[PATCH] D54841: [LoopSimplifyCFG] Don't delete LCSSA Phis

[Max Kazantsev via Phabricator via llvm-commits]

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

When removing edges, we also update Phi inputs and may end up removing
a Phi if it has only one input. We should not do it for edges that leave the current
loop because these Phis are LCSSA Phis and need to be preserved.


https://reviews.llvm.org/D54841

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


Index: test/Transforms/LoopSimplifyCFG/lcssa.ll
===================================================================
--- test/Transforms/LoopSimplifyCFG/lcssa.ll
+++ test/Transforms/LoopSimplifyCFG/lcssa.ll
@@ -0,0 +1,52 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -loop-simplifycfg -verify-loop-info -verify-dom-info -verify-loop-lcssa < %s | FileCheck %s
+; RUN: opt -S -passes='require<domtree>,loop(simplify-cfg)' -verify-loop-info -verify-dom-info -verify-loop-lcssa < %s | FileCheck %s
+; RUN: opt -S -loop-simplifycfg -enable-mssa-loop-dependency=true -verify-memoryssa -verify-loop-info -verify-dom-info -verify-loop-lcssa < %s | FileCheck %s
+
+target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+
+define void @c() {
+; CHECK-LABEL: @c(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[D:%.*]]
+; CHECK:       d.loopexit:
+; CHECK-NEXT:    [[DOTLCSSA:%.*]] = phi i32 [ [[TMP1:%.*]], [[FOR_COND:%.*]] ]
+; CHECK-NEXT:    br label [[D]]
+; CHECK:       d:
+; CHECK-NEXT:    [[TMP0:%.*]] = phi i32 [ undef, [[ENTRY:%.*]] ], [ [[DOTLCSSA]], [[D_LOOPEXIT:%.*]] ]
+; CHECK-NEXT:    br label [[FOR_COND]]
+; CHECK:       for.cond:
+; CHECK-NEXT:    [[TMP1]] = phi i32 [ [[TMP0]], [[D]] ], [ 0, [[IF_END:%.*]] ]
+; CHECK-NEXT:    [[TOBOOL2:%.*]] = icmp eq i32 [[TMP1]], 0
+; CHECK-NEXT:    br i1 [[TOBOOL2]], label [[IF_END]], label [[D_LOOPEXIT]]
+; CHECK:       if.end:
+; CHECK-NEXT:    br label [[FOR_COND]]
+;
+entry:
+  br label %d
+
+d.loopexit:                                       ; preds = %if.end.7, %for.body
+  %.lcssa = phi i32 [ %1, %for.body ], [ 0, %if.end.7 ]
+  br label %d
+
+d:                                                ; preds = %d.loopexit, %entry
+  %0 = phi i32 [ undef, %entry ], [ %.lcssa, %d.loopexit ]
+  br label %for.cond
+
+for.cond:                                         ; preds = %if.end.8, %d
+  %1 = phi i32 [ %0, %d ], [ 0, %if.end.8 ]
+  br label %for.body
+
+for.body:                                         ; preds = %for.cond
+  %tobool2 = icmp eq i32 %1, 0
+  br i1 %tobool2, label %if.end, label %d.loopexit
+
+if.end:                                           ; preds = %for.body
+  br label %if.end.7
+
+if.end.7:                                         ; preds = %if.end
+  br i1 true, label %if.end.8, label %d.loopexit
+
+if.end.8:                                         ; preds = %if.end.7
+  br label %for.cond
+}
Index: lib/Transforms/Scalar/LoopSimplifyCFG.cpp
===================================================================
--- lib/Transforms/Scalar/LoopSimplifyCFG.cpp
+++ lib/Transforms/Scalar/LoopSimplifyCFG.cpp
@@ -249,7 +249,10 @@
       for (auto *Succ : successors(BB))
         if (Succ != TheOnlySucc) {
           DeadSuccessors.insert(Succ);
-          Succ->removePredecessor(BB);
+          // If our successor lies in a different loop, we don't want to remove
+          // the one-input Phi with because it is an LCSSA Phi.
+          bool PreserveLCSSAPhi = !L.contains(Succ);
+          Succ->removePredecessor(BB, PreserveLCSSAPhi);
         }
 
       IRBuilder<> Builder(BB->getContext());


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