[LoopFlatten] Fix invalid assertion (PR49571)

[TaWeiTu]

Summary

The InductionPHI is not necessarily the increment instruction, as
demonstrated in pr49571.ll.
This patch removes the assertion and instead bails out from the
LoopFlatten pass if that happens.

This fixes https://bugs.llvm.org/show_bug.cgi?id=49571

[SjoerdMeijer]

Yep, agreed, seems reasonable. Thanks for fixing.

[TaWeiTu]

In D99252#2647105, @SjoerdMeijer wrote:

Yep, agreed, seems reasonable. Thanks for fixing.

Thanks!

[fhahn]

llvm/lib/Transforms/Scalar/LoopFlatten.cpp

181

The message seems a bit confusing, perhaps it should say Incoming value from latch is not the increment inst?

llvm/test/Transforms/LoopFlatten/pr49571.ll

4

given that the test is very simple, I think it would be better to explicitly check the full output. Also, it would be helpful if you could add a comment explaining what scenario this tests

[TaWeiTu]

Hi Florian,

Sorry for the haste commit.
I will fix these issues in a NFC patch shortly.
Is that OK or should I revert this patch for now?

[SjoerdMeijer]

llvm/test/Transforms/LoopFlatten/pr49571.ll

4

Ah yeah, agreed, I missed that. A follow up commit should do, if you're comfortable I don't think that needs another review, but I guess we are happy to look again of course. Definitely not needed to revert the first commit.

[TaWeiTu]

The debug message and test description are improved in rG4d9d7368759

llvm/lib/Transforms/Scalar/LoopFlatten.cpp

if (!Increment || Increment->hasNUsesOrMore(3)) {
     LLVM_DEBUG(dbgs() << "Cound not find valid increment\n");
     return false;
   }
   IterationInstructions.insert(Increment);
   LLVM_DEBUG(dbgs() << "Found increment: "; Increment->dump());
   LLVM_DEBUG(dbgs() << "Found limit: "; Limit->dump());
 
   assert(InductionPHI->getNumIncomingValues() == 2);
-  assert(InductionPHI->getIncomingValueForBlock(Latch) == Increment &&
-         "PHI value is not increment inst");
+
+  if (InductionPHI->getIncomingValueForBlock(Latch) != Increment) {
+    LLVM_DEBUG(dbgs() << "PHI value is not increment inst");
+    return false;
+  }
 
   auto *CI = dyn_cast<ConstantInt>(
       InductionPHI->getIncomingValueForBlock(L->getLoopPreheader()));
   if (!CI || !CI->isZero()) {
     LLVM_DEBUG(dbgs() << "PHI value is not zero: "; CI->dump());
     return false;
   }
 

llvm/test/Transforms/LoopFlatten/pr49571.ll

+; RUN: opt < %s -S -loop-flatten -verify-loop-info -verify-dom-info -verify-scev -verify | FileCheck %s
+
+; CHECK-LABEL: @main
+
+define dso_local void @main() {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.end, %entry
+  br label %for.body
+
+for.body:                                         ; preds = %for.inc, %for.cond
+  %a.03 = phi i32 [ 0, %for.cond ], [ %inc, %for.inc ]
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %0 = add i32 %a.03, 1
+  %cmp = icmp slt i32 %0, 10
+  %inc = add nsw i32 %a.03, 1
+  br i1 %cmp, label %for.body, label %for.end
+
+for.end:                                          ; preds = %for.inc
+  br label %for.cond
+}