Fix and improve SCEV ComputeBackedgeTankCount.

This is a follow-up to r209358: PR19799: Indvars miscompile due to an
incorrect max backedge taken count from SCEV.

That fix was incomplete as pointed out by Arnold and Michael Z. The
code was also too confusing. It needed a careful rewrite with more
unit tests. This version will also happen to optimize more cases.

<rdar://17005101> PR19799: Indvars miscompile...

llvm-svn: 209545

1 files changed, 56 insertions, 0 deletions

diff --git a/llvm/test/Analysis/ScalarEvolution/max-trip-count.ll b/llvm/test/Analysis/ScalarEvolution/max-trip-count.ll
index 43a54b4f303..d67d2fab2e3 100644
--- a/llvm/test/Analysis/ScalarEvolution/max-trip-count.ll
+++ b/llvm/test/Analysis/ScalarEvolution/max-trip-count.ll
@@ -124,3 +124,59 @@ for.cond.i:                                       ; preds = %for.body.i
 bar.exit:                                         ; preds = %for.cond.i, %for.body.i
   ret i32 0
 }
+
+; Here we have a must-exit loop latch that is not computabe and a
+; may-exit early exit that can only have one non-exiting iteration
+; before the check is forever skipped.
+;
+; CHECK-LABEL: @cannot_compute_mustexit
+; CHECK: Loop %for.body.i: <multiple exits> Unpredictable backedge-taken count. 
+; CHECK: Loop %for.body.i: Unpredictable max backedge-taken count. 
+@b = common global i32 0, align 4
+
+define i32 @cannot_compute_mustexit() {
+entry:
+  store i32 -1, i32* @a, align 4
+  br label %for.body.i
+
+for.body.i:                                       ; preds = %for.cond.i, %entry
+  %storemerge1.i = phi i32 [ -1, %entry ], [ %add.i.i, %for.cond.i ]
+  %tobool.i = icmp eq i32 %storemerge1.i, 0
+  %add.i.i = add nsw i32 %storemerge1.i, 2
+  br i1 %tobool.i, label %bar.exit, label %for.cond.i
+
+for.cond.i:                                       ; preds = %for.body.i
+  store i32 %add.i.i, i32* @a, align 4
+  %ld = load volatile i32* @b
+  %cmp.i = icmp ne i32 %ld, 0
+  br i1 %cmp.i, label %for.body.i, label %bar.exit
+
+bar.exit:                                         ; preds = %for.cond.i, %for.body.i
+  ret i32 0
+}
+
+; This loop has two must-exits, both of with dominate the latch. The
+; MaxBECount should be the minimum of them.
+;
+; CHECK-LABEL: @two_mustexit
+; CHECK: Loop %for.body.i: <multiple exits> Unpredictable backedge-taken count. 
+; CHECK: Loop %for.body.i: max backedge-taken count is 1
+define i32 @two_mustexit() {
+entry:
+  store i32 -1, i32* @a, align 4
+  br label %for.body.i
+
+for.body.i:                                       ; preds = %for.cond.i, %entry
+  %storemerge1.i = phi i32 [ -1, %entry ], [ %add.i.i, %for.cond.i ]
+  %tobool.i = icmp sgt i32 %storemerge1.i, 0
+  %add.i.i = add nsw i32 %storemerge1.i, 2
+  br i1 %tobool.i, label %bar.exit, label %for.cond.i
+
+for.cond.i:                                       ; preds = %for.body.i
+  store i32 %add.i.i, i32* @a, align 4
+  %cmp.i = icmp slt i32 %storemerge1.i, 3
+  br i1 %cmp.i, label %for.body.i, label %bar.exit
+
+bar.exit:                                         ; preds = %for.cond.i, %for.body.i
+  ret i32 0
+}