, ...)
+
+define i8 @f_basic(i1* %c_ptr) {
+; CHECK-LABEL: @f_basic(
+
+  %c = load volatile i1, i1* %c_ptr
+  call void(i1, ...) @llvm.experimental.guard(i1 %c, i32 1) [ "deopt"(i32 1) ]
+  ret i8 5
+
+; CHECK:  br i1 %c, label %guarded, label %deopt
+; CHECK: deopt:
+; CHECK-NEXT:  %deoptcall = call i8 (...) @llvm.experimental.deoptimize.i8(i32 1) [ "deopt"(i32 1) ]
+; CHECK-NEXT:  ret i8 %deoptcall
+; CHECK: guarded:
+; CHECK-NEXT:  ret i8 5
+}
+
+define void @f_void_return_ty(i1* %c_ptr) {
+; CHECK-LABEL: @f_void_return_ty(
+
+  %c = load volatile i1, i1* %c_ptr
+  call void(i1, ...) @llvm.experimental.guard(i1 %c, i32 1) [ "deopt"() ]
+  ret void
+
+; CHECK:  br i1 %c, label %guarded, label %deopt
+; CHECK: deopt:
+; CHECK-NEXT:  call void (...) @llvm.experimental.deoptimize.isVoid(i32 1) [ "deopt"() ]
+; CHECK-NEXT:  ret void
+; CHECK: guarded:
+; CHECK-NEXT:  ret void
+}
+
+define void @f_multiple_args(i1* %c_ptr) {
+; CHECK-LABEL: @f_multiple_args(
+
+  %c = load volatile i1, i1* %c_ptr
+  call void(i1, ...) @llvm.experimental.guard(i1 %c, i32 1, i32 2, double 500.0) [ "deopt"(i32 2, i32 3) ]
+  ret void
+
+; CHECK: br i1 %c, label %guarded, label %deopt
+; CHECK: deopt:
+; CHECK-NEXT:  call void (...) @llvm.experimental.deoptimize.isVoid(i32 1, i32 2, double 5.000000e+02) [ "deopt"(i32 2, i32 3) ]
+; CHECK-NEXT:  ret void
+; CHECK: guarded:
+; CHECK-NEXT:  ret void
+}
+
+define i32 @f_zero_args(i1* %c_ptr) {
+; CHECK-LABEL: @f_zero_args(
+  %c = load volatile i1, i1* %c_ptr
+  call void(i1, ...) @llvm.experimental.guard(i1 %c) [ "deopt"(i32 2, i32 3) ]
+  ret i32 500
+
+; CHECK: br i1 %c, label %guarded, label %deopt
+; CHECK: deopt:
+; CHECK-NEXT:  %deoptcall = call i32 (...) @llvm.experimental.deoptimize.i32() [ "deopt"(i32 2, i32 3) ]
+; CHECK-NEXT:  ret i32 %deoptcall
+; CHECK: guarded:
+; CHECK-NEXT:  ret i32 500
+}

test/Verifier/guard-intrinsic.ll

@@ -0,0 +1,26 @@
+; RUN: not opt -S -verify < %s 2>&1 | FileCheck %s
+
+declare void @llvm.experimental.guard(i1, ...)
+
+declare void @unknown()
+
+define void @f_nodeopt() {
+entry:
+  call void(i1, ...) @llvm.experimental.guard(i1 undef, i32 1, i32 2)
+; CHECK: guard must have exactly one "deopt" operand bundle
+  ret void
+}
+
+define void @f_invoke() personality i8 3 {
+entry:
+  invoke void(i1, ...) @llvm.experimental.guard(i1 undef, i32 0, float 0.0) [ "deopt"() ] to label %ok unwind label %not_ok
+; CHECK: guard cannot be invoked
+
+ok:
+  ret void
+
+not_ok:
+  %0 = landingpad { i8*, i32 }
+          filter [0 x i8*] zeroinitializer
+  ret void
+}