[PATCH] D48421: [IPSCCP] Try to replace phis before they are removed by changeToUnreachable.

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
fhahn added reviewers: efriedma, davide.

If a successor of a dead block has a PHI node with 2 incoming values,
this PHI node will be replaced by the other incoming value by
changeToUnreachable. If we found a constant for the PHI node, we have to
replace it before changeToUnreachable.

Fixes PR37780.


https://reviews.llvm.org/D48421

Files:
  lib/Transforms/Scalar/SCCP.cpp
  test/Transforms/SCCP/ipsccp-phi-one-pred-dead.ll


Index: test/Transforms/SCCP/ipsccp-phi-one-pred-dead.ll
===================================================================
--- /dev/null
+++ test/Transforms/SCCP/ipsccp-phi-one-pred-dead.ll
@@ -0,0 +1,39 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -S -ipsccp | FileCheck %s
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @test() {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label %Flow5.pre
+; CHECK:       Flow6:
+; CHECK-NEXT:    br label %end2
+; CHECK:       Flow5.pre:
+; CHECK-NEXT:    br label %Flow5
+; CHECK:       Flow5:
+; CHECK-NEXT:    br label %Flow6
+; CHECK:       end2:
+; CHECK-NEXT:    unreachable
+;
+entry:
+  br i1 true, label %Flow5.pre, label %Flow5.pre.unreachable
+
+Flow5.pre.unreachable:
+  br label %Flow5
+
+Flow6:
+  br i1 %0, label %end1, label %end2
+
+Flow5.pre:
+  br label %Flow5
+
+Flow5:
+  %0 = phi i1 [ undef, %Flow5.pre ], [ false, %Flow5.pre.unreachable ]
+  br label %Flow6
+
+end1:
+  unreachable
+
+end2:
+  unreachable
+}
Index: lib/Transforms/Scalar/SCCP.cpp
===================================================================
--- lib/Transforms/Scalar/SCCP.cpp
+++ lib/Transforms/Scalar/SCCP.cpp
@@ -2019,8 +2019,25 @@
     for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
       if (!Solver.isBlockExecutable(&*BB)) {
         LLVM_DEBUG(dbgs() << "  BasicBlock Dead:" << *BB);
-
         ++NumDeadBlocks;
+
+        // By using changeToUnreachable below, PHI nodes in successors of
+        // BB with 2 predecessors are replaced by the incoming value. Therefore
+        // we have to try and replace them with constants here, before they
+        // are removed.
+        for (BasicBlock *Successor : successors(&*BB)) {
+          for (PHINode &PN : Successor->phis()) {
+            if (PN.getNumIncomingValues() != 2 ||
+                !Solver.isBlockExecutable(Successor))
+              continue;
+
+            // There is no need to remove PN here, as changeToUnreachable will
+            // take care of that.
+            if (tryToReplaceWithConstant(Solver, &PN))
+              ++IPNumInstRemoved;
+            }
+        }
+
         NumInstRemoved +=
             changeToUnreachable(BB->getFirstNonPHI(), /*UseLLVMTrap=*/false);
 


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