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-rw-r--r--llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp88
-rw-r--r--llvm/lib/Transforms/InstCombine/InstCombineInternal.h1
-rw-r--r--llvm/test/Transforms/InstCombine/compare-alloca.ll97
3 files changed, 186 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 3afb5ed6d8a..a333c3e18e5 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -730,6 +730,83 @@ Instruction *InstCombiner::FoldGEPICmp(GEPOperator *GEPLHS, Value *RHS,
return nullptr;
}
+Instruction *InstCombiner::FoldAllocaCmp(ICmpInst &ICI, AllocaInst *Alloca,
+ Value *Other) {
+ assert(ICI.isEquality() && "Cannot fold non-equality comparison.");
+
+ // It would be tempting to fold away comparisons between allocas and any
+ // pointer not based on that alloca (e.g. an argument). However, even
+ // though such pointers cannot alias, they can still compare equal.
+ //
+ // But LLVM doesn't specify where allocas get their memory, so if the alloca
+ // doesn't escape we can argue that it's impossible to guess its value, and we
+ // can therefore act as if any such guesses are wrong.
+ //
+ // The code below checks that the alloca doesn't escape, and that it's only
+ // used in a comparison once (the current instruction). The
+ // single-comparison-use condition ensures that we're trivially folding all
+ // comparisons against the alloca consistently, and avoids the risk of
+ // erroneously folding a comparison of the pointer with itself.
+
+ unsigned MaxIter = 32; // Break cycles and bound to constant-time.
+
+ SmallVector<Use *, 32> Worklist;
+ for (Use &U : Alloca->uses()) {
+ if (Worklist.size() >= MaxIter)
+ return nullptr;
+ Worklist.push_back(&U);
+ }
+
+ unsigned NumCmps = 0;
+ while (!Worklist.empty()) {
+ assert(Worklist.size() <= MaxIter);
+ Use *U = Worklist.pop_back_val();
+ Value *V = U->getUser();
+ --MaxIter;
+
+ if (isa<BitCastInst>(V) || isa<GetElementPtrInst>(V) || isa<PHINode>(V) ||
+ isa<SelectInst>(V)) {
+ // Track the uses.
+ } else if (isa<LoadInst>(V)) {
+ // Loading from the pointer doesn't escape it.
+ continue;
+ } else if (auto *SI = dyn_cast<StoreInst>(V)) {
+ // Storing *to* the pointer is fine, but storing the pointer escapes it.
+ if (SI->getValueOperand() == U->get())
+ return nullptr;
+ continue;
+ } else if (isa<ICmpInst>(V)) {
+ if (NumCmps++)
+ return nullptr; // Found more than one cmp.
+ continue;
+ } else if (auto *Intrin = dyn_cast<IntrinsicInst>(V)) {
+ switch (Intrin->getIntrinsicID()) {
+ // These intrinsics don't escape or compare the pointer. Memset is safe
+ // because we don't allow ptrtoint. Memcpy and memmove are safe because
+ // we don't allow stores, so src cannot point to V.
+ case Intrinsic::lifetime_start: case Intrinsic::lifetime_end:
+ case Intrinsic::dbg_declare: case Intrinsic::dbg_value:
+ case Intrinsic::memcpy: case Intrinsic::memmove: case Intrinsic::memset:
+ continue;
+ default:
+ return nullptr;
+ }
+ } else {
+ return nullptr;
+ }
+ for (Use &U : V->uses()) {
+ if (Worklist.size() >= MaxIter)
+ return nullptr;
+ Worklist.push_back(&U);
+ }
+ }
+
+ Type *CmpTy = CmpInst::makeCmpResultType(Other->getType());
+ return ReplaceInstUsesWith(
+ ICI,
+ ConstantInt::get(CmpTy, !CmpInst::isTrueWhenEqual(ICI.getPredicate())));
+}
+
/// FoldICmpAddOpCst - Fold "icmp pred (X+CI), X".
Instruction *InstCombiner::FoldICmpAddOpCst(Instruction &ICI,
Value *X, ConstantInt *CI,
@@ -3211,6 +3288,17 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
ICmpInst::getSwappedPredicate(I.getPredicate()), I))
return NI;
+ // Try to optimize equality comparisons against alloca-based pointers.
+ if (Op0->getType()->isPointerTy() && I.isEquality()) {
+ assert(Op1->getType()->isPointerTy() && "Comparing pointer with non-pointer?");
+ if (auto *Alloca = dyn_cast<AllocaInst>(GetUnderlyingObject(Op0, DL)))
+ if (Instruction *New = FoldAllocaCmp(I, Alloca, Op1))
+ return New;
+ if (auto *Alloca = dyn_cast<AllocaInst>(GetUnderlyingObject(Op1, DL)))
+ if (Instruction *New = FoldAllocaCmp(I, Alloca, Op0))
+ return New;
+ }
+
// Test to see if the operands of the icmp are casted versions of other
// values. If the ptr->ptr cast can be stripped off both arguments, we do so
// now.
