[llvm] r246476 - [FunctionAttr] Infer nonnull attributes on returns
Philip Reames via llvm-commits
llvm-commits at lists.llvm.org
Mon Aug 31 12:44:38 PDT 2015
Author: reames
Date: Mon Aug 31 14:44:38 2015
New Revision: 246476
URL: http://llvm.org/viewvc/llvm-project?rev=246476&view=rev
Log:
[FunctionAttr] Infer nonnull attributes on returns
Teach FunctionAttr to infer the nonnull attribute on return values of functions which never return a potentially null value. This is done both via a conservative local analysis for the function itself and a optimistic per-SCC analysis. If no function in the SCC returns anything which could be null (other than values from other functions in the SCC), we can conclude no function returned a null pointer. Even if some function within the SCC returns a null pointer, we may be able to locally conclude that some don't.
Differential Revision: http://reviews.llvm.org/D9688
Added:
llvm/trunk/test/Transforms/FunctionAttrs/nonnull.ll
Modified:
llvm/trunk/lib/Transforms/IPO/FunctionAttrs.cpp
Modified: llvm/trunk/lib/Transforms/IPO/FunctionAttrs.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/IPO/FunctionAttrs.cpp?rev=246476&r1=246475&r2=246476&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/IPO/FunctionAttrs.cpp (original)
+++ llvm/trunk/lib/Transforms/IPO/FunctionAttrs.cpp Mon Aug 31 14:44:38 2015
@@ -27,10 +27,12 @@
#include "llvm/Analysis/CallGraph.h"
#include "llvm/Analysis/CallGraphSCCPass.h"
#include "llvm/Analysis/CaptureTracking.h"
+#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/LLVMContext.h"
+#include "llvm/Support/Debug.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
using namespace llvm;
@@ -42,6 +44,7 @@ STATISTIC(NumNoCapture, "Number of argum
STATISTIC(NumReadNoneArg, "Number of arguments marked readnone");
STATISTIC(NumReadOnlyArg, "Number of arguments marked readonly");
STATISTIC(NumNoAlias, "Number of function returns marked noalias");
+STATISTIC(NumNonNullReturn, "Number of function returns marked nonnull");
STATISTIC(NumAnnotated, "Number of attributes added to library functions");
namespace {
@@ -67,6 +70,13 @@ namespace {
// AddNoAliasAttrs - Deduce noalias attributes for the SCC.
bool AddNoAliasAttrs(const CallGraphSCC &SCC);
+ /// \brief Does this function return null?
+ bool ReturnsNonNull(Function *F, SmallPtrSet<Function*, 8> &,
+ bool &Speculative) const;
+
+ /// \brief Deduce nonnull attributes for the SCC.
+ bool AddNonNullAttrs(const CallGraphSCC &SCC);
+
// Utility methods used by inferPrototypeAttributes to add attributes
// and maintain annotation statistics.
@@ -832,6 +842,143 @@ bool FunctionAttrs::AddNoAliasAttrs(cons
return MadeChange;
}
+bool FunctionAttrs::ReturnsNonNull(Function *F,
+ SmallPtrSet<Function*, 8> &SCCNodes,
+ bool &Speculative) const {
+ assert(F->getReturnType()->isPointerTy() &&
+ "nonnull only meaningful on pointer types");
+ Speculative = false;
+
+ SmallSetVector<Value *, 8> FlowsToReturn;
+ for (BasicBlock &BB : *F)
+ if (auto *Ret = dyn_cast<ReturnInst>(BB.getTerminator()))
+ FlowsToReturn.insert(Ret->getReturnValue());
+
+ for (unsigned i = 0; i != FlowsToReturn.size(); ++i) {
+ Value *RetVal = FlowsToReturn[i];
+
+ // If this value is locally known to be non-null, we're good
+ if (isKnownNonNull(RetVal, TLI))
+ continue;
+
+ // Otherwise, we need to look upwards since we can't make any local
+ // conclusions.
+ Instruction *RVI = dyn_cast<Instruction>(RetVal);
+ if (!RVI)
+ return false;
+ switch (RVI->getOpcode()) {
+ // Extend the analysis by looking upwards.
+ case Instruction::BitCast:
+ case Instruction::GetElementPtr:
+ case Instruction::AddrSpaceCast:
+ FlowsToReturn.insert(RVI->getOperand(0));
+ continue;
+ case Instruction::Select: {
+ SelectInst *SI = cast<SelectInst>(RVI);
+ FlowsToReturn.insert(SI->getTrueValue());
+ FlowsToReturn.insert(SI->getFalseValue());
+ continue;
+ }
+ case Instruction::PHI: {
+ PHINode *PN = cast<PHINode>(RVI);
+ for (int i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
+ FlowsToReturn.insert(PN->getIncomingValue(i));
+ continue;
+ }
+ case Instruction::Call:
+ case Instruction::Invoke: {
+ CallSite CS(RVI);
+ Function *Callee = CS.getCalledFunction();
+ // A call to a node within the SCC is assumed to return null until
+ // proven otherwise
+ if (Callee && SCCNodes.count(Callee)) {
+ Speculative = true;
+ continue;
+ }
+ return false;
+ }
+ default:
+ return false; // Unknown source, may be null
+ };
+ llvm_unreachable("should have either continued or returned");
+ }
+
+ return true;
+}
+
+bool FunctionAttrs::AddNonNullAttrs(const CallGraphSCC &SCC) {
+ SmallPtrSet<Function*, 8> SCCNodes;
+
+ // Fill SCCNodes with the elements of the SCC. Used for quickly
+ // looking up whether a given CallGraphNode is in this SCC.
