[SCEV] Clarify behavior around max backedge taken count

This is a re-application of a r303497 that was reverted in r303498.
I thought it had broken a bot when it had not (the breakage did not
go away with the revert).

This change makes the split between the "exact" backedge taken count
and the "maximum" backedge taken count a bit more obvious.  Both of
these are upper bounds on the number of times the loop header
executes (since SCEV does not account for most kinds of abnormal
control flow), but the latter is guaranteed to be a constant.

There were a few places where the max backedge taken count *was* a
non-constant; I've changed those to compute constants instead.

At this point, I'm not sure if the constant max backedge count can be
computed by calling `getUnsignedRange(Exact).getUnsignedMax()` without
losing precision.  If it can, we can simplify even further by making
`getMaxBackedgeTakenCount` a thin wrapper around
`getBackedgeTakenCount` and `getUnsignedRange`.

llvm-svn: 303531

2 files changed, 6 insertions, 6 deletions

diff --git a/llvm/test/Analysis/ScalarEvolution/nsw.ll b/llvm/test/Analysis/ScalarEvolution/nsw.ll
index a3752919d33..39b958d3ea0 100644
--- a/llvm/test/Analysis/ScalarEvolution/nsw.ll
+++ b/llvm/test/Analysis/ScalarEvolution/nsw.ll
@@ -102,7 +102,7 @@ for.body.i.i:                                     ; preds = %entry, %for.body.i.
   %cmp.i.i = icmp eq i32* %ptrincdec.i.i, %end
   br i1 %cmp.i.i, label %_ZSt4fillIPiiEvT_S1_RKT0_.exit, label %for.body.i.i
 ; CHECK: Loop %for.body.i.i: backedge-taken count is ((-4 + (-1 * %begin) + %end) /u 4)
-; CHECK: Loop %for.body.i.i: max backedge-taken count is ((-4 + (-1 * %begin) + %end) /u 4)
+; CHECK: Loop %for.body.i.i: max backedge-taken count is 4611686018427387903
 _ZSt4fillIPiiEvT_S1_RKT0_.exit:                   ; preds = %for.body.i.i, %entry
   ret void
 }
diff --git a/llvm/test/Analysis/ScalarEvolution/trip-count-pow2.ll b/llvm/test/Analysis/ScalarEvolution/trip-count-pow2.ll
index 8d053060b50..04d1b9544ab 100644
--- a/llvm/test/Analysis/ScalarEvolution/trip-count-pow2.ll
+++ b/llvm/test/Analysis/ScalarEvolution/trip-count-pow2.ll
@@ -14,7 +14,7 @@ exit:
 
 ; CHECK-LABEL: @test1
 ; CHECK: Loop %loop: backedge-taken count is ((-32 + (96 * %n)) /u 32)
-; CHECK: Loop %loop: max backedge-taken count is ((-32 + (96 * %n)) /u 32)
+; CHECK: Loop %loop: max backedge-taken count is 134217727
 }
 
 ; PR19183
@@ -32,7 +32,7 @@ exit:
 
 ; CHECK-LABEL: @test2
 ; CHECK: Loop %loop: backedge-taken count is ((-32 + (32 * (%n /u 32))) /u 32)
-; CHECK: Loop %loop: max backedge-taken count is ((-32 + (32 * (%n /u 32))) /u 32)
+; CHECK: Loop %loop: max backedge-taken count is 134217727
 }
 
 define void @test3(i32 %n) {
@@ -49,7 +49,7 @@ exit:
 
 ; CHECK-LABEL: @test3
 ; CHECK: Loop %loop: backedge-taken count is ((-32 + (32 * %n)) /u 32)
-; CHECK: Loop %loop: max backedge-taken count is ((-32 + (32 * %n)) /u 32)
+; CHECK: Loop %loop: max backedge-taken count is 134217727
 }
 
 define void @test4(i32 %n) {
@@ -66,7 +66,7 @@ exit:
 
 ; CHECK-LABEL: @test4
 ; CHECK: Loop %loop: backedge-taken count is ((-4 + (-1431655764 * %n)) /u 4)
-; CHECK: Loop %loop: max backedge-taken count is ((-4 + (-1431655764 * %n)) /u 4)
+; CHECK: Loop %loop: max backedge-taken count is 1073741823
 }
 
 define void @test5(i32 %n) {
@@ -83,5 +83,5 @@ exit:
 
 ; CHECK-LABEL: @test5
 ; CHECK: Loop %loop: backedge-taken count is ((-4 + (4 * %n)) /u 4)
-; CHECK: Loop %loop: max backedge-taken count is ((-4 + (4 * %n)) /u 4)
+; CHECK: Loop %loop: max backedge-taken count is 1073741823
 }