[CallSiteSplitting] Report edge deletion to DomTreeUpdater

[JosephTremoulet]

Summary

When splitting musttail calls, the split blocks' original terminators
get removed; inform the DTU when this happens.

Also add a testcase that fails an assertion in the DTU without this fix.

[fhahn]

LGTM, thanks.

lib/Transforms/Scalar/CallSiteSplitting.cpp

static void splitCallSite(
   // FIXME: remove TI in `copyMustTailReturn`
   if (IsMustTailCall) {
     // Remove superfluous `br` terminators from the end of the Split blocks
     // NOTE: Removing terminator removes the SplitBlock from the TailBB's
     // predecessors. Therefore we must get complete list of Splits before
     // attempting removal.
     SmallVector<BasicBlock *, 2> Splits(predecessors((TailBB)));
     assert(Splits.size() == 2 && "Expected exactly 2 splits!");
-    for (unsigned i = 0; i < Splits.size(); i++)
+    for (unsigned i = 0; i < Splits.size(); i++) {
       Splits[i]->getTerminator()->eraseFromParent();
+      DTU.deleteEdge(Splits[i], TailBB);
+    }
 
     // Erase the tail block once done with musttail patching
     DTU.deleteBB(TailBB);
     return;
   }
 
   auto *OriginalBegin = &*TailBB->begin();
   // Replace users of the original call with a PHI mering call-sites split.

test/Transforms/CallSiteSplitting/musttail.ll

Tail:
   ret void
 End:
   ret void
 }
 
 define void @void_callee(i8* %a, i8* %b) noinline {
   ret void
 }
+
+;   Include a test with a larger CFG that exercises the DomTreeUpdater
+;   machinery a bit more.
+;CHECK-LABEL: @larger_cfg_caller
+;CHECK-LABEL: Top.split:
+;CHECK: %r1 = musttail call i8* @callee(i8* null, i8* %b)
+;CHECK: ret i8* %r1
+;CHECK-LABEL: TBB.split
+;CHECK: %r2 = musttail call i8* @callee(i8* nonnull %a, i8* null)
+;CHECK: ret i8* %r2
+define i8* @larger_cfg_caller(i8* %a, i8* %b) {
+Top:
+  %cond1 = icmp eq i8* %a, null
+  br i1 %cond1, label %Tail, label %ExtraTest
+ExtraTest:
+  %a0 = load i8, i8* %a
+  %cond2 = icmp eq i8 %a0, 0
+  br i1 %cond2, label %TBB_pred, label %End
+TBB_pred:
+  br label %TBB
+TBB:
+  %cond3 = icmp eq i8* %b, null
+  br i1 %cond3, label %Tail, label %End
+Tail:
+  %r = musttail call i8* @callee(i8* %a, i8* %b)
+  ret i8* %r
+End:
+  ret i8* null
+}