[llvm] r309684 - [PostDom] document the current handling of infinite loops and unreachables
Tobias Grosser via llvm-commits
llvm-commits at lists.llvm.org
Tue Aug 1 07:40:55 PDT 2017
Author: grosser
Date: Tue Aug 1 07:40:55 2017
New Revision: 309684
URL: http://llvm.org/viewvc/llvm-project?rev=309684&view=rev
Log:
[PostDom] document the current handling of infinite loops and unreachables
Summary:
As we are in the process of changing the behavior of how the post-dominator tree
is computed, make sure we have some more test coverage in this area.
Current inconsistencies:
- Newly unreachable nodes are not added as new roots, in case the PDT is updated
but not rebuilt.
- Newly unreachable loops are not added to the CFG at all (neither when
building from scratch nor when updating the CFG). This is inconsistent with
the fact that unreachables are added to the PDT, but unreachable loops not.
On the other side, PDT relationships are not loosened at the moment in
cases where new unreachable loops are built.
This commit is providing additional test coverage for
https://reviews.llvm.org/D35851
Reviewers: dberlin, kuhar
Reviewed By: kuhar
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D36107
Modified:
llvm/trunk/unittests/IR/DominatorTreeTest.cpp
Modified: llvm/trunk/unittests/IR/DominatorTreeTest.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/IR/DominatorTreeTest.cpp?rev=309684&r1=309683&r2=309684&view=diff
==============================================================================
--- llvm/trunk/unittests/IR/DominatorTreeTest.cpp (original)
+++ llvm/trunk/unittests/IR/DominatorTreeTest.cpp Tue Aug 1 07:40:55 2017
@@ -326,6 +326,274 @@ TEST(DominatorTree, NonUniqueEdges) {
});
}
+// Verify that the PDT is correctly updated in case an edge removal results
+// in a new unreachable CFG node.
+//
+// For the following input code and initial PDT:
+//
+// CFG PDT
+//
+// A Exit
+// | |
+// _B D
+// / | \ |
+// ^ v \ B
+// \ / D / \
+// C \ C A
+// v
+// Exit
+//
+// we verify that CFG' and PDT-updated is obtained after removal of edge C -> B.
+//
+// CFG' PDT-updated
+//
+// A Exit
+// | |
+// B D
+// | \ |
+// v \ B
+// / D \
+// C \ A
+// | v
+// unreachable Exit
+//
+// WARNING: PDT-updated is inconsistent with PDT-recalculated, which is
+// constructed from CFG' when recalculating the PDT from scratch.
+//
+// PDT-recalculated
+//
+// Exit
+// / | \
+// C B D
+// |
+// A
+//
+// TODO: document the wanted behavior after resolving this inconsistency.
+TEST(DominatorTree, DeletingEdgesIntroducesUnreachables) {
+ StringRef ModuleString =
+ "define void @f() {\n"
+ "A:\n"
+ " br label %B\n"
+ "B:\n"
+ " br i1 undef, label %D, label %C\n"
+ "C:\n"
+ " br label %B\n"
+ "D:\n"
+ " ret void\n"
+ "}\n";
+
+ // Parse the module.
+ LLVMContext Context;
+ std::unique_ptr<Module> M = makeLLVMModule(Context, ModuleString);
+
+ runWithDomTree(
+ *M, "f", [&](Function &F, DominatorTree *DT, PostDomTree *PDT) {
+ Function::iterator FI = F.begin();
+
+ FI++;
+ BasicBlock *B = &*FI++;
+ BasicBlock *C = &*FI++;
+ BasicBlock *D = &*FI++;
+
+ assert(PDT->dominates(PDT->getNode(D), PDT->getNode(B)));
+
+ C->getTerminator()->eraseFromParent();
+ new UnreachableInst(C->getContext(), C);
+
+ DT->deleteEdge(C, B);
+ PDT->deleteEdge(C, B);
+
+ EXPECT_TRUE(PDT->dominates(PDT->getNode(D), PDT->getNode(B)));
+ EXPECT_EQ(PDT->getNode(C), nullptr);
+
+ PDT->recalculate(F);
+
+ EXPECT_FALSE(PDT->dominates(PDT->getNode(D), PDT->getNode(B)));
+ EXPECT_NE(PDT->getNode(C), nullptr);
+ });
+}
+
+// Verify that the PDT is correctly updated in case an edge removal results
+// in an infinite loop.
+//
+// Test case:
+//
+// CFG PDT
+//
+// A Exit
+// | |
+// _B D
+// / | \ |
+// ^ v \ B
+// \ / D / \
+// C \ C A
+// / \ v
+// ^ v Exit
+// \_/
+//
+// After deleting the edge C->B, C is part of an infinite reverse-unreachable
+// loop:
+//
+// CFG' PDT'
+//
+// A Exit
+// | |
+// B D
+// | \ |
+// v \ B
+// / D \
+// C \ A
+// / \ v
+// ^ v Exit
+// \_/
+//
+// In PDT, D post-dominates B. We verify that this post-dominance
+// relation is preserved _after_ deleting the edge C->B from CFG.
+//
+// As C now becomes reverse-unreachable, it is not anymore part of the
+// PDT. We also verify this property.
+//
+// TODO: Can we change the PDT definition such that C remains part of the
+// CFG, at best without loosing the dominance relation D postdom B.
+TEST(DominatorTree, DeletingEdgesIntroducesInfiniteLoop) {
+ StringRef ModuleString =
+ "define void @f() {\n"
+ "A:\n"
+ " br label %B\n"
+ "B:\n"
+ " br i1 undef, label %D, label %C\n"
+ "C:\n"
+ " switch i32 undef, label %C [\n"
+ " i32 0, label %B\n"
+ " ]\n"
+ "D:\n"
+ " ret void\n"
+ "}\n";
+
+ // Parse the module.
+ LLVMContext Context;
+ std::unique_ptr<Module> M = makeLLVMModule(Context, ModuleString);
+
+ runWithDomTree(
+ *M, "f", [&](Function &F, DominatorTree *DT, PostDomTree *PDT) {
+ Function::iterator FI = F.begin();
+
+ FI++;
+ BasicBlock *B = &*FI++;
+ BasicBlock *C = &*FI++;
+ BasicBlock *D = &*FI++;
+
+ assert(PDT->dominates(PDT->getNode(D), PDT->getNode(B)));
+
+ auto SwitchC = cast<SwitchInst>(C->getTerminator());
+ SwitchC->removeCase(SwitchC->case_begin());
+ DT->deleteEdge(C, B);
+ PDT->deleteEdge(C, B);
+
+ EXPECT_TRUE(PDT->dominates(PDT->getNode(D), PDT->getNode(B)));
+ EXPECT_EQ(PDT->getNode(C), nullptr);
+
+ PDT->recalculate(F);
+
+ EXPECT_TRUE(PDT->dominates(PDT->getNode(D), PDT->getNode(B)));
+ EXPECT_EQ(PDT->getNode(C), nullptr);
+ });
+}
+
+// Verify that the PDT is correctly updated in case an edge removal results
+// in an infinite loop.
+//
+// Test case:
+//
+// CFG PDT
+//
+// A Exit
+// | / | \
+// B-- C B D
+// | \ |
+// v \ A
+// / D
+// C--C2 \
+// / \ \ v
+// ^ v --Exit
+// \_/
+//
+// After deleting the edge C->E, C is part of an infinite reverse-unreachable
+// loop:
+//
+// CFG' PDT'
+//
+// A Exit
+// | |
+// B D
+// | \ |
+// v \ B
+// / D \
+// C \ A
+// / \ v
+// ^ v Exit
+// \_/
+//
+// In PDT, D does not post-dominate B. After the edge C->E is removed, a new
+// post-dominance relation is introduced.
+//
+// As C now becomes reverse-unreachable, it is not anymore part of the
+// PDT. We also verify this property.
+//
+// TODO: Can we change the PDT definition such that C remains part of the
+// CFG, at best without loosing the dominance relation D postdom B.
+TEST(DominatorTree, DeletingEdgesIntroducesInfiniteLoop2) {
+ StringRef ModuleString =
+ "define void @f() {\n"
+ "A:\n"
+ " br label %B\n"
+ "B:\n"
+ " br i1 undef, label %D, label %C\n"
+ "C:\n"
+ " switch i32 undef, label %C [\n"
+ " i32 0, label %C2\n"
+ " ]\n"
+ "C2:\n"
+ " ret void\n"
+ "D:\n"
+ " ret void\n"
+ "}\n";
+
+ // Parse the module.
+ LLVMContext Context;
+ std::unique_ptr<Module> M = makeLLVMModule(Context, ModuleString);
+
+ runWithDomTree(
+ *M, "f", [&](Function &F, DominatorTree *DT, PostDomTree *PDT) {
+ Function::iterator FI = F.begin();
+
+ FI++;
+ BasicBlock *B = &*FI++;
+ BasicBlock *C = &*FI++;
+ BasicBlock *C2 = &*FI++;
+ BasicBlock *D = &*FI++;
+
+ auto SwitchC = cast<SwitchInst>(C->getTerminator());
+ SwitchC->removeCase(SwitchC->case_begin());
+ DT->deleteEdge(C, C2);
+ PDT->deleteEdge(C, C2);
+ C2->eraseFromParent();
+
+ EXPECT_EQ(DT->getNode(C2), nullptr);
+ PDT->eraseNode(C2);
+
+ EXPECT_TRUE(PDT->dominates(PDT->getNode(D), PDT->getNode(B)));
+ EXPECT_EQ(PDT->getNode(C), nullptr);
+ EXPECT_EQ(PDT->getNode(C2), nullptr);
+
+ PDT->recalculate(F);
+
+ EXPECT_TRUE(PDT->dominates(PDT->getNode(D), PDT->getNode(B)));
+ EXPECT_EQ(PDT->getNode(C), nullptr);
+ EXPECT_EQ(PDT->getNode(C2), nullptr);
+ });
+}
+
namespace {
const auto Insert = CFGBuilder::ActionKind::Insert;
const auto Delete = CFGBuilder::ActionKind::Delete;
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