[llvm] 29d054b - [SimplifyCFG] Preserve knowledge about guarding condition by adding assume

[Max Kazantsev via llvm-commits]

Author: Max Kazantsev
Date: 2021-09-08T14:05:17+07:00
New Revision: 29d054bf126870706f909a821895770bd9af42b3

URL: https://github.com/llvm/llvm-project/commit/29d054bf126870706f909a821895770bd9af42b3
DIFF: https://github.com/llvm/llvm-project/commit/29d054bf126870706f909a821895770bd9af42b3.diff

LOG: [SimplifyCFG] Preserve knowledge about guarding condition by adding assume

This improvement adds "assume" after removal of branch basing on UB in successor block.

Consider the following example:

```
pred:
  x = ...
  cond = x > 10
  br cond, bb, other.succ

bb:
  phi [nullptr, pred], ... // other possible preds
  load(phi) // UB if we came from pred

other.succ:
  // here we know that x <= 10, but this knowledge is lost
  // after the branch is turned to unconditional unless we
  // preserve it with assume.
```

If we remove the branch basing on knowledge about UB in a successor block,
then the fact that x <= 10 is other.succ might be lost if this condition is
not inferrable from any dominating condition. To preserve this knowledge, we
can add assume intrinsic with (possibly inverted) branch condition.

Patch by Dmitry Bakunevich!

Differential Revision: https://reviews.llvm.org/D109054
Reviewed By: lebedev.ri

Added: 
    

Modified: 
    llvm/lib/Transforms/Utils/SimplifyCFG.cpp
    llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index ba3129f5581fe..737b4f97a97a3 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -6598,9 +6598,17 @@ static bool removeUndefIntroducingPredecessor(BasicBlock *BB,
           // destination from conditional branches.
           if (BI->isUnconditional())
             Builder.CreateUnreachable();
-          else
+          else {
+            // Preserve guarding condition in assume, because it might not be
+            // inferrable from any dominating condition.
+            Value *Cond = BI->getCondition();
+            if (BI->getSuccessor(0) == BB)
+              Builder.CreateAssumption(Builder.CreateNot(Cond));
+            else
+              Builder.CreateAssumption(Cond);
             Builder.CreateBr(BI->getSuccessor(0) == BB ? BI->getSuccessor(1)
                                                        : BI->getSuccessor(0));
+          }
           BI->eraseFromParent();
           if (DTU)
             DTU->applyUpdates({{DominatorTree::Delete, Predecessor, BB}});

diff  --git a/llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll b/llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll
index 912598197080e..91ab3e54c2308 100644
--- a/llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll
+++ b/llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll
@@ -14,8 +14,12 @@ define void @eggs(i1 %arg, i1 %arg16, %0* %arg17, %0* %arg18, %0* %arg19) {
 ; CHECK-NEXT:    unreachable
 ; CHECK:       bb21:
 ; CHECK-NEXT:    [[I:%.*]] = icmp eq %0* [[ARG17:%.*]], null
+; CHECK-NEXT:    [[TMP0:%.*]] = xor i1 [[I]], true
+; CHECK-NEXT:    call void @llvm.assume(i1 [[TMP0]])
 ; CHECK-NEXT:    call void @hoge()
-; CHECK-NEXT:    [[I27:%.*]] = getelementptr inbounds [[TMP0:%.*]], %0* [[ARG19:%.*]], i64 0, i32 0
+; CHECK-NEXT:    [[TMP1:%.*]] = xor i1 [[ARG16:%.*]], true
+; CHECK-NEXT:    call void @llvm.assume(i1 [[TMP1]])
+; CHECK-NEXT:    [[I27:%.*]] = getelementptr inbounds [[TMP0]], %0* [[ARG19:%.*]], i64 0, i32 0
 ; CHECK-NEXT:    [[I28:%.*]] = load %1*, %1** [[I27]], align 8
 ; CHECK-NEXT:    call void @pluto.1(%1* [[I28]])
 ; CHECK-NEXT:    call void @pluto()