From 4cad61adb32337b27bcc17827d51ccbb160ed492 Mon Sep 17 00:00:00 2001
From: Sanjoy Das <sanjoy@playingwithpointers.com>
Date: Tue, 1 Aug 2017 22:37:58 +0000
Subject: [PATCH] [SCEV/IndVars] Always compute loop exiting values if the
 backedge count is 0

If SCEV can prove that the backedge taken count for a loop is zero, it does not
need to "understand" a recursive PHI to compute its exiting value.

This should fix PR33885.

llvm-svn: 309758
---
 llvm/lib/Analysis/ScalarEvolution.cpp         | 19 +++++++++++++
 .../IndVarSimplify/exit_value_test2.ll        | 28 +++++++++++++++++--
 2 files changed, 44 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index b1cbf917dee7..d5abccb8dbb1 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -7597,6 +7597,25 @@ const SCEV *ScalarEvolution::computeSCEVAtScope(const SCEV *V, const Loop *L) {
             const SCEV *BackedgeTakenCount = getBackedgeTakenCount(LI);
             if (const SCEVConstant *BTCC =
                   dyn_cast<SCEVConstant>(BackedgeTakenCount)) {
+
+              // This trivial case can show up in some degenerate cases where
+              // the incoming IR has not yet been fully simplified.
+              if (BTCC->getValue()->isZero()) {
+                Value *InitValue = nullptr;
+                bool MultipleInitValues = false;
+                for (unsigned i = 0; i < PN->getNumIncomingValues(); i++) {
+                  if (!LI->contains(PN->getIncomingBlock(i))) {
+                    if (!InitValue)
+                      InitValue = PN->getIncomingValue(i);
+                    else if (InitValue != PN->getIncomingValue(i)) {
+                      MultipleInitValues = true;
+                      break;
+                    }
+                  }
+                  if (!MultipleInitValues && InitValue)
+                    return getSCEV(InitValue);
+                }
+              }
               // Okay, we know how many times the containing loop executes.  If
               // this is a constant evolving PHI node, get the final value at
               // the specified iteration number.
diff --git a/llvm/test/Transforms/IndVarSimplify/exit_value_test2.ll b/llvm/test/Transforms/IndVarSimplify/exit_value_test2.ll
index ee641667506c..7b6e91a742b2 100644
--- a/llvm/test/Transforms/IndVarSimplify/exit_value_test2.ll
+++ b/llvm/test/Transforms/IndVarSimplify/exit_value_test2.ll
@@ -3,15 +3,14 @@
 
 ; Check IndVarSimplify should not replace exit value because or else
 ; udiv will be introduced by expand and the cost will be high.
-;
-; CHECK-LABEL: @_Z3fooPKcjj(
-; CHECK-NOT: udiv
 
 declare void @_Z3mixRjj(i32* dereferenceable(4), i32)
 declare void @llvm.lifetime.start.p0i8(i64, i8* nocapture)
 declare void @llvm.lifetime.end.p0i8(i64, i8* nocapture)
 
 define i32 @_Z3fooPKcjj(i8* nocapture readonly %s, i32 %len, i32 %c) {
+; CHECK-LABEL: @_Z3fooPKcjj(
+; CHECK-NOT: udiv
 entry:
   %a = alloca i32, align 4
   %tmp = bitcast i32* %a to i8*
@@ -50,3 +49,26 @@ while.end:                                        ; preds = %while.cond.while.en
   call void @llvm.lifetime.end.p0i8(i64 4, i8* %tmp)
   ret i32 %tmp4
 }
+
+define i32 @zero_backedge_count_test(i32 %unknown_init, i32* %unknown_mem) {
+; CHECK-LABEL: @zero_backedge_count_test(
+entry:
+  br label %loop
+
+loop:
+  %iv = phi i32 [ 0, %entry], [ %iv.inc, %loop ]
+  %unknown_phi = phi i32 [ %unknown_init, %entry ], [ %unknown_next, %loop ]
+  %iv.inc = add i32 %iv, 1
+  %be_taken = icmp ne i32 %iv.inc, 1
+  %unknown_next = load volatile i32, i32* %unknown_mem
+  br i1 %be_taken, label %loop, label %leave
+
+leave:
+; We can fold %unknown_phi even though the backedge value for it is completely
+; unknown, since we can prove that the loop's backedge taken count is 0.
+
+; CHECK: leave:
+; CHECK: ret i32 %unknown_init
+  %exit_val = phi i32 [ %unknown_phi, %loop ]
+  ret i32 %exit_val
+}