[PATCH] D54849: [LoopSimplifyCFG] Fix corner case with duplicating successors

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.
mkazantsev added reviewers: uabelho, dmgreen, anna, skatkov.

It fixes a bug that doesn't update Phi inputs of the only live successor that
is in the list of block's successors more than once.


https://reviews.llvm.org/D54849

Files:
  lib/Transforms/Scalar/LoopSimplifyCFG.cpp
  test/Transforms/LoopSimplifyCFG/phi_with_duplicating_inputs.ll


Index: test/Transforms/LoopSimplifyCFG/phi_with_duplicating_inputs.ll
===================================================================
--- test/Transforms/LoopSimplifyCFG/phi_with_duplicating_inputs.ll
+++ test/Transforms/LoopSimplifyCFG/phi_with_duplicating_inputs.ll
@@ -1,17 +1,28 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; This is currently failing because of bug in LoopSimplifyCFG. It does not update
 ; duplicating Phi inputs properly.
-; XFAIL: *
 ; RUN: opt -S -enable-loop-simplifycfg-term-folding=true -loop-simplifycfg -debug-only=loop-simplifycfg -verify-loop-info -verify-dom-info -verify-loop-lcssa 2>&1 < %s | FileCheck %s
 ; RUN: opt -S -enable-loop-simplifycfg-term-folding=true -passes='require<domtree>,loop(simplify-cfg)' -debug-only=loop-simplifycfg -verify-loop-info -verify-dom-info -verify-loop-lcssa 2>&1 < %s | FileCheck %s
 ; RUN: opt -S -enable-loop-simplifycfg-term-folding=true -loop-simplifycfg -enable-mssa-loop-dependency=true -verify-memoryssa -debug-only=loop-simplifycfg -verify-loop-info -verify-dom-info -verify-loop-lcssa 2>&1 < %s | FileCheck %s
 
 target datalayout = "P40"
 
 @a = external global i16, align 1
 
-; CHECK-LABEL: @f1(
-
 define void @f1(i1 %cond) {
+; CHECK-LABEL: @f1(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_COND:%.*]]
+; CHECK:       for.cond:
+; CHECK-NEXT:    br i1 [[COND:%.*]], label [[IF_THEN:%.*]], label [[FOR_INC:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    [[TMP0:%.*]] = load i16, i16* @a, align 1
+; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp ne i16 [[TMP0]], 0
+; CHECK-NEXT:    br label [[FOR_INC]]
+; CHECK:       for.inc:
+; CHECK-NEXT:    [[C_1:%.*]] = phi i16 [ 2, [[IF_THEN]] ], [ 1, [[FOR_COND]] ]
+; CHECK-NEXT:    br label [[FOR_COND]]
+;
 entry:
   br label %for.cond
 
Index: lib/Transforms/Scalar/LoopSimplifyCFG.cpp
===================================================================
--- lib/Transforms/Scalar/LoopSimplifyCFG.cpp
+++ lib/Transforms/Scalar/LoopSimplifyCFG.cpp
@@ -249,14 +249,24 @@
 
       SmallPtrSet<BasicBlock *, 2> DeadSuccessors;
       // Remove all BB's successors except for the live one.
+      unsigned TheOnlySuccDuplicates = 0;
       for (auto *Succ : successors(BB))
         if (Succ != TheOnlySucc) {
           DeadSuccessors.insert(Succ);
           // If our successor lies in a different loop, we don't want to remove
           // the one-input Phi because it is a LCSSA Phi.
           bool PreserveLCSSAPhi = !L.contains(Succ);
           Succ->removePredecessor(BB, PreserveLCSSAPhi);
-        }
+        } else
+          ++TheOnlySuccDuplicates;
+
+      assert(TheOnlySuccDuplicates > 0 && "Should be!");
+      // If TheOnlySucc was BB's successor more than once, after transform it
+      // will be its successor only once. Remove redundant inputs from
+      // TheOnlySucc's Phis.
+      bool PreserveLCSSAPhi = !L.contains(TheOnlySucc);
+      for (unsigned Dup = 1; Dup < TheOnlySuccDuplicates; ++Dup)
+        TheOnlySucc->removePredecessor(BB, PreserveLCSSAPhi);
 
       IRBuilder<> Builder(BB->getContext());
       Instruction *Term = BB->getTerminator();


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