[llvm] r174122 - Rewrite instsimplify's handling if icmp on pointer values to remove the
Dan Gohman
dan433584 at gmail.com
Thu Jan 31 16:11:13 PST 2013
Author: djg
Date: Thu Jan 31 18:11:13 2013
New Revision: 174122
URL: http://llvm.org/viewvc/llvm-project?rev=174122&view=rev
Log:
Rewrite instsimplify's handling if icmp on pointer values to remove the
remaining use of AliasAnalysis concepts such as isIdentifiedObject to
prove pointer inequality.
@external_compare in test/Transforms/InstSimplify/compare.ll shows a simple
case where a noalias argument can be equal to a global variable address, and
while AliasAnalysis can get away with saying that these pointers don't alias,
instsimplify cannot say that they are not equal.
Modified:
llvm/trunk/lib/Analysis/InstructionSimplify.cpp
llvm/trunk/test/Transforms/InstSimplify/compare.ll
Modified: llvm/trunk/lib/Analysis/InstructionSimplify.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/InstructionSimplify.cpp?rev=174122&r1=174121&r2=174122&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/InstructionSimplify.cpp (original)
+++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp Thu Jan 31 18:11:13 2013
@@ -21,10 +21,10 @@
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/Statistic.h"
-#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/GlobalAlias.h"
#include "llvm/IR/Operator.h"
@@ -667,8 +667,8 @@ Value *llvm::SimplifyAddInst(Value *Op0,
/// This is very similar to GetPointerBaseWithConstantOffset except it doesn't
/// follow non-inbounds geps. This allows it to remain usable for icmp ult/etc.
/// folding.
-static Constant *stripAndComputeConstantOffsets(const DataLayout *TD,
- Value *&V) {
+static ConstantInt *stripAndComputeConstantOffsets(const DataLayout *TD,
+ Value *&V) {
assert(V->getType()->isPointerTy());
// Without DataLayout, just be conservative for now. Theoretically, more could
@@ -701,7 +701,7 @@ static Constant *stripAndComputeConstant
} while (Visited.insert(V));
Type *IntPtrTy = TD->getIntPtrType(V->getContext());
- return ConstantInt::get(IntPtrTy, Offset);
+ return cast<ConstantInt>(ConstantInt::get(IntPtrTy, Offset));
}
/// \brief Compute the constant difference between two pointer values.
@@ -1689,8 +1689,19 @@ static Value *ExtractEquivalentCondition
}
static Constant *computePointerICmp(const DataLayout *TD,
+ const TargetLibraryInfo *TLI,
CmpInst::Predicate Pred,
Value *LHS, Value *RHS) {
+ // First, skip past any trivial no-ops.
+ LHS = LHS->stripPointerCasts();
+ RHS = RHS->stripPointerCasts();
+
+ // A non-null pointer is not equal to a null pointer.
+ if (llvm::isKnownNonNull(LHS) && isa<ConstantPointerNull>(RHS) &&
+ (Pred == CmpInst::ICMP_EQ || Pred == CmpInst::ICMP_NE))
+ return ConstantInt::get(GetCompareTy(LHS),
+ !CmpInst::isTrueWhenEqual(Pred));
+
// We can only fold certain predicates on pointer comparisons.
switch (Pred) {
default:
@@ -1713,15 +1724,80 @@ static Constant *computePointerICmp(cons
break;
}
- Constant *LHSOffset = stripAndComputeConstantOffsets(TD, LHS);
- Constant *RHSOffset = stripAndComputeConstantOffsets(TD, RHS);
+ // Strip off any constant offsets so that we can reason about them.
+ // It's tempting to use getUnderlyingObject or even just stripInBoundsOffsets
+ // here and compare base addresses like AliasAnalysis does, however there are
+ // numerous hazards. AliasAnalysis and its utilities rely on special rules
+ // governing loads and stores which don't apply to icmps. Also, AliasAnalysis
+ // doesn't need to guarantee pointer inequality when it says NoAlias.
+ ConstantInt *LHSOffset = stripAndComputeConstantOffsets(TD, LHS);
+ ConstantInt *RHSOffset = stripAndComputeConstantOffsets(TD, RHS);
+
+ // If LHS and RHS are related via constant offsets to the same base
+ // value, we can replace it with an icmp which just compares the offsets.
+ if (LHS == RHS)
+ return ConstantExpr::getICmp(Pred, LHSOffset, RHSOffset);
+
+ // Various optimizations for (in)equality comparisons.
+ if (Pred == CmpInst::ICMP_EQ || Pred == CmpInst::ICMP_NE) {
+ // Different non-empty allocations that exist at the same time have
+ // different addresses (if the program can tell). Global variables always
+ // exist, so they always exist during the lifetime of each other and all
+ // allocas. Two different allocas usually have different addresses...
+ //
+ // However, if there's an @llvm.stackrestore dynamically in between two
+ // allocas, they may have the same address. It's tempting to reduce the
+ // scope of the problem by only looking at *static* allocas here. That would
+ // cover the majority of allocas while significantly reducing the likelihood
+ // of having an @llvm.stackrestore pop up in the middle. However, it's not
+ // actually impossible for an @llvm.stackrestore to pop up in the middle of
+ // an entry block. Also, if we have a block that's not attached to a
+ // function, we can't tell if it's "static" under the current definition.
+ // Theoretically, this problem could be fixed by creating a new kind of
+ // instruction kind specifically for static allocas. Such a new instruction
+ // could be required to be at the top of the entry block, thus preventing it
+ // from being subject to a @llvm.stackrestore. Instcombine could even
+ // convert regular allocas into these special allocas. It'd be nifty.
+ // However, until then, this problem remains open.
+ //
+ // So, we'll assume that two non-empty allocas have different addresses
+ // for now.
+ //
+ // With all that, if the offsets are within the bounds of their allocations
+ // (and not one-past-the-end! so we can't use inbounds!), and their
+ // allocations aren't the same, the pointers are not equal.
+ //
+ // Note that it's not necessary to check for LHS being a global variable
+ // address, due to canonicalization and constant folding.
+ if (isa<AllocaInst>(LHS) &&
+ (isa<AllocaInst>(RHS) || isa<GlobalVariable>(RHS))) {
+ uint64_t LHSSize, RHSSize;
+ if (getObjectSize(LHS, LHSSize, TD, TLI) &&
+ getObjectSize(RHS, RHSSize, TD, TLI)) {
+ const APInt &LHSOffsetValue = LHSOffset->getValue();
+ const APInt &RHSOffsetValue = RHSOffset->getValue();
+ if (!LHSOffsetValue.isNegative() &&
+ !RHSOffsetValue.isNegative() &&
+ LHSOffsetValue.ult(LHSSize) &&
+ RHSOffsetValue.ult(RHSSize)) {
+ return ConstantInt::get(GetCompareTy(LHS),
+ !CmpInst::isTrueWhenEqual(Pred));
+ }
+ }
- // If LHS and RHS are not related via constant offsets to the same base
- // value, there is nothing we can do here.
- if (LHS != RHS)
- return 0;
+ // Repeat the above check but this time without depending on DataLayout
+ // or being able to compute a precise size.
+ if (!cast<PointerType>(LHS->getType())->isEmptyTy() &&
+ !cast<PointerType>(RHS->getType())->isEmptyTy() &&
+ LHSOffset->isNullValue() &&
+ RHSOffset->isNullValue())
+ return ConstantInt::get(GetCompareTy(LHS),
+ !CmpInst::isTrueWhenEqual(Pred));
+ }
+ }
- return ConstantExpr::getICmp(Pred, LHSOffset, RHSOffset);
+ // Otherwise, fail.
+ return 0;
}
/// SimplifyICmpInst - Given operands for an ICmpInst, see if we can
@@ -1786,50 +1862,6 @@ static Value *SimplifyICmpInst(unsigned
}
}
- // icmp <object*>, <object*/null> - Different identified objects have
- // different addresses (unless null), and what's more the address of an
- // identified local is never equal to another argument (again, barring null).
- // Note that generalizing to the case where LHS is a global variable address
- // or null is pointless, since if both LHS and RHS are constants then we
- // already constant folded the compare, and if only one of them is then we
- // moved it to RHS already.
- Value *LHSPtr = LHS->stripPointerCasts();
- Value *RHSPtr = RHS->stripPointerCasts();
- if (LHSPtr == RHSPtr)
- return ConstantInt::get(ITy, CmpInst::isTrueWhenEqual(Pred));
-
- // Be more aggressive about stripping pointer adjustments when checking a
- // comparison of an alloca address to another object. We can rip off all
- // inbounds GEP operations, even if they are variable.
- LHSPtr = LHSPtr->stripInBoundsOffsets();
- if (llvm::isIdentifiedObject(LHSPtr)) {
- RHSPtr = RHSPtr->stripInBoundsOffsets();
- if (llvm::isKnownNonNull(LHSPtr) || llvm::isKnownNonNull(RHSPtr)) {
- // If both sides are different identified objects, they aren't equal
- // unless they're null.
- if (LHSPtr != RHSPtr && llvm::isIdentifiedObject(RHSPtr) &&
- Pred == CmpInst::ICMP_EQ)
- return ConstantInt::get(ITy, false);
-
- // A local identified object (alloca or noalias call) can't equal any
- // incoming argument, unless they're both null or they belong to
- // different functions. The latter happens during inlining.
- if (Instruction *LHSInst = dyn_cast<Instruction>(LHSPtr))
- if (Argument *RHSArg = dyn_cast<Argument>(RHSPtr))
- if (LHSInst->getParent()->getParent() == RHSArg->getParent() &&
- Pred == CmpInst::ICMP_EQ)
- return ConstantInt::get(ITy, false);
- }
-
- // Assume that the constant null is on the right.
- if (llvm::isKnownNonNull(LHSPtr) && isa<ConstantPointerNull>(RHSPtr)) {
- if (Pred == CmpInst::ICMP_EQ)
- return ConstantInt::get(ITy, false);
- else if (Pred == CmpInst::ICMP_NE)
- return ConstantInt::get(ITy, true);
- }
- }
-
// If we are comparing with zero then try hard since this is a common case.
if (match(RHS, m_Zero())) {
bool LHSKnownNonNegative, LHSKnownNegative;
@@ -2457,7 +2489,7 @@ static Value *SimplifyICmpInst(unsigned
// Simplify comparisons of related pointers using a powerful, recursive
// GEP-walk when we have target data available..
if (LHS->getType()->isPointerTy())
- if (Constant *C = computePointerICmp(Q.TD, Pred, LHS, RHS))
+ if (Constant *C = computePointerICmp(Q.TD, Q.TLI, Pred, LHS, RHS))
return C;
if (GetElementPtrInst *GLHS = dyn_cast<GetElementPtrInst>(LHS)) {
Modified: llvm/trunk/test/Transforms/InstSimplify/compare.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstSimplify/compare.ll?rev=174122&r1=174121&r2=174122&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstSimplify/compare.ll (original)
+++ llvm/trunk/test/Transforms/InstSimplify/compare.ll Thu Jan 31 18:11:13 2013
@@ -660,3 +660,25 @@ define i1 @alloca_argument_compare(i64*
; CHECK: alloca_argument_compare
; CHECK: ret i1 %cmp
}
+
+; As above, but with the operands reversed.
+
+define i1 @alloca_argument_compare_swapped(i64* %arg) {
+ %alloc = alloca i64
+ %cmp = icmp eq i64* %alloc, %arg
+ ret i1 %cmp
+ ; CHECK: alloca_argument_compare_swapped
+ ; CHECK: ret i1 %cmp
+}
+
+; Don't assume that a noalias argument isn't equal to a global variable's
+; address. This is an example where AliasAnalysis' NoAlias concept is
+; different from actual pointer inequality.
+
+ at y = external global i32
+define zeroext i1 @external_compare(i32* noalias %x) {
+ %cmp = icmp eq i32* %x, @y
+ ret i1 %cmp
+ ; CHECK: external_compare
+ ; CHECK: ret i1 %cmp
+}
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