[llvm] 6c99e63 - [SCEV] By more careful when traversing phis in isImpliedViaMerge.

[Florian Hahn via llvm-commits]

Author: Florian Hahn
Date: 2021-05-07T19:52:29+01:00
New Revision: 6c99e631201aaea0a75708749cbaf2ba08a493f9

URL: https://github.com/llvm/llvm-project/commit/6c99e631201aaea0a75708749cbaf2ba08a493f9
DIFF: https://github.com/llvm/llvm-project/commit/6c99e631201aaea0a75708749cbaf2ba08a493f9.diff

LOG: [SCEV] By more careful when traversing phis in isImpliedViaMerge.

I think currently isImpliedViaMerge can incorrectly return true for phis
in a loop/cycle, if the found condition involves the previous value of

Consider the case in exit_cond_depends_on_inner_loop.

At some point, we call (modulo simplifications)
isImpliedViaMerge(<=, %x.lcssa, -1, %call, -1).

The existing code tries to prove IncV <= -1 for all incoming values
InvV using the found condition (%call <= -1). At the moment this succeeds,
but only because it does not compare the same runtime value. The found
condition checks the value of the last iteration, but the incoming value
is from the *previous* iteration.

Hence we incorrectly determine that the *previous* value was <= -1,
which may not be true.

I think we need to be more careful when looking at the incoming values
here. In particular, we need to rule out that a found condition refers to
any value that may refer to one of the previous iterations. I'm not sure
there's a reliable way to do so (that also works of irreducible control
flow).

So for now this patch adds an additional requirement that the incoming
value must properly dominate the phi block. This should ensure the
values do not change in a cycle. I am not entirely sure if will catch
all cases and I appreciate a through second look in that regard.

Alternatively we could also unconditionally bail out in this case,
instead of checking the incoming values

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D101829

Added: 
    

Modified: 
    llvm/lib/Analysis/ScalarEvolution.cpp
    llvm/test/Transforms/IRCE/decrementing-loop.ll
    llvm/test/Transforms/IndVarSimplify/eliminate-exit.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 4005597de792b..e8c3f15083022 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -10785,6 +10785,10 @@ bool ScalarEvolution::isImpliedViaMerge(ICmpInst::Predicate Pred,
       if (!dominates(RHS, IncBB))
         return false;
       const SCEV *L = getSCEV(LPhi->getIncomingValueForBlock(IncBB));
+      // Make sure L does not refer to a value from a potentially previous
+      // iteration of a loop.
+      if (!properlyDominates(L, IncBB))
+        return false;
       if (!ProvedEasily(L, RHS))
         return false;
     }

diff  --git a/llvm/test/Transforms/IRCE/decrementing-loop.ll b/llvm/test/Transforms/IRCE/decrementing-loop.ll
index 462607162bcbe..5ae4412b34bd7 100644
--- a/llvm/test/Transforms/IRCE/decrementing-loop.ll
+++ b/llvm/test/Transforms/IRCE/decrementing-loop.ll
@@ -210,16 +210,17 @@ exit:
   ret void
 }
 
+; TODO: we need to be more careful when trying to look through phi nodes in
+; cycles, because the condition to prove may reference the previous value of
+; the phi. So we currently fail to optimize this case.
 ; Check that we can figure out that IV is non-negative via implication through
 ; two Phi nodes, one being AddRec.
 define void @test_05(i32* %a, i32* %a_len_ptr, i1 %cond) {
 
 ; CHECK-LABEL: test_05
-; CHECK:       mainloop:
-; CHECK-NEXT:    br label %loop
-; CHECK:       loop:
-; CHECK:         br i1 true, label %in.bounds, label %out.of.bounds
-; CHECK:       loop.preloop:
+; CHECK: entry:
+; CHECK:   br label %merge
+; CHECK-NOT: mainloop
 
  entry:
   %len.a = load i32, i32* %a_len_ptr, !range !0

diff  --git a/llvm/test/Transforms/IndVarSimplify/eliminate-exit.ll b/llvm/test/Transforms/IndVarSimplify/eliminate-exit.ll
index eec7908b6a8b9..e574c2f84ea33 100644
--- a/llvm/test/Transforms/IndVarSimplify/eliminate-exit.ll
+++ b/llvm/test/Transforms/IndVarSimplify/eliminate-exit.ll
@@ -453,7 +453,8 @@ define i32 @exit_cond_depends_on_inner_loop() {
 ; CHECK-NEXT:    br i1 [[INNER_COND]], label [[INNER]], label [[OUTER_EXITING_1:%.*]]
 ; CHECK:       outer.exiting.1:
 ; CHECK-NEXT:    [[X_LCSSA:%.*]] = phi i32 [ [[X]], [[INNER]] ]
-; CHECK-NEXT:    br i1 false, label [[EXIT:%.*]], label [[OUTER_LATCH]]
+; CHECK-NEXT:    [[OUTER_COND_1:%.*]] = icmp sgt i32 [[X_LCSSA]], -1
+; CHECK-NEXT:    br i1 [[OUTER_COND_1]], label [[EXIT:%.*]], label [[OUTER_LATCH]]
 ; CHECK:       outer.latch:
 ; CHECK-NEXT:    [[IV_OUTER_NEXT]] = add nuw nsw i32 [[IV_OUTER]], 1
 ; CHECK-NEXT:    [[OUTER_COND_2:%.*]] = icmp ult i32 [[IV_OUTER]], 100