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
index 9e58c7428bd..79cb5f25dc6 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -281,6 +281,7 @@ public:
ICmpInst::Predicate Pred);
Instruction *FoldGEPICmp(GEPOperator *GEPLHS, Value *RHS,
ICmpInst::Predicate Cond, Instruction &I);
+ Instruction *FoldAllocaCmp(ICmpInst &ICI, AllocaInst *Alloca, Value *Other);
Instruction *FoldShiftByConstant(Value *Op0, Constant *Op1,
BinaryOperator &I);
Instruction *commonCastTransforms(CastInst &CI);
diff --git a/llvm/test/Transforms/InstCombine/compare-alloca.ll b/llvm/test/Transforms/InstCombine/compare-alloca.ll
new file mode 100644
index 00000000000..ca24da19177
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/compare-alloca.ll
@@ -0,0 +1,97 @@
+; RUN: opt -instcombine -S %s | FileCheck %s
+target datalayout = "p:32:32"
+
+
+define i1 @alloca_argument_compare(i64* %arg) {
+ %alloc = alloca i64
+ %cmp = icmp eq i64* %arg, %alloc
+ ret i1 %cmp
+ ; CHECK-LABEL: alloca_argument_compare
+ ; CHECK: ret i1 false
+}
+
+define i1 @alloca_argument_compare_swapped(i64* %arg) {
+ %alloc = alloca i64
+ %cmp = icmp eq i64* %alloc, %arg
+ ret i1 %cmp
+ ; CHECK-LABEL: alloca_argument_compare_swapped
+ ; CHECK: ret i1 false
+}
+
+define i1 @alloca_argument_compare_ne(i64* %arg) {
+ %alloc = alloca i64
+ %cmp = icmp ne i64* %arg, %alloc
+ ret i1 %cmp
+ ; CHECK-LABEL: alloca_argument_compare_ne
+ ; CHECK: ret i1 true
+}
+
+define i1 @alloca_argument_compare_derived_ptrs(i64* %arg, i64 %x) {
+ %alloc = alloca i64, i64 8
+ %p = getelementptr i64, i64* %arg, i64 %x
+ %q = getelementptr i64, i64* %alloc, i64 3
+ %cmp = icmp eq i64* %p, %q
+ ret i1 %cmp
+ ; CHECK-LABEL: alloca_argument_compare_derived_ptrs
+ ; CHECK: ret i1 false
+}
+
+declare void @escape(i64*)
+define i1 @alloca_argument_compare_escaped_alloca(i64* %arg) {
+ %alloc = alloca i64
+ call void @escape(i64* %alloc)
+ %cmp = icmp eq i64* %alloc, %arg
+ ret i1 %cmp
+ ; CHECK-LABEL: alloca_argument_compare_escaped_alloca
+ ; CHECK: %cmp = icmp eq i64* %alloc, %arg
+ ; CHECK: ret i1 %cmp
+}
+
+declare void @check_compares(i1, i1)
+define void @alloca_argument_compare_two_compares(i64* %p) {
+ %q = alloca i64, i64 8
+ %r = getelementptr i64, i64* %p, i64 1
+ %s = getelementptr i64, i64* %q, i64 2
+ %cmp1 = icmp eq i64* %p, %q
+ %cmp2 = icmp eq i64* %r, %s
+ call void @check_compares(i1 %cmp1, i1 %cmp2)
+ ret void
+ ; We will only fold if there is a single cmp.
+ ; CHECK-LABEL: alloca_argument_compare_two_compares
+ ; CHECK: call void @check_compares(i1 %cmp1, i1 %cmp2)
+}
+
+define i1 @alloca_argument_compare_escaped_through_store(i64* %arg, i64** %ptr) {
+ %alloc = alloca i64
+ %cmp = icmp eq i64* %alloc, %arg
+ %p = getelementptr i64, i64* %alloc, i64 1
+ store i64* %p, i64** %ptr
+ ret i1 %cmp
+ ; CHECK-LABEL: alloca_argument_compare_escaped_through_store
+ ; CHECK: %cmp = icmp eq i64* %alloc, %arg
+ ; CHECK: ret i1 %cmp
+}
+
+declare void @llvm.lifetime.start(i64, i8* nocapture)
+declare void @llvm.lifetime.end(i64, i8* nocapture)
+define i1 @alloca_argument_compare_benign_instrs(i8* %arg) {
+ %alloc = alloca i8
+ call void @llvm.lifetime.start(i64 1, i8* %alloc)
+ %cmp = icmp eq i8* %arg, %alloc
+ %x = load i8, i8* %arg
+ store i8 %x, i8* %alloc
+ call void @llvm.lifetime.end(i64 1, i8* %alloc)
+ ret i1 %cmp
+ ; CHECK-LABEL: alloca_argument_compare_benign_instrs
+ ; CHECK: ret i1 false
+}
+
+declare i64* @allocator()
+define i1 @alloca_call_compare() {
+ %p = alloca i64
+ %q = call i64* @allocator()
+ %cmp = icmp eq i64* %p, %q
+ ret i1 %cmp
+ ; CHECK-LABEL: alloca_call_compare
+ ; CHECK: ret i1 false
+}
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