+ for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
+ SCCNodes.insert((*I)->getFunction());
+
+ // Speculative that all functions in the SCC return only nonnull
+ // pointers. We may refute this as we analyze functions.
+ bool SCCReturnsNonNull = true;
+
+ bool MadeChange = false;
+
+ // Check each function in turn, determining which functions return nonnull
+ // pointers.
+ for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
+ Function *F = (*I)->getFunction();
+
+ if (!F || F->hasFnAttribute(Attribute::OptimizeNone))
+ // External node or node we don't want to optimize - skip it;
+ return false;
+
+ // Already nonnull.
+ if (F->getAttributes().hasAttribute(AttributeSet::ReturnIndex,
+ Attribute::NonNull))
+ continue;
+
+ // Definitions with weak linkage may be overridden at linktime, so
+ // treat them like declarations.
+ if (F->isDeclaration() || F->mayBeOverridden())
+ return false;
+
+ // We annotate nonnull return values, which are only applicable to
+ // pointer types.
+ if (!F->getReturnType()->isPointerTy())
+ continue;
+
+ bool Speculative = false;
+ if (ReturnsNonNull(F, SCCNodes, Speculative)) {
+ if (!Speculative) {
+ // Mark the function eagerly since we may discover a function
+ // which prevents us from speculating about the entire SCC
+ DEBUG(dbgs() << "Eagerly marking " << F->getName() << " as nonnull\n");
+ F->addAttribute(AttributeSet::ReturnIndex, Attribute::NonNull);
+ ++NumNonNullReturn;
+ MadeChange = true;
+ }
+ continue;
+ }
+ // At least one function returns something which could be null, can't
+ // speculate any more.
+ SCCReturnsNonNull = false;
+ }
+
+ if (SCCReturnsNonNull) {
+ for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
+ Function *F = (*I)->getFunction();
+ if (F->getAttributes().hasAttribute(AttributeSet::ReturnIndex,
+ Attribute::NonNull) ||
+ !F->getReturnType()->isPointerTy())
+ continue;
+
+ DEBUG(dbgs() << "SCC marking " << F->getName() << " as nonnull\n");
+ F->addAttribute(AttributeSet::ReturnIndex, Attribute::NonNull);
+ ++NumNonNullReturn;
+ MadeChange = true;
+ }
+ }
+
+ return MadeChange;
+}
+
/// inferPrototypeAttributes - Analyze the name and prototype of the
/// given function and set any applicable attributes. Returns true
/// if any attributes were set and false otherwise.
@@ -1707,5 +1854,6 @@ bool FunctionAttrs::runOnSCC(CallGraphSC
Changed |= AddReadAttrs(SCC);
Changed |= AddArgumentAttrs(SCC);
Changed |= AddNoAliasAttrs(SCC);
+ Changed |= AddNonNullAttrs(SCC);
return Changed;
}
Added: llvm/trunk/test/Transforms/FunctionAttrs/nonnull.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/FunctionAttrs/nonnull.ll?rev=246476&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/FunctionAttrs/nonnull.ll (added)
+++ llvm/trunk/test/Transforms/FunctionAttrs/nonnull.ll Mon Aug 31 14:44:38 2015
@@ -0,0 +1,74 @@
+; RUN: opt -S -functionattrs %s | FileCheck %s
+declare nonnull i8* @ret_nonnull()
+
+; Return a pointer trivially nonnull (call return attribute)
+define i8* @test1() {
+; CHECK: define nonnull i8* @test1
+ %ret = call i8* @ret_nonnull()
+ ret i8* %ret
+}
+
+; Return a pointer trivially nonnull (argument attribute)
+define i8* @test2(i8* nonnull %p) {
+; CHECK: define nonnull i8* @test2
+ ret i8* %p
+}
+
+; Given an SCC where one of the functions can not be marked nonnull,
+; can we still mark the other one which is trivially nonnull
+define i8* @scc_binder() {
+; CHECK: define i8* @scc_binder
+ call i8* @test3()
+ ret i8* null
+}
+
+define i8* @test3() {
+; CHECK: define nonnull i8* @test3
+ call i8* @scc_binder()
+ %ret = call i8* @ret_nonnull()
+ ret i8* %ret
+}
+
+; Given a mutual recursive set of functions, we can mark them
+; nonnull if neither can ever return null. (In this case, they
+; just never return period.)
+define i8* @test4_helper() {
+; CHECK: define noalias nonnull i8* @test4_helper
+ %ret = call i8* @test4()
+ ret i8* %ret
+}
+
+define i8* @test4() {
+; CHECK: define noalias nonnull i8* @test4
+ %ret = call i8* @test4_helper()
+ ret i8* %ret
+}
+
+; Given a mutual recursive set of functions which *can* return null
+; make sure we haven't marked them as nonnull.
+define i8* @test5_helper() {
+; CHECK: define noalias i8* @test5_helper
+ %ret = call i8* @test5()
+ ret i8* null
+}
+
+define i8* @test5() {
+; CHECK: define noalias i8* @test5
+ %ret = call i8* @test5_helper()
+ ret i8* %ret
+}
+
+; Local analysis, but going through a self recursive phi
+define i8* @test6() {
+entry:
+; CHECK: define nonnull i8* @test6
+ %ret = call i8* @ret_nonnull()
+ br label %loop
+loop:
+ %phi = phi i8* [%ret, %entry], [%phi, %loop]
+ br i1 undef, label %loop, label %exit
+exit:
+ ret i8* %phi
+}
+
+
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