[llvm] [SandboxIR] Preserve the order of switch cases after revert. (PR #115577)

[Jorge Gorbe Moya via llvm-commits]

https://github.com/slackito created https://github.com/llvm/llvm-project/pull/115577

Preserving the case order is not strictly necessary to preserve semantics (for example, operations like SwitchInst::removeCase will happily swap cases around). However, I'm planning to introduce an optional verification step for SandboxIR that will use StructuralHash to compare IR after a revert to the original IR to help catch tracker bugs, and the order difference triggers a difference there.

>From b266ea739a7678804c14c4752dbfd8f7d1beeebe Mon Sep 17 00:00:00 2001
From: Jorge Gorbe Moya <jgorbe at google.com>
Date: Fri, 8 Nov 2024 18:58:59 -0800
Subject: [PATCH] [SandboxIR] Preserve the order of switch cases after revert.

Preserving the case order is not strictly necessary to preserve
semantics (for example, operations like SwitchInst::removeCase will
happily swap cases around). However, I'm planning to introduce an
optional verification step for SandboxIR that will use StructuralHash to
compare IR after a revert to the original IR to help catch tracker bugs,
and the order difference triggers a difference there.
---
 llvm/include/llvm/SandboxIR/Tracker.h    |  6 ++++--
 llvm/lib/SandboxIR/Instruction.cpp       |  2 +-
 llvm/lib/SandboxIR/Tracker.cpp           | 14 +++++++++++++-
 llvm/unittests/SandboxIR/TrackerTest.cpp | 13 +++++++++++++
 4 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/llvm/include/llvm/SandboxIR/Tracker.h b/llvm/include/llvm/SandboxIR/Tracker.h
index 3e3e539a8c7c16..7db0fc23c75fb9 100644
--- a/llvm/include/llvm/SandboxIR/Tracker.h
+++ b/llvm/include/llvm/SandboxIR/Tracker.h
@@ -315,12 +315,14 @@ class SwitchAddCase : public IRChangeBase {
 
 class SwitchRemoveCase : public IRChangeBase {
   SwitchInst *Switch;
+  unsigned Index;
   ConstantInt *Val;
   BasicBlock *Dest;
 
 public:
-  SwitchRemoveCase(SwitchInst *Switch, ConstantInt *Val, BasicBlock *Dest)
-      : Switch(Switch), Val(Val), Dest(Dest) {}
+  SwitchRemoveCase(SwitchInst *Switch, unsigned Index, ConstantInt *Val,
+                   BasicBlock *Dest)
+      : Switch(Switch), Index(Index), Val(Val), Dest(Dest) {}
   void revert(Tracker &Tracker) final;
   void accept() final {}
 #ifndef NDEBUG
diff --git a/llvm/lib/SandboxIR/Instruction.cpp b/llvm/lib/SandboxIR/Instruction.cpp
index df941b2fa81efe..2df51a5947c173 100644
--- a/llvm/lib/SandboxIR/Instruction.cpp
+++ b/llvm/lib/SandboxIR/Instruction.cpp
@@ -1133,7 +1133,7 @@ void SwitchInst::addCase(ConstantInt *OnVal, BasicBlock *Dest) {
 SwitchInst::CaseIt SwitchInst::removeCase(CaseIt It) {
   auto &Case = *It;
   Ctx.getTracker().emplaceIfTracking<SwitchRemoveCase>(
-      this, Case.getCaseValue(), Case.getCaseSuccessor());
+      this, Case.getCaseIndex(), Case.getCaseValue(), Case.getCaseSuccessor());
 
   auto *LLVMSwitch = cast<llvm::SwitchInst>(Val);
   unsigned CaseNum = It - case_begin();
diff --git a/llvm/lib/SandboxIR/Tracker.cpp b/llvm/lib/SandboxIR/Tracker.cpp
index abcad39330094d..a6f25670c55207 100644
--- a/llvm/lib/SandboxIR/Tracker.cpp
+++ b/llvm/lib/SandboxIR/Tracker.cpp
@@ -170,7 +170,19 @@ void CatchSwitchAddHandler::revert(Tracker &Tracker) {
   LLVMCSI->removeHandler(LLVMCSI->handler_begin() + HandlerIdx);
 }
 
-void SwitchRemoveCase::revert(Tracker &Tracker) { Switch->addCase(Val, Dest); }
+void SwitchRemoveCase::revert(Tracker &Tracker) {
+  // removeCase swaps the last case with the deleted one. To revert it, we use
+  // addCase (which adds the new case to the end), and swap the newly-added
+  // value and successor operands to the positions for the original case index.
+  Switch->addCase(Val, Dest);
+  auto ValUseA = Switch->getOperandUse(2 + Index * 2);
+  auto SucUseA = Switch->getOperandUse(2 + Index * 2 + 1);
+  unsigned NumOps = Switch->getNumOperands();
+  auto ValUseB = Switch->getOperandUse(NumOps - 2);
+  auto SucUseB = Switch->getOperandUse(NumOps - 2 + 1);
+  ValUseA.swap(ValUseB);
+  SucUseA.swap(SucUseB);
+}
 
 #ifndef NDEBUG
 void SwitchRemoveCase::dump() const {
diff --git a/llvm/unittests/SandboxIR/TrackerTest.cpp b/llvm/unittests/SandboxIR/TrackerTest.cpp
index 6d060854949e1f..71c4c3d0ea9210 100644
--- a/llvm/unittests/SandboxIR/TrackerTest.cpp
+++ b/llvm/unittests/SandboxIR/TrackerTest.cpp
@@ -963,6 +963,19 @@ define void @foo(i32 %cond0, i32 %cond1) {
   EXPECT_EQ(Switch->getNumCases(), 2u);
   EXPECT_EQ(Switch->findCaseDest(BB0), Zero);
   EXPECT_EQ(Switch->findCaseDest(BB1), One);
+
+  // Check order is preserved after reverting multiple removeCase invocations.
+  Ctx.save();
+  Switch->removeCase(Switch->findCaseValue(Zero));
+  Switch->removeCase(Switch->findCaseValue(One));
+  EXPECT_EQ(Switch->getNumCases(), 0u);
+  Ctx.revert();
+  EXPECT_EQ(Switch->getNumCases(), 2u);
+  EXPECT_EQ(Switch->findCaseDest(BB0), Zero);
+  EXPECT_EQ(Switch->findCaseDest(BB1), One);
+  EXPECT_EQ(Switch->getSuccessor(0), OrigDefaultDest);
+  EXPECT_EQ(Switch->getSuccessor(1), BB0);
+  EXPECT_EQ(Switch->getSuccessor(2), BB1);
 }
 
 TEST_F(TrackerTest, SelectInst) {