[llvm] r347484 - [LoopSimplifyCFG] Don't delete LCSSA Phis

[Max Kazantsev via llvm-commits]

Author: mkazantsev
Date: Thu Nov 22 23:56:47 2018
New Revision: 347484

URL: http://llvm.org/viewvc/llvm-project?rev=347484&view=rev
Log:
[LoopSimplifyCFG] Don't delete LCSSA Phis

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.

Thanks @dmgreen	for finding this!

Differential Revision: https://reviews.llvm.org/D54841

Added:
    llvm/trunk/test/Transforms/LoopSimplifyCFG/lcssa.ll
Modified:
    llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp

Modified: llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp?rev=347484&r1=347483&r2=347484&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp Thu Nov 22 23:56:47 2018
@@ -249,7 +249,10 @@ private:
       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 because it is a LCSSA Phi.
+          bool PreserveLCSSAPhi = !L.contains(Succ);
+          Succ->removePredecessor(BB, PreserveLCSSAPhi);
         }
 
       IRBuilder<> Builder(BB->getContext());

Added: llvm/trunk/test/Transforms/LoopSimplifyCFG/lcssa.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopSimplifyCFG/lcssa.ll?rev=347484&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/LoopSimplifyCFG/lcssa.ll (added)
+++ llvm/trunk/test/Transforms/LoopSimplifyCFG/lcssa.ll Thu Nov 22 23:56:47 2018
@@ -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
+}