Partial fix for bug 22589

Don't spend the entire iteration space in the scalar loop prologue if
computing the trip count overflows.  This change also gets rid of the
backedge check in the prologue loop and the extra check for
overflowing trip-count.

Differential Revision: http://reviews.llvm.org/D7715



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229731 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/LoopUnroll/tripcount-overflow.ll

@@ -1,19 +1,28 @@
 ; RUN: opt < %s -S -unroll-runtime -unroll-count=2 -loop-unroll | FileCheck %s
 target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
 
-; When prologue is fully unrolled, the branch on its end is unconditional.
-; Unrolling it is illegal if we can't prove that trip-count+1 doesn't overflow,
-; like in this example, where it comes from an argument.
-;
-; This test is based on an example from here:
-; http://stackoverflow.com/questions/23838661/why-is-clang-optimizing-this-code-out
-;
+; This test case documents how runtime loop unrolling handles the case
+; when the backedge-count is -1.
+
+; If %N, the backedge-taken count, is -1 then %0 unsigned-overflows
+; and is 0.  %xtraiter too is 0, signifying that the total trip-count
+; is divisible by 2.  The prologue then branches to the unrolled loop
+; and executes the 2^32 iterations there, in groups of 2.
+
+
+; CHECK: entry:
+; CHECK-NEXT: %0 = add i32 %N, 1
+; CHECK-NEXT: %xtraiter = and i32 %0, 1
+; CHECK-NEXT: %lcmp.mod = icmp ne i32 %xtraiter, 0
+; CHECK-NEXT: br i1 %lcmp.mod, label %while.body.prol, label %entry.split
+
 ; CHECK: while.body.prol:
-; CHECK: br i1
+; CHECK: br label %entry.split
+
 ; CHECK: entry.split:
 
 ; Function Attrs: nounwind readnone ssp uwtable
-define i32 @foo(i32 %N) #0 {
+define i32 @foo(i32 %N) {
 entry:
   br label %while.body
 
@@ -26,5 +35,3 @@ while.body:                                       ; preds = %while.body, %entry
 while.end:                                        ; preds = %while.body
   ret i32 %i
 }
-
-attributes #0 = { nounwind readnone ssp uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }