Fix return status of SimplifyCFG

[serge-sans-paille]

Summary

When a switch case is folded into default's case, that's an IR change that should be reported, update ConstantFoldTerminator accordingly.

[serge-sans-paille]

This fixes this build: https://firefox-ci-tc.services.mozilla.com/tasks/HwRa82dySq2_04Qth6nPQQ/runs/0/logs/https%3A%2F%2Ffirefox-ci-tc.services.mozilla.com%2Fapi%2Fqueue%2Fv1%2Ftask%2FHwRa82dySq2_04Qth6nPQQ%2Fruns%2F0%2Fartifacts%2Fpublic%2Flogs%2Flive.log

[fhahn]

Is it possible to add a test for that?

[serge-sans-paille]

Test case added

[nikic]

LGTM

llvm/lib/Transforms/Utils/Local.cpp

if (auto *SI = dyn_cast<SwitchInst>(T)) {
     BasicBlock *TheOnlyDest = DefaultDest;
 
     // If the default is unreachable, ignore it when searching for TheOnlyDest.
     if (isa<UnreachableInst>(DefaultDest->getFirstNonPHIOrDbg()) &&
         SI->getNumCases() > 0) {
       TheOnlyDest = SI->case_begin()->getCaseSuccessor();
     }
 
+    bool Changed = false;
+
     // Figure out which case it goes to.
     for (auto i = SI->case_begin(), e = SI->case_end(); i != e;) {
       // Found case matching a constant operand?
       if (i->getCaseValue() == CI) {
         TheOnlyDest = i->getCaseSuccessor();
         break;
       }
 
for (auto i = SI->case_begin(), e = SI->case_end(); i != e;) {
                           MDBuilder(BB->getContext()).
                           createBranchWeights(Weights));
         }
         // Remove this entry.
         BasicBlock *ParentBB = SI->getParent();
         DefaultDest->removePredecessor(ParentBB);
         i = SI->removeCase(i);
         e = SI->case_end();
+        Changed = true;
         if (DTU)
           DTU->applyUpdatesPermissive(
               {{DominatorTree::Delete, ParentBB, DefaultDest}});
         continue;
       }
 
       // Otherwise, check to see if the switch only branches to one destination.
       // We do this by reseting "TheOnlyDest" to null when we find two non-equal
if (SI->getNumCases() == 1) {
       MDNode *MakeImplicitMD = SI->getMetadata(LLVMContext::MD_make_implicit);
       if (MakeImplicitMD)
         NewBr->setMetadata(LLVMContext::MD_make_implicit, MakeImplicitMD);
 
       // Delete the old switch.
       SI->eraseFromParent();
       return true;
     }
-    return false;
+    return Changed;
   }
 
   if (auto *IBI = dyn_cast<IndirectBrInst>(T)) {
     // indirectbr blockaddress(@F, @BB) -> br label @BB
     if (auto *BA =
           dyn_cast<BlockAddress>(IBI->getAddress()->stripPointerCasts())) {
       BasicBlock *TheOnlyDest = BA->getBasicBlock();
       std::vector <DominatorTree::UpdateType> Updates;

llvm/test/Transforms/SimplifyCFG/merge-default.ll

+; RUN: opt -simplifycfg -S < %s | FileCheck %s
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare void @g()
+declare void @f()
+
+define void @foo(i32 %Kind) {
+; CHECK-LABEL: @foo(
+; CHECK-NEXT:entry:
+; CHECK-NEXT:  switch i32 %Kind, label %sw.epilog [
+; CHECK-NEXT:    i32 15, label %sw.bb2
+; CHECK-NEXT:    i32 2, label %sw.bb
+; CHECK-NEXT:  ]
+; CHECK:     sw.bb:
+; CHECK-NEXT:  call void @g()
+; CHECK-NEXT:  call void @g()
+; CHECK-NEXT:  br label %sw.epilog
+; CHECK:     sw.bb2:
+; CHECK-NEXT:  call void @f()
+; CHECK-NEXT:  br label %sw.epilog
+; CHECK:     sw.epilog:
+; CHECK-NEXT:  ret void
+; CHECK-NEXT:}
+
+entry:
+  switch i32 %Kind, label %sw.epilog [
+    i32 1, label %sw.epilog
+    i32 2, label %sw.bb
+    i32 15, label %sw.bb2
+  ]
+
+sw.bb:
+  call void @g()
+  call void @g()
+  br label %sw.epilog
+
+sw.bb2:
+  call void @f()
+  br label %sw.epilog
+
+sw.epilog:
+  ret void
